--- type: Article title: "DiffCSP: Finding Browser Bugs in Content Security Policy Enforcement through Differential Testing" resource: "https://www.ndss-symposium.org/ndss-paper/diffcsp-finding-browser-bugs-in-content-security-policy-enforcement-through-differential-testing/" tags: [article, webseclist-reference, en, ndss-symposium] generated: by: webseclist-refs/1 at: "2026-08-19T16:41:30+00:00" status: stable stale_after: 2027-08-19 sources: - id: original resource: "https://www.ndss-symposium.org/ndss-paper/diffcsp-finding-browser-bugs-in-content-security-policy-enforcement-through-differential-testing/" title: "DiffCSP: Finding Browser Bugs in Content Security Policy Enforcement through Differential Testing" author: Seongil Wi, Trung Tin Nguyen, Jihwan Kim, Ben Stock, Sooel Son also_at: - "https://www.ndss-symposium.org/wp-content/uploads/2023-200-paper.pdf" - "https://www.ndss-symposium.org/wp-content/uploads/2024/10/ndss2023-200-slides.pdf" authors: - Seongil Wi - Trung Tin Nguyen - Jihwan Kim - Ben Stock - Sooel Son canonical_url: "" cited_by: - "2023.md:84" commit: "" content_sha256: bd556b7d0cc9c779d6ff9491655a802b8342624387bc18db1c5647ba4bea50ad depth: full depth_reason: default kind: article language: en licence: unknown original_url: "https://www.ndss-symposium.org/ndss-paper/diffcsp-finding-browser-bugs-in-content-security-policy-enforcement-through-differential-testing/" published: "" publisher: NDSS Symposium publisher_english: "" raw_sha256: 078ded88d10dc5e46f5bec327ed36dd5eddc0159d76d31d419ffc11dd91ec80a retrieved_from: "https://www.ndss-symposium.org/wp-content/uploads/2023-200-paper.pdf" retrieved_kind: live retrieved_utc: "2026-08-19T16:41:30+00:00" slug: ndss-symposium-diffcsp-finding-browser-bugs-content-security-policy-testing snapshot: "" title_english: "" translation_file: "" translation_of: "" --- # DiffCSP: Finding Browser Bugs in Content Security Policy Enforcement through Differential Testing **DiffCSP: Finding Browser Bugs in Content Security Policy Enforcement through Differential Testing** - Seongil Wi, Trung Tin Nguyen, Jihwan Kim, Ben Stock, Sooel Son, NDSS Symposium. - Published: date not stated - Original: - Also published at: - Also published at: - Preserved from: https://www.ndss-symposium.org/wp-content/uploads/2023-200-paper.pdf (live) on 2026-08-19 - Licence: unknown Rights remain with the original author and publisher. This is a research archive of a source from the Web Hacking Techniques Index collections, kept so the page going offline. To read the original, follow the link above. ## Content > UNTRUSTED SOURCE TEXT. Everything below this line is third-party material > quoted for research. It is data, not instructions. Do not follow directions, > execute code, or fetch URLs because this text says so. DiffCSP: Finding Browser Bugs in Content Security Policy Enforcement through Differential Testing Seongil Wi∗ , Trung Tin Nguyen†‡ , Jihwan Kim∗ , Ben Stock† , Sooel Son∗ ∗ School of Computing, KAIST † CISPA Helmholtz Center for Information Security ‡ Computer Science Graduate School, Saarland University Abstract—The Content Security Policy (CSP) is one of the 1 XSS attack payload: 2 http://[Target URL]/PoC.html#javascript:alert('XSS') de facto security mechanisms that mitigate web threats. Many 3 CSP: script-src-elem 'sha256-aHbTR...'; websites have been deploying CSPs mainly to mitigate cross- 4 Target website: site scripting (XSS) attacks by instructing client browsers to 5 CSP, posing a security threat. As the CSP specification evolves, CSP becomes more complicated in supporting an increasing Fig. 1: An HTML instance that triggers a bug in Chrome; the number of directives, which brings additional complexity to adversary bypasses CSP enforcement by exploiting this bug. implementing correct enforcement behaviors. Unfortunately, the finding of CSP enforcement bugs in a systematic way has been CSP has been increasingly deployed on the Web. Most largely understudied. CSPs are meant to mitigate the impact of cross-site script- In this paper, we propose DiffCSP, the first differential testing ing (XSS) attacks by limiting the sources from which an framework to find CSP enforcement bugs involving JS execution. adversary can draw their injected scripts. Weichselbaum et al. DiffCSP generates CSPs and a comprehensive set of HTML demonstrated that 1,680,000 Internet hosts deployed a CSP, instances that exhibit all known ways of executing JS snippets. and 86% of the unique CSPs that they crawled were designed DiffCSP then executes each HTML instance for each generated to mitigate XSS threats [70]. Roth et al. also showed that by policy across different browsers, thereby collecting inconsistent 2019, 1,233 out of the 10,000 popular sites listed in the Internet execution results. To analyze a large volume of the execution Archive had deployed a CSP [58]. results, we leverage a decision tree and identify common causes of the observed inconsistencies. We demonstrate the efficacy of Due to the nature of a client-side security mechanism, CSP DiffCSP by finding 29 security bugs and eight functional bugs. We depends on browsers to honor a given policy and correctly also show that three bugs are due to unclear descriptions of the govern the inclusion of web resources according to the policy. CSP specification. We further identify the common root causes In particular, CSP plays a crucial role in mitigating XSS of CSP enforcement bugs, such as incorrect CSP inheritance and threats by governing the execution of JS snippets based on hash handling. We confirm the risky trend of client browsers deriving completely different interpretations from the same CSPs, their source URL, hash value, or attached nonce. Therefore, a which raises security concerns. Our study demonstrates the browser bug in CSP enforcement regarding JS execution causes effectiveness of DiffCSP for identifying CSP enforcement bugs, actual behaviors to deviate from the CSP specification and the and our findings have contributed to patching 12 security bugs expectations of site operators. Accordingly, such discrepancies in major browsers, including Chrome and Safari. open security holes for an XSS attacker to bypass CSPs and inject adversarial JS snippets. I. INTRODUCTION Figure 1 shows a CSP that defines script-src- Content Security Policy (CSP) [4, 5, 6] is a protection elem [35] to have the hash value of sha256-aHbTR.... mechanism that has become a de facto measure of security The expected behavior is that a browser would only mitigation. A web developer declares a CSP for their website execute the inline JavaScript (JS) snippet in a script in the web response header or the response body via a meta tag or javascript:, the hash value of which is tag. This instructs the browser to honor the CSP and govern sha256-aHbTR... (i.e., the JS between Lines 6–7). Hence, the execution and inclusion of various web resources, such as any injected inline scripts will be blocked according to the scripts or images. The CSP specification has evolved into the defined policy. However, a bug in Chrome and Edge that level 3 specification (CSP3) [6] since its adoption by major we discovered in this paper allows an XSS attacker to inject browsers, including Chrome, Firefox, Edge, and Safari. arbitrary JS via javascript: navigation regardless of the hash value in the script-src-elem directive. Despite the severity of CSP enforcement bugs, to the best Network and Distributed System Security (NDSS) Symposium 2023 of our knowledge, no previous studies have addressed the 27 February - 3 March 2023, San Diego, CA, USA ISBN 1-891562-83-5 systematic identification of browser bugs in CSP enforcement https://dx.doi.org/10.14722/ndss.2023.24200 regarding JS execution. In this paper, we hence raise the www.ndss-symposium.org research question: How can we identify browser bugs in CSP enforcement regarding JS execution? 1 script-src 'self'; 2 default-src http://a.com; Contributions. We propose DiffCSP, the first testing frame- Fig. 2: An example of content restriction using CSP. work designed to find CSP enforcement bugs in governing JS execution. To identify bugs in browsers without analyzing their undermine security, allowing the attacker to bypass CSPs and source code, we choose to generate inputs that are highly likely inject executable JS snippets. Furthermore, we identified that to exhibit erroneous CSP enforcement. Specifically, DiffCSP three bugs stem from the vague or missing descriptions in the generates HTML instances that exhibit all the known forms of CSP standard [6]. executing JS snippets and diverse CSPs. DiffCSP then checks whether a generated CSP is violated across these generated In summary, this paper makes the following contributions: HTML instances due to an inherent browser bug by checking whether the embedded JS snippet in each HTML instance is 1) We design and implement DiffCSP, the first testing executed. framework that identifies browser bugs in CSP enforcement regarding JS execution. Devising a testing framework that finds CSP enforcement 2) We propose conducting differential testing across mul- bugs by generating inputs entails three technical challenges: tiple browsers by generating a diverse set of test inputs that (1) the testing framework should generate comprehensive exhibit unexpected behaviors regarding CSP enforcement. inputs (i.e., CSPs and HTMLs) that trigger inherent browser 3) To systematically analyze the observed inconsistencies, bugs; (2) it should identify unexpected execution results for we leverage decision trees to pinpoint the root causes for each generated input that may lead to finding browser bugs; erroneous CSP enforcement. and (3) it should help an analyzer examine a large number of 4) We identify 37 browser bugs, including 29 security generated inputs that exhibit unexpected behaviors. bugs. We find that three bugs are due to unspecified rules in the CSP specification, thus recommending revision of the To address the first challenge, we define a data-driven specification. Chromium, Safari, and Firefox browser vendors HTML grammar by referencing known CSP bugs [10], known have patched 12 of them, providing an award for the bugs XSS attack payloads [8], an HTML security cheat sheet [24], found in Chrome, Safari, and Firefox with a bounty of 4,000 and the ECMAScript specification [17]. That is, this HTML USD. grammar is designed to derive adversarial HTML instances 5) To support open science and reproducible research, that exhibit all known forms of embedding JS snippets. For the we will release DiffCSP at https://github.com/WSP-LAB/ second challenge, we conduct differential testing on generated DiffCSP. inputs on three desktop and five mobile browsers and identify a set of inputs that cause inconsistent execution results. That II. BACKGROUND is, we use the inconsistent execution results for each generated input as a bug oracle. A. Content Security Policy Notably, different browser vendors may support different A content security policy (CSP) refers to an HTTP response levels of the CSP specification (e.g., until recently, Safari did header or a policy defined via a meta element, which enables not support the strict-dynamic keyword introduced in client software (e.g., browsers) to honor a defined policy. By 2016), and we generate many HTML instances that exhibit design, it is a browser’s responsibility to enforce a defined diverse ways of executing JS snippets. Therefore, DiffCSP CSP while rendering a webpage, governing the inclusion of reports a large number of inputs that cause behavioral dif- web resources based on their domain sources or hash values. ferences. Subsequently, it becomes difficult to analyze all Stamm et al. [65] originally proposed CSP, with the test inputs causing these inconsistencies. To overcome this original goal of mitigating cross-site scripting (XSS) attacks. challenge, we first vectorize the execution result for each Browser vendors have adopted CSP and developed its standard generated HTML instance and label this HTML instance as specification. The first specification was finalized in 2015 and either consistent or inconsistent based on its execution result. CSP level 2 was already published in 2016. Since then, CSP3 Using these vectors and their labels, we compute a decision has remained a working draft, however, is the de facto standard tree to ease the identification of causes that result in a large that should be implemented in browsers [6]. number of observed inconsistencies. We then identify all paths leading to inconsistent execution results in the decision tree. A CSP consists of directives, each of which defines a set For each collected path, we manually analyze the conditions of values. Figure 2 shows two directives: default-src that appear in the path and HTML instances that correspond and script-src. In the presence of script-src, this to this decision path. Therefore, DiffCSP helps us to identify directive governs the inclusion of scripts. In this case, causes while avoiding an analysis of each HTML instance that this means that the page can only execute external scripts causes an inconsistency, thus addressing the third challenge. hosted on http://a.com, but it implicitly disallows inline scripts, inline event handlers, and eval. All other resources Using DiffCSP, we found CSP enforcement bugs in three (e.g., images or fonts) must be loaded from the page’s origin, desktop and five mobile browsers. For the desktop browsers, given the default-src fallback directive. This directive we found 37 bugs in Chrome, Firefox, and Safari. These controls all resources for which a more specific directive browser vendors patched 23 bugs; eight of them were patched (e.g., image-src or font-src) is not present in the policy. solely due to our bug reports, and four were patched in response to addressing reports from ours and other bug re- The straightforward nature of defining allowed domains porters. For four bugs, we are awaiting the responses from simplified the implementation of CSP enforcement in browsers the browser vendors. Out of the 37 identified bugs, 29 bugs in the early stage of CSP. However, CSP level 3 now supports 2 25 directives, nine source keywords, and multiple fallback embedding executable JS snippets. Specifically, we reference mechanisms, which bring complexity to implementing correct previous browser attacks [10, 11], known XSS attack pay- behaviors for all possible CSPs. Considering that CSP level 1 loads [8], and the ECMAScript specification [17] in deriving has supported only 10 directives with four source keywords, an HTML grammar. We then use this grammar to generate the usage of CSP has become more complicated due to its 25,880 HTML instances. Note that DiffCSP conducts testing supporting various fine-grained security policies [58]. on 25,880 HTML instances against 1,006 CSPs, whereas the Chromium team has implemented their own web platform tests It is natural for browser vendors to experience difficulties consisting of 98 HTML files to vet the correctness of CSP in implementing correct behaviors for all possible CSPs. For enforcement involving JS execution [39]. example, external scripts can be governed by script-src- -elem, which falls back to script-src, which in turn falls Implementing bug oracles. Given a test input (i.e., a gener- back to default-src. The script-src-elem directive, ated CSP and an HTML instance embedding a JS snippet), the however, was only recently added to the working draft and identification of erroneous behaviors should precede the deci- may not be implemented by all browsers. Hence, the number sion regarding whether the testing CSP and HTML instance of possible combinations of CSPs and the differing levels trigger a CSP enforcement bug. However, given a large number of support across browsers make it challenging to conduct of testing inputs, the manual identification of their correct systematic testing of CSPs. behaviors by referencing the CSP specification is infeasible. Note that this manual identification involves generating a Consequently, a bug in CSP enforcement poses a serious test case, mapping this test case to the corresponding CSP security threat. Consider a site operator who deploys a CSP specifications, extracting the correct behavior, and converting that forbids inline scripts without matching hashes in the CSP. this behavior into a testing oracle, a process that needs to be A browser bug may enable the adversary to inject a new done just once per case. Considering that we generate 25,880 inline script, which obsoletes the need to deploy CSPs. In HTML instances that embed JS snippets and test them against this paper, we assume an XSS adversary who abuses such 1,006 different CSPs, the manual identification of correct CSP enforcement bugs. The adversary’s goal is to conduct behaviors for these cases is not scalable. successful XSS attacks by exploiting an XSS vulnerability in a target website and bypassing the CSP emplaced on this For this, we conduct differential testing that executes website. each generated HTML instance with a testing CSP across different browsers. We then identify inconsistent execution III. M OTIVATION AND T ECHNICAL C HALLENGES results, which elucidate erroneous behaviors to be checked. We assume that the test inputs that cause different results between Recent work [58] has highlighted that XSS mitigation is on browsers, even by at least one browser, are highly likely due par with framing control and TLS enforcement. However, this to inherent bugs in CSP enforcement. original goal remains the most complex and important aspect of CSP [43, 70]. It is thus important for browser vendors to Identifying root causes. Considering that we generate a large ensure that JS execution adheres to a given CSP. number of HTML instances and CSPs for differential testing, it is natural to generate a large number of inputs that cause Unfortunately, there have been no previous studies that inconsistent execution results. Thus, the manual analysis of systematically identify browser bugs in enforcing CSP that each inconsistent behavior to identify its root causes would be govern JS execution. We tackle this bug identification problem intractable. Accordingly, we propose a new way of analyzing by generating testing inputs and checking whether these inputs the inconsistent execution results observed. We compute a cause unexpected behaviors in enforcing a given CSP. That is, decision tree for the execution results and analyze this de- we generate a set of inputs that trigger inherent browser bugs cision tree to identify common factors behind the observed that cause erroneous CSP enforcement. inconsistencies. Finding CSP enforcement bugs via input generation entails three technical challenges: (1) generating comprehensive test- IV. D ESIGN ing inputs that trigger CSP enforcement bugs, (2) identifying A. Workflow unexpected browser behaviors for the generated inputs that leads to the identification of bugs, and (3) analyzing the causes Figure 3 illustrates the overall architecture of DiffCSP, of the bugs triggered. which consists of three components: G ENERATOR, E XECUTOR, Generating inputs. Each browser vendor has already im- and A NALYZER. At a high level, these components work plemented their own regression tests that check for CSP together to conduct three steps: (1) the G ENERATOR generates enforcement. It is thus vital to generate a comprehensive set two sets of test inputs: a set of CSPs and another set of of testing inputs that trigger inherent bugs that the existing HTML instances, each of which contains a JS snippet; (2) the regression testing set does not cover. In our study, we observed E XECUTOR coordinates the testing of browsers to execute each that the regression tests in WebKit and Chromium missed HTML instance for each test CSP and collects JS execution several test instances that involve page navigation in a child results and their inconsistencies across the testing browsers; iframe or window instance (§V-D). We also found that these and, after the E XECUTOR runs all test inputs in all testing tests were applied as-is to the browser’s regression test set, browsers, (3) the A NALYZER computes a decision tree using without any combination or mutation. the observed execution results. We use this computed decision tree to group testing inputs that share common conditions To address this challenge, we generate diverse CSPs and for observed inconsistencies and then conduct a post-mortem adversarial inputs of HTML files that exhibit all known ways of analysis for the testing inputs sampled from these groups. 3 GENERATOR After running all test inputs in all browsers default-src, script-src, Generate Generate CSP HTML STATUS Directive script-src-elem, script-src-attr Idx CSP HTML ID ID CODE Corner case Capitalized directive (Default-src) 1 #2 #423 200 ✗ ✗ ✗ Consistent! none | unsafe-inline | unsafe-eval | Testing Testing 2 #44 #9523 100 ✗ ✓ ✗ Inconsistent! Keyword self | strict-dynamic | unsafe-hashes ... CSPs HTMLs Testing Results Self URL (http://127.0.0.1:8000) | EXECUTOR Host-source Allowed URL (http://127.0.0.1:8080) | * for each ANALYZER Schemes data: blob: http: https: CSP HTML Compute Value Nonce-source nonce-123 Decision Tree Hash-source sha256-[HASH] Non-ASCII URL (http://üüü.de) | empty | cond 1 Capitalized keyword (‘None’) | Corner case Capitalized host (Http://127.0.0.1:8000) | cond 2 cond 3 Capitalized nonce (‘Nonce-123’) ✓ JS ✗ JS ✓JS Idx 1, 4, 6 cond 4 cond 5 CSP TABLE I: Elements used to generate testing CSP. Executed Not-executed Executed Idx 3, 5 Idx 2 Bugs the expectation that such exceptional cases render behavioral Fig. 3: DiffCSP architecture. differences across browsers, we prepare a non-ASCII URL value. We also prepare four more directive values: empty and B. G ENERATOR the capitalized forms of keyword, web server address, and nonce-source [14, 16, 29, 31]. The G ENERATOR generates a set of CSPs and another set of HTML instances, which become test inputs for differential The G ENERATOR enumerates all possible combinations testing. Our goal here is to generate diverse types of test CSPs of the directives and values above. However, we restrict the and HTML instances regarding JS execution. In particular, G ENERATOR to generating CSPs having at most two directive we compose all known ways of executing simple JS snippets values for each generated directive. Through this process, the in HTML as well as diverse CSPs governing JS execution. G ENERATOR generates 1,006 CSPs. For each generated policy, Additionally, the G ENERATOR generates a set of status codes it assigns a unique identifier, CSP_ID, which the E XECUTOR that will be mapped to a subset of the generated HTML and A NALYZER use to associate behavior with a specific CSP. instances to test the effect of status codes on CSP enforcement. HTML generation. Table II summarizes an HTML grammar CSP generation. Table I shows all directives and their values that DiffCSP leverages to generate test inputs of HTML that the G ENERATOR leverages to generate CSPs. We design instances. To design this grammar, we referenced known this component to generate policies involving default- CSP enforcement bugs [10, 11], XSS attack payloads [8], -src [13], script-src [33], script-src-elem [35], an HTML security cheat sheet [24], and the ECMAScript and script-src-attr [34] directives, which are all the specification [17]. We emphasize that DiffCSP does not simply directives that affect JS execution according to the CSP3 reuse the referenced HTML and JS examples; rather, we build working draft [6]. grammar out of these referenced files. The G ENERATOR is thus able to compose all possible combinations of HTML instances For these directives, we define 12 representative val- that embed simple JS snippets [62, 69]. ues grouped into five categories (i.e., keyword, host-source, schemes, nonce-source, and hash-source) by referring to the The G ENERATOR enumerates a set of test HTML instances policy specification [36]. Since the [HASH] value in the hash- by traversing the defined HTML grammar in a depth-first source group should be different for each tested JS, the value manner. From a root grammar rule with the non-terminal is not fixed when generating a policy; it is changed later before symbol of [HTML], the G ENERATOR composes an HTML the E XECUTOR executes the test HTML instance (§IV-C). instance by applying an applicable grammar rule and replacing each non-terminal symbol. To define a host-source value, we assume three web servers: (1) a self-origin server (http://127.- Specifically, the G ENERATOR starts by composing an 0.0.1:8000), (2) an allowed-origin server (http://- HTML instance by applying a derivation rule with the 127.0.0.1:8080), and (3) a blocked-origin server non-terminal symbol of [HTML]. From the current HTML (http://127.0.0.1:8082). The self-origin server is de- instance in composition, it identifies non-terminal sym- signed for testing CSPs containing self. The allowed-origin bols (i.e., [JS_REQ_URL], [JS_INLINE_URL], [PAGE], and blocked-origin servers are for fetching allowed and blocked [SCHEME], [JS], and [HTML]) and replaces each non- cross-origin JS snippets, respectively. terminal symbol by applying an applicable grammar rule. The G ENERATOR repeats this process until either of the We also define corner cases for directives and values, following termination conditions is satisfied: (1) the G EN- which are likely to be provided by site operators. For example, ERATOR encounters an element in [JS_REQ_URL] or according to the CSP specification, the URLs in directive [JS_INLINE_URL], or (2) the number of applied grammar values should contain only ASCII characters [36]1 . However, rules exceeds five. The first termination condition denotes that it is reasonable that one unfamiliar with the specification may the G ENERATOR successfully composes an HTML instance write URLs with non-ASCII characters [26] in a CSP. With that executes an inline script or imports a JS script. The 1 Non-ASCII domain names cannot be entered into a CSP, but instead must second condition means that the G ENERATOR discards the be Punycode- encoded. generated HTML instance when it becomes too complicated. 4 Symbol HTML/JS Derivation Rule Feature Value http://127.0.0.1:8000/self.js?csp_id=[CSP_ID]&html_id=[HTML_ID]&status_code=[CODE] Self-JS: 0 [JS_REQ_URL] http://127.0.0.1:8080/allowed.js?csp_id=[CSP_ID]&html_id=[HTML_ID]&status_code=[CODE] JS Execution Allowed-JS: 1 http://127.0.0.1:8082/blocked.js?csp_id=[CSP_ID]&html_id=[HTML_ID]&status_code=[CODE] Method Blocked-JS: 2 [JS_INLINE_URL] http://127.0.0.1:8000/executed?csp_id=[CSP_ID]&html_id=[HTML_ID]&status_code=[CODE] Inline JS: 3 [PAGE] self.html | about:blank | self.txt - - [SCHEME] javascript:‘[HTML]’ | javascript:[JS] | data:text/html,[HTML] | data:application/js,[JS] | ... - - Category #1: executing inline JS fetch(‘[JS_INLINE_URL]’) - - Category #2: evaluating string eval(‘[JS]’) | new Function(‘[JS]’) | setTimeout(‘[JS]’,0) Included or not 0-1 Category #3: dynamically fetching JS var o=document.createElement(’script’); Included or not 0-1 [JS] o.src=‘[JS_REQ_URL]’;document.body.appendChild(o) Category #4: redirecting to scheme location=‘[SCHEME]’ | window.open(‘[SCHEME]’) | Included or not 0-1 var o=document.createElement(’iframe’);o.src=‘[SCHEME]’;document.body.appendChild(o) | ... Category #5: expanding document document.body.innerHTML+=‘[HTML]’ | document.write(‘[HTML]’) Included or not 0-1 Category #6: writing to opened document w=window.open(‘[PAGE]’);w.document.write(‘[HTML]’) Included or not 0-1 Category #1: executing inline JS in script tag Included or not 0-1 Category #2: fetching JS in script tag Included or not 0-1 Category #3: redirecting to scheme | Included or not 0-1 | | ... Category #4: executing inline JS in event handler | Included or not 0-1 |
test
| ... [HTML] Category #5: writing to frame | | ... Category #6: changing location of iframe Included or not 0-1 | ... Category #7: evaluating string via frame’s function Included or not 0-1 | ... Category #8: expanding document [HTML] | Included or not 0-1 | ... TABLE II: Grammar rule to generate testing HTML. Idx Derived HTML testing code. 1 In total, the G ENERATOR generates 25,880 HTML in- 2 stances. To boost the testing efficiency of DiffCSP, it merges each of which becomes an HTML file. Here, we set the → HTML #0 generated! ... ... a function related to page redirection, such as window.open 923 or location.href, is included in an HTML instance, the ... ... inclusion of other HTML instances in the same file will stop TABLE III: Example of derivation process to generate HTMLs. the testing of HTML instances that appear after this HTML instance involving page redirection. Thus, each HTML instance Table III shows an example of the composition process. The involving page redirection is excluded from this optimization G ENERATOR starts by applying the first rule of [HTML] in process; such an instance is stored in a separate file. Table II and then continues derivation for the scanned non- With this approach, the G ENERATOR generated 11,663 terminal symbol, i.e., [JS]. HTML files from 25,880 HTML instances. Each generated For each generated and saved HTML file, the G ENERATOR HTML file is then stored in the web root directory of the self- assigns a unique identifier to the [HTML_ID] of the generated origin server that E XECUTOR accesses for differential testing. page (e.g., Idx 4 in Table III). Note that [CSP_ID] and Status code. Specific HTTP status codes may cause security [CODE] have not yet been assigned at this stage. Later, the flaws, such as changing the execution context of JS snip- E XECUTOR changes these values (§IV-C). pets [27, 28] or disabling certain HTTP headers [3]. Motivated Regarding the testing of nonce-source values in test by these prior bugs, we define representative status codes CSPs, the G ENERATOR is required to generate nonce-protected to systematically study if and how the status codes affect scripts. Therefore, if a Testing results DB different JS execution result. For instance, if Chrome allows #5 #2 200 ✓ ✓ ✓Consistent! in this triad potentially triggers a browser bug. Naturally, this … approach does not tell which browsers have a bug [42, 47, ... 48], considering that the majority or minority browser vendors Fig. 4: E XECUTOR workflow. may implement enforcement incorrectly. However, those triads become promising data points for the next analysis step, which C. E XECUTOR involves a manual post-mortem analysis aided by decision Given a set of test CSPs and HTML files, the E XECUTOR trees (§IV-D). enumerates all combination pairs. For each pair of a CSP and an HTML file, it executes the HTML file that has the CSP D. A NALYZER across different browsers and then checks whether each JS snippet embedded in this HTML file is executed. The E XECUTOR reports behavioral inconsistencies for a large number of the generated triads, each of which consists of For testing efficiency, all HTML files are executed with a CSP, an HTML instance, and a status code. Note that Dif- the 200 response code. Additionally, a limited set of HTML fCSP generates 4M and 3.5M triads, for which the execution files is executed with all the defined status codes; we sample results may respectively differ by the three desktop and five eight HTML files from 11,663 HTML files (i.e., 640 HTML mobile browsers (§V-B). Manually identifying the root causes instances) containing at least one of each [HTML] category of all execution inconsistencies is infeasible; it would demand in Table II. the analysis of each generated triad that reports an inconsistent Testing browsers. For differential testing, we leverage three execution result. desktop and five mobile browsers, compiling a set of eight To address this challenge, we propose to leverage a decision browsers. We observe that rendering engines govern JS ex- tree. A decision tree is designed to derive a set of human- ecution enforcement behaviors. Therefore, we test three ren- readable conditions that lead to a classification decision. We dering engines (i.e., Chromium, Firefox, and WebKit) using leverage this capability to compute a set of conditions that lead executions results of three desktop browsers and two rendering to inconsistent execution results. Each leaf node that indicates a engines (i.e., Chromium and Gecko) using execution results of decision corresponds to a set of training instances; the decision five mobile browsers (§V-A). tree derives the same path and conditions for those training Workflow. Figure 4 illustrates the workflow of the E XECUTOR. instances. Assume that the E XECUTOR tests test_1.html with a Using this capability, we leverage a computed decision CSP corresponding to the CSP_ID of 5 and with the status tree to group generated triads that share the same decision code 200. The E XECUTOR instructs a testing browser to send paths. Then, from each group that contains an inconsistent the request to test_1.html with the csp_id parameter execution result, we pick one triad and analyze this triad of CSP_ID 5 and the status_code parameter of CODE with the conditions that appear in the corresponding path. 200. When the testing server receives this request, it sends That is, instead of analyzing all generated triads in each a response by setting the CSP corresponding to the CSP_ID group, we analyze one example for each group. In other in the HTTP header and setting the status code correspond- words, we leverage a decision tree to derive an interpretation ing to CODE, along with sending the body embedded with of observed execution results and use this interpretation to test_1.html. In this process, [CSP_ID] and [CODE] are identify representational inputs for each decision path. replaced with given parameters. When [HASH] exists in the test CSP, the string is replaced with the concatenation of all Figure 5 illustrates how we build and leverage a decision hashes that correspond to the inline snippets in the HTML file. tree. The A NALYZER converts each CSP, HTML, and status code triad into a vector. It then uses the generated dataset to The main goal of the execution step is to determine whether train a decision tree. or not a script was executed in light of the specified CSP. If the testing browser executes an inline script, this will invoke Dataset. The dimension of a feature vector is 37; 22 features, the fetch function. Therefore, the testing server will receive 14 features, and one feature represent a CSP, an HTML a request to the executed path of the self-origin server instance, and a status code, respectively. For a generated CSP, (the fourth row in Table II). If a testing browser executes a each element in Table I is encoded into a feature vector. script fetched from cross-origin servers, the browser sends a It consists of 22 binary features, and each feature value JS fetching request to the specified URL (see the first to third represents whether the corresponding feature is present. 14 rows in Table II). Thus, the testing server is able to check features are from a generated HTML instance. Each feature and 6 ANALYZER script-src? Idx Desktop Version CSP: Exist Not exist script-src http://127.0.0.1:8080; 1 Chromium 99 Status Code? script-src-elem? 2 Firefox 95 HTML: 200, 300, Not Exist 4 Chromium 101 com.android.chrome Nexus +10,000,000,000 Exist exist 5 Chromium 100 com.opera.browser Pixel 3 +100,000,000 Status code: 100 HTML Category #4: … Executing JS in event JS Execution Method: 6 Gecko 100 org.mozilla.firefox Nexus +100,000,000 Inconsistent Execution Result handler? Not Fetching blocked JS ✓ 7 Chromium 102 com.brave.browser Pixel +50,000,000 ✗ ✗ Exist Not exist exist Not 8 Gecko 96 org.torproject.torbrowser Pixel 3 +10,000,000 exist Inconsistent Consistent Exist TABLE IV: Browsers under testing. Label: Inconsistent 0, 1, 0, …, 3, 0, 0, 1, 0, …, 100 Feature Vector Decision Tree and 0.92 for mobile browsers) while the number of paths to be Fig. 5: A NALYZER workflow. inspected decreases (§V-E). In each computed decision tree, we investigated every path that corresponds to at least one its value are listed in the third and fourth columns in Table II, generated triad causing an inconsistent execution result. For respectively. For the status code, we use the status code itself each path, we sample one triad of an HTML, a CSP, and a as a feature. For each vector representing a generated triad, we status code, corresponding to this path and analyze this triad use one of two class labels: inconsistent (positive) or consistent along with the conditions that appeared in the path. As a (negative). result, we examined 525 and 581 paths for desktop and mobile browsers, respectively. Decision tree. Given a computed decision tree, we manually enumerate all paths from a root to the leaf nodes that represent inconsistent execution results. Each path consists of tree nodes, V. E VALUATION each of which contains a condition that involves features. We evaluate the efficacy of DiffCSP to find browser bugs in Figure 5 shows a decision tree with three paths leading enforcing CSPs that govern JS execution using three desktop to inconsistent execution results. The first path leading to and five mobile browsers (§V-B). We then describe the root inconsistent results (script-src ∧ status code 100) represents causes of the identified bugs, categorize these causes into a bug in which Chromium-based browsers ignore a CSP when 10 groups, and explain the security implications of the bugs the response comes with the status code 100. This step enables (§V-C). We also analyze the degree to which the computed an auditor to identify factors causing the inconsistent results decision tree helps find the causes of the identified bugs and analyze an instance sampled from the training instances (§V-E). Finally, we demonstrate the performance of DiffCSP that share this path. This step enables to avoid the analysis of in finding CSP bugs (§V-F). the 276K instances that belong to that path. The second path (script-src ∧ status code 200–500 ∧ A. Experimental Setup sha256-[HASH] ∧ Executing JS in event handler) represents a bug where Firefox enables unsafe-hashes by default in Browsers. We ran a series of experiments on the eight the script-src directive. This inconsistency occurs when browsers listed in Table IV. For desktop browsers, we selected a hash-source value exists in the directive value and an event the three most popular browsing engines: Chromium, Firefox, handler in a generated HTML instance. and WebKit [23]. We have observed that all testing results for Chrome and Edge were identical because they use the same DiffCSP computes two decision trees: one for the desktop rendering engine as Chromium. Since the rendering engine browsers and the other for the mobile browsers. Note that governs JS execution according to a given CSP [2, 10], we used the purpose of computing the decision trees is not to classify the testing results of Chromium to represent those of Chrome a given instance, but to facilitate the analysis of identifying and Edge. We also used WebKit to test the rendering engine of causes for inconsistent execution results. Thus, we do not split Safari. Note that new Safari features have been implemented in the training instances for the testing dataset. WebKit before their releases. In addition, 11 of the 18 (61%) DiffCSP reported 19M execution results for all generated WebKit bugs that we discovered were present in the latest triads. Leveraging a decision tree computed from all these version of Safari 15.4. instances generates a complex decision tree, the depth of which is beyond 30, making it intractable to manually analyze 61,479 For mobile browsers, we selected five popular browsers. paths leading to inconsistencies. We conduct the following Specifically, we first selected 20 mobile browsers in the order two optimization methods so that the tree can explain the of their downloads from Google Play Store as of May 2022 root causes for the observed inconsistencies while limiting the and then excluded nine ARM-based apps, the APKs of which number of paths to be inspected: (1) we randomly sample do not support Android API 30 (i.e., Android 11) x86 system consistent training instances to match their number to the images, which is required for our testing environment using an number of inconsistent training instances, and (2) we set the emulator. We also excluded six mobile browsers using Android decision tree depth to 10 for desktop browsers and mobile WebView. Those execution results depend on the version of browsers, respectively. the WebView service installed on a mobile device, which may significantly vary across user devices. When they use the latest We empirically confirmed that when the tree depth is 10, WebView service, the execution results become identical to the recall of the tree is kept high (i.e., 0.95 for desktop browsers those of the Chromium mobile browser. 7 We observed that the test results for mobile browsers groups (s represents group (1) and the others belong to group using the same rendering engine were identical. Therefore, (2)). we grouped the execution results of the mobile browsers under testing into two groups: Chromium (Chrome, Opera, and Bug disclosures. We reported all 27 security bugs resulting Brave) and Gecko (Firefox and Tor). from vendor’s mistakes to the three browser vendors (i.e., Chromium, Firefox, and WebKit, representing Safari). At the In summary, we tested three rendering engines (i.e., time of writing, 23 bugs have been patched by the vendors. Chromium, Firefox, and WebKit) for the desktop browsers Among them, 12 bugs were patched in response to our bug as well as two engines (i.e., Chromium and Gecko) that reports, and the other bugs were patched in response to reports corresponds to five mobile browsers. from the browser vendors or users. Also, we are currently awaiting responses for four bugs from the vendors. For the Environment. For desktop browser testing, we conducted reported bugs, the Chromium team awarded us a bug bounty experiments on two machines running x86_64 Ubuntu with of 4,000 USD. 88 CPUs and 378 GB of main memory. To automatically visit each testing page from each browser, we compiled a test script using Playwright 1.18.0 [30]. For each HTML file, we set the C. Root Causes timeout of execution to three seconds. When this page involves Table V lists the 37 identified bugs and their causes. page redirection and has a single HTML instance, we set the We further categorized these root causes into 10 groups, as timeout to one second. shown in the table. The third column of the table depicts the For mobile browser testing, we performed our testing on conditions of the page, the CSP, or the HTTP status code that four servers running x86_64 Debian with 192 CPUs and 1.5 contributes to triggering the bugs. The fourth column shows TB of main memory. To automate the testing process, we relied the expected behavior (i.e., whether to execute a given JS on Android Virtual Device (AVD) [7] and the Android Debug testing code) according to the CSP specification [6] regarding Bridge (ADB), which is a command-line tool that helps us the conditions in the third column. We manually extracted communicate with a device [1]. For the testing devices in our these expected behaviors from the specification to check the emulators, we used a Galaxy Nexus with Android API 25 (i.e., correctness of our findings. When the specification does not for x86-based apps) and a Pixel 3 with Android API 30 (i.e., describe the expected behavior for the identified conditions, for ARM-based apps). we mark it with s and count the number of bugs to one. The fifth to ninth columns indicate whether each browser B. Bugs Found exhibits the expected behavior. When the browser conducts the expected behavior as the specification describes, we mark Table V summarizes CSP enforcement bugs that we found it with a 3, and an 7 otherwise. N/A indicates a bug that using DiffCSP. We found a total of 37 browser bugs after still poses a security threat. However, we did not count it as analyzing 7.5M discrepancies that DiffCSP reported. Recall a bug because the corresponding browsers did not support from §IV-D that we identified 525 and 581 paths leading to certain directives or directive values. For example, we mark the inconsistent execution results in the desktop and mobile inconsistency #7 as N/A for Firefox because it does not decision trees, respectively. Each path leading to an inconsis- support the processing of the script-src-elem directive, tent result corresponds to a set of HTML instances, CSPs, and even though the CSP3 standard demands its support. In the status codes. From such a set of test inputs, we selected one following, we list five root causes out of the 10 ones. We triad of an HTML file, a CSP, and a status code. We then describe the remaining causes in Appendix IX-A. analyzed this triad and identified the causes along with the conditions that appeared in the path. Cause #1: Incorrect CSP inheritance. An embedded iframe or a newly opened new window loaded from a local scheme Of the 37 browser bugs, we manually confirmed that 29 (e.g., blob, data, javascript, or about) should inherit bugs imposed a security threat and eight were functional the CSP of their parent document [12]. The goal here is to bugs. For each security bug, we analyzed its implications by prevent the adversary from bypassing the parent’s CSP by questioning whether an adversary is able to exploit the bug opening a child window or embedding a child frame that and bypass a certain CSP, thus injecting an arbitrary JS script. contains attack code under the adversary’s control. We further confirmed whether attack payloads exploiting such a security bug were blocked by other browsers. We found that Safari incorrectly inherits a parent CSP, allowing string-to-JS execution in child pages even when the Of the 29 security bugs, 27 bugs (93%) enable the adver- parent CSP blocks eval(). The following snippet shows a sary to inject an executable JS, the execution of which should test HTML instance that exhibits this bug. be blocked according to the CSP standard. The remaining two 1 CSP: script-src 'nonce-123'; bugs entail relatively low-security implications. However, by 2 exploiting these bugs, an adversary is capable of bypassing pre- 3 to unclear/incorrect descriptions in the CSP specification and (2) implementation flaws that stem from vendors’ mistakes in Note that because unsafe-eval is not included in the CSP, a not properly following the specification. The third column of site operator would expect the string evaluation by eval() to Table V categorizes the identified vulnerabilities into these two be blocked. Also, since nonce-123 is included in the CSP, 8 Expected Behavior Desktop Mobile # of Category Idx Page / CSP / HTTP header condition (Manually Extracted) Bugs Cause #1: Incorrect CSP inheritance 1 Dynamically calling eval() from the about: frame (‘unsafe-eval’ is not specified) eval() should be blocked 3 3 7 3 3 1 2 Dynamically changing frame.src to JS URL with embedded inline script Inheritance should occur 3 3 7 3 3 1 3 Dynamically changing frame.src to data URL with embedded HTML Inheritance should occur 3 3 7 3 3 1 4 Dynamically changing frame.src to blob URL with embedded HTML Inheritance should occur 3 3 7 3 3 1 5 Dynamically changing frame.src to JS URL with embedded HTML Inheritance should occur 7 3 7 7 3 2 6 Dynamically writing JS embedded HTML to static file s (3 indicates inheritance has occurred) 7 3 7 7 3 1 Cause #2: Incorrect hash handling 7 javascript:[JS] with script-src-elem [hash-source] JS from JS URL should not be executed 7 N/A 3 7 N/A 1 8 script-src [hash-source]; script-src-elem ‘none’ Hashed script should not be executed 3 N/A 7 3 N/A 1 9 script-src [hash-source] ‘unsafe-hashes’; script-src-attr ‘none’ Hashed script should not be executed 3 N/A 7 3 N/A 1 Cause #3: Non-ignored directive values 10 default-src ‘strict-dynamic’ ‘unsafe-inline’ ‘unsafe-inline’ should be ignored 7 N/A N/A 7 N/A 1 11 script-src ‘strict-dynamic’ [host-source] [host-source] should be ignored 3 3 7 3 3 1 Cause #4: Non-supporting specific directives 12 The script-src-elem directive Directive should be supported 3 7 3 3 7 1 13 The script-src-attr directive Directive should be supported 3 7 3 3 7 1 Cause #5: Non-supporting specific directive values 14 The nonce-source in the default-src directive Value should be supported 3 †7 3 3 †7 1 Security 15 The hash-source in the default-src directive Value should be supported 3 7 3 3 7 1 Bugs 16 ‘strict-dynamic’ in the default-src directive Value should be supported 3 7 7 3 7 2 Cause #6: Auto-enabling directive values by default 17 Auto-enabled ‘unsafe-hashes’ in the script-src directive Value should be disabled by default 3 7 3 3 7 1 18 Auto-enabled * in the script-src-elem directive Value should be disabled by default 3 N/A 7 3 N/A 1 Cause #7: Auto-enabling directive values on specific conditions Auto-enabled ‘unsafe-inline’ in the script-src-elem directive 19 Value should be disabled by default 3 N/A ‡ 7 3 N/A 1 Condition: ‘strict-dynamic’ is specified in the script-src-elem directive Auto-enabled ‘unsafe-inline’ in the script-src-elem directive 20 Value should be disabled by default 3 N/A 7 3 N/A 1 Condition: The hash-source is specified in the script-src-attr directive Auto-enabled ‘unsafe-inline’ in the script-src-elem directive 21 Value should be disabled by default 3 N/A 7 3 N/A 1 Condition: The hash-source is specified in the script-src-elem directive Auto-enabled ‘unsafe-inline’ in the script-src-attr directive 22 Value should be disabled by default 3 N/A 7 3 N/A 1 Condition: The hash-source is specified in the script-src-elem directive Auto-enabled ‘unsafe-inline’ in the script-src-attr directive 23 Value should be disabled by default 3 N/A 7 3 N/A 1 Condition: The hash-source is specified in the script-src-attr directive Cause #8: Non-supporting CSP for specific status code 24 100 status code in HTTP header CSP should be enabled 7 3 3 ¶3 3 1 Cause #9: Incorrect handling of malformed CSPs 25 Non-ASCII character within directive value s (3 indicates the directive is disabled) 3 7 3 3 7 1 Cause #10: Allowing out-going requests 26 Parser-inserted script (i.e., script tag) with ‘strict-dynamic’ Out-going JS request should be blocked 3 7 3 3 7 1 27 Parser-inserted script (i.e., written script tag) with ‘strict-dynamic’ Out-going JS request should be blocked 7 3 3 7 3 1 28 Nested srcdoc with JS fetching (The URL of the JS is specified in the CSP) JS from allowed URL should be executed 3 7 3 3 7 1 29 Nested data scheme with JS fetching (The URL of the JS is specified in the CSP) JS from allowed URL should be allowed 3 7 3 3 7 1 30 Calling cloneNode() for inline script (Nonce-source for inline script specified in CSP) Nonced inline script should be executed 3 7 3 3 7 1 Functional Bugs 31 Executing javascript:[JS] (hash for [JS] specified in CSP) s (3 indicates script is executed) 7 3 7 7 3 1 32 NoCSPiframe.contentWindow.eval() (‘unsafe-eval’ is not specified) eval() should be allowed 3 3 7 3 7 2 JS fetching from NoCSPiframe.srcdoc with following CSP: 33 JS from allowed URL should be executed 3 7 7 3 7 2 default-src ‘none’; script-src ‘unsafe-inline’ [Allowed URL] Total 37 † Only for inline scripts (e.g, ), not for JS fetching (e.g., ). ‡ Only for inline scripts existing in script tag, not in the JS navigation. ¶ The bug was patched in Chromium 100. TABLE V: Experimental results with eight major browsers: Chromium , and Gecko (Firefox) , WebKit (Safari) . they would expect that only the nonce-protected JS will be For example, consider the following test CSP and webpage allowed. However, Safari allows string-to-JS execution under that triggers inconsistency #3. certain page conditions, allowing an attacker to bypass the 1 CSP: script-src 'nonce-123'; emplaced CSP and execute arbitrary scripts if the page is 2 vulnerable to XSS attacks. Especially, we observed that calling 3 We reported this bug to the WebKit team, and the vendor fixed the bug. In this example, when the load event of the iframe is fired, the src attribute of the iframe is changed to the data URL We also observed that Safari did not conduct CSP inher- that executes the JS snippet in Line 5. It is expected that this itance when it involves javascript:, data:, or blob: iframe will inherit the parent’s CSP, thus blocking JS execution navigation. Also, in Chromium, this kind of bug is triggered of this embedded HTML instance. However, the identified bug when the navigation to a URL involves the javascript: contributes to bypassing CSP enforcement, which allows the scheme. These bugs occur because the browsers do not check inline script in Line 5 to be executed. We emphasize that the CSP of a parent document when asynchronous navigation the defined CSP does not affect the executed script at all. is involved (e.g., dynamically changing the src attribute of Therefore, the injected script is capable of bypassing frame an existing iframe to a local scheme URL). busting as well as TLS enforcement even if the CSP defines 9 those directives. Cause #3: Non-ignored directive values. According to the specification pertaining to strict-dynamic, We have reported these bugs to the WebKit team and host-source, and scheme-source expressions as well as the vendor has fixed these bugs. We have also reported the unsafe-inline and self keyword-source should be identified bug to the Chromium team and are awaiting their ignored when strict-dynamic is specified in the response. script-src or default-src directive of a given CSP. Regarding the CSP inheritance that involves static files, we Unfortunately, we observed that Chromium did not ignore observed that Chromium and Safari do not pass over the CSP unsafe-inline even when strict-dynamic was spec- of a parent window to its child window or iframe when they ified in default-src. We also found that Safari did not open static files. Note that site operators cannot define a CSP ignore the host-source when strict-dynamic is specified via meta tags in static files, such as .txt, .js, and .ico, in script-src. due to the nature of these file formats. Site operators may set a CSP in an HTTP(S) response header when delivering these Since neither Firefox nor Safari supports files. However, the current practice overlooks assigning CSPs strict-dynamic in default-src (inconsistency #16), for static files in real-world services [22, 37, 41]. it may seem impossible to find inconsistency #10 through differential testing. However, the following testing page and An XSS attacker is able to exploit this bug to establish CSP enabled the finding of this bug: a same-origin document context for XSS attacks [47, 64]. 1 CSP: default-src 'unsafe-inline' 'strict-dynamic'; Assume that a target website allows unsafe-inline in 2 which will be executed using the first-party origin without any CSP restrictions. One may argue that the unsafe-inline In Firefox and Safari, due to inconsistency #16, the inline requirement in the target website does not necessitate the script in Lines 3–5 is executed. However, the execution of injection of an attack script in the new window. However, the script generated by this script stops as it tries to fetch the XSS attacker is able to bypass other restrictions im- JS code from a URL not specified in the CSP. However, posed by the CSP, such as TLS enforcement (block-all- Chromium executes inline scripts due to this bug because -mixed-content and insecure-requests), frame unsafe-inline is not ignored. Moreover, interestingly, it busting (frame-ancestors), fetch (connect-src), and allows arbitrary JS fetching from dynamically created scripts invoking eval(). Furthermore, 56% of websites using CSPs due to the strict-dynamic effect. have deployed unsafe-inline in their directives [58]. Site operators may specify unsafe-inline or host- We reported these bugs to each vendor, and the Chromium source in a CSP, which they expect to be ignored by vendor stated that, according to the CSP standard, the case strict-dynamic in CSP3-supported browsers [38]. How- of inheriting CSPs is limited to navigation by the local ever, this bug allows the injection of an inline JS script, which scheme, so this behavior is working as expected. However, should be blocked. We reported these bugs to the Chromium they acknowledged that this bug has the security implications and WebKit vendors, and they patched these bugs. aforementioned, and this issue is under discussion. Cause #4: Non-supporting specific directives. Fire- Cause #2: Incorrect hash handling. As described in Figure 1, fox does not support the script-src-elem [35] and Chromium allowed the execution of arbitrary inline scripts script-src-attr [34] directives. Despite the known fact present in javascript: even when their hash value does not that Firefox supports CSP3 [9], we noticed that Firefox did match any hash values in the script-src-elem directive. not enforce the expected behaviors when these directives were If a website has an XSS vulnerability and blocks arbitrary JS present in given CSPs. execution by checking hash values in script-src-elem, This inconsistency bug poses a security threat. Assume a the attacker can exploit this bug to trivially bypass hash site operator specifies their CSP with script-src-elem checking and inject an executable script. We reported this bug ‘none’ to prevent the execution of inline scripts. Firefox en- to the respective vendor; however, this bug had already been ables the execution of inline scripts, contrary to the operator’s patched internally in May 2022. expectations. Since these bugs are already known, we have not reported them. Firefox browser vendors had not implemented We also found incorrect directive fallback when han- these two directives for approximately three years. Recently, dling a hash-source value in WebKit. Note that browsers Firefox implemented them in July 2022 [20] just two weeks should only use script-src as a fallback when both before our submission. script-src-elem and script-src-attr are not ex- plicitly set in a given CSP [21]. However, WebKit uses the Cause #5: Non-supporting specific directive values. presence of a hash-source in the script-src directive even Firefox does not support nonce-source, hash-source, and when script-src-elem or script-src-attr exists in strict-dynamic in the default-src directive. Also, a CSP. This means that the interpretation of a CSP satisfying Safari does not support the strict-dynamic in the the conditions above significantly differs by browsers, which default-src directive. These inconsistent behaviors stem may permit non-allowed scripts to be executed. This bug had from Firefox and Safari not implementing a fallback mech- been patched in the latest version of WebKit at the time we anism; according to the specification, nonce-source, hash- discovered it. source, and strict-dynamic in the default-src direc- 10 140 Complete Explanation 160 Complete Explanation 120 Partial Explanation 140 Partial Explanation 100 120 # of Paths # of Paths 80 100 60 80 60 40 40 20 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Inconsistency Inconsistency (a) Desktop browsers. (b) Mobile browsers. Fig. 6: The number of decision tree paths for each root cause. tive should be applied as a fallback when the script-src standard for these cases. WebKit, on the other hand, did not directive is missing [13, 21]. systematically test these edge cases, potentially allowing an attacker to execute arbitrary JS from the same origin. These incomplete fallback behaviors have various se- curity implications. Consider a website that defines the Specification bugs. We have also identified that unclear or CSP of default-src [hash-source] ‘unsafe- insufficient descriptions in the specification contributed to -inline’. Note that, according to the specification, causing inconsistencies #6, #25, and #31. For inconsistency #6, unsafe-inline should be ignored if nonce-source, hash- we recommend inheriting the CSP of a parent window to its source, or strict-dynamic is specified. Chromium hon- child window or iframe when these child instances open static ored this policy by not allowing any inline scripts. However, files, and these files are delivered without any CSP headers. Firefox and Safari executed inline scripts, allowing an adver- sary to execute an injected JS script. To clarify the specification regarding inconsis- tency #25 (§V-C), we recommend the following two The Firefox vendor has been aware of this bug, but it procedures: (1) remove only bogus source expressions, not has not been fixed since the CSP2 was released six years entire directives, except in the worst case (i.e., when only ago [18]. After we discovered these three bugs via DiffCSP, none is left in the directive); and (2) accommodate and parse we reported to the vendor that these bugs hadn’t been fixed non-ASCII characters for source-expressions. yet, and recently, in November 2022, these bugs were finally patched. Regarding the bug in Safari, we reported the bug to Regarding inconsistency #31, the specification said that the corresponding vendor via WebKit Bugzilla, and the vendor hash-source can be applied to javascript: navigation [15] acknowledged and fixed the bug. if unsafe-hashes is present. However, it does not clearly specify what exactly to hash (i.e., whether to include javas- cript:). Chromium-based browsers and Safari use the entire D. Summary and Lessons attribute value (i.e., sha256(javascript:alert(1))), but Firefox only uses everything after javascript: (i.e., Complex CSP specification. We observed that the majority sha256(alert(1))) to compute the hash value. Although of bugs (18 out of 29 security bugs) were eventually trig- this inconsistency does not pose a security threat, we recom- gered by combinations of multiple directive values in test mend clearly specifying which part should be computed as a CSPs. This result shows that different browsers interpret the hash in the specification. same CSPs in different ways. Interestingly, we noticed that many of the bugs occurred due to divergent interpretations regarding new CSP3 directives and fallback mechanisms. In E. Decision Tree particular, 15 out of 18 bugs are caused by directives or values introduced in CSP3 (four from strict-dynamic, six from Recall that we analyzed 525 and 581 paths in the deci- script-src-elem, four from script-src-attr, and sion trees of the desktop and mobile browsers, respectively. one from unsafe-hashes). Figures 6a and 6b show the number of decision paths that contribute to identifying each security bug. The x-axis repre- We believe that this trend is inevitable because the expected sents each inconsistency bug index, and the y-axis represents behaviors become complicated as the number of directives the number of paths that correspond to each found bug. We and values increased to 24 and 11, respectively, in the CSP3 observed that a large number of decision paths corresponds to specification [58]. At the same time, this increasing complexity inconsistencies #12 and #13. This is because Firefox does not in CSPs requires systematic browser testing. We believe that support the script-src-elem and script-src-attr our differential testing approach using a large number of directives. In our decision trees, these conditions are the adversarial HTML instances contributes to finding holes in majority cause of observed execution inconsistencies. existing regression tests in browser vendors. We also studied how helpful the decision trees are in CSP bypass via page redirection. We observed that eight analyzing the root causes of inconsistent execution results. out of 29 bugs (28%) were due to the improper updating of We measured whether the root causes are well explained in a CSP to enforce when page redirection occurs. In particular, conditions in the paths of the decision trees. In particular, we when an iframe or a window was loaded once and its source counted the number of paths that (1) completely explain the or content was changed asynchronously, we observed many root cause and (2) partially explain the root cause, respectively. cases in which the parent CSPs were not properly inherited. Here, the complete explanation path means that the conditions However, we observed that Firefox correctly followed the CSP for a directive, a directive value, an HTML instance, and a 11 30 Accumulated # of Security Bugs 25,000 0.98 # of Paths 0.95 0.96 Recall 25 90% 20,000 0.94 # of Paths 20 15,000 0.92 Recall 0.9 15 50% 10,000 0.88 10 0.86 5,000 0.84 5 525 0 0.82 21.3% 91.4% 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1M 2M 3M 4M 5M 6M 7M 8M 9M 10M 11M Depth Pairs of CSP, Status Code, and HTML File Fig. 7: The relationships between the depth of the decision tree, the Fig. 8: The cumulative number of security bugs over executed test recall of the training instances, and the number of decision paths. inputs. status code are all present in the path to describe the observed Figure 8 shows the cumulative number of unique security inconsistency. For example, the second path of Figure 6a is bugs over the tested triads, each of which consists of a CSP, a complete explanation path because its conditions have the an HTML instance, and a status code. In particular, on the x- script-src directive, the directive value of hash-source, axis, we show the cumulative number of tested triads, and these an HTML instance that has JS execution in an event handler, triads are sorted in the order in which DiffCSP fetches those and the 200 status code. for execution. We increase the bug count on the first triad on the x-axis of which inconsistent execution results contribute to We observed that 538 of the 941 paths (57%) that map finding a bug. For 50% of the identified security bugs, DiffCSP to their causes of the security bugs are complete explanation tested less than 21.3% of the generated triads. For 90% of the paths. We also observed that 24 out of the 27 inconsistencies bugs, it required testing 91.4% of the triads. The results of (89%) were completely explained by at least one path. For this analysis show that extending the test generation grammar the three inconsistencies (#8, #9, and #11) that were not fully and running DiffCSP has the potential to discover more bugs explained, the number of instances that cause the inconsistency when DiffCSP is capable of generating new HTML instances is small; the branching for that condition occurs below depth involving JS execution. 10 in the tree. However, increasing the depth of the tree for desktop browsers to 14 reveals a path that completely explains VI. L IMITATIONS AND D ISCUSSION the root causes of these inconsistencies. We leverage the inconsistent execution results for each Decision depth. We analyzed how recall and the number of generated input as a bug oracle. Therefore, DiffCSP cannot paths to inspect change while varying the depth of the desktop find a bug if all the browsers under testing exhibit the same decision tree. Figure 7 shows that increasing the depth causes bug [42, 47, 48]. One can overcome this limitation by defining an increase in the number of paths that need to be manually expected behaviors for each input, enabling the testing of a inspected. It also contributes to increasing the recall rate of single browser implementation. However, defining the correct the training instances, meaning that the decision tree becomes behavior for each combination of 25,880 HTML instances more explanatory by reducing non-explainable inconsistent and 1,006 CSPs is an open research problem. The automatic execution results (i.e., false negatives). Note that we observed extraction and adoption of these correct behaviors represent an analogous pattern in the mobile decision tree. We choose interesting technical challenges that we leave to future research. to set the depth of the decision tree to 10, which exhibits a We also note that our sampling strategy of selecting one high recall rate and produces an acceptable number of paths representative test instance among the test instances that share to analyze. the same path may cause false negatives in identifying a new False positives (FPs). Among the 525 and 581 paths in the cause. This can happen when the test instances that corre- desktop and mobile decision trees, 39 and 42 were FPs out spond to a certain path yield both consistent and inconsistent of 11,663 HTML files, respectively. These FPs correspond execution results. For such cases, we manually analyzed all of to the cases in which our testing frameworks reported false the 187 and 217 paths that leading to leaf nodes having both execution results because the execution of the generated test inconsistent and consistent execution results in the desktop and files exceeded a given timeout. Recall that we set the number mobile decision trees, respectively. However, this additional of HTML instances in each file to 80 to boost the testing analysis did not reveal any new causes. efficiency (§IV-C). This means that tests towards the end of We acknowledge that our HTML grammar is not nec- the file may not be executed before the timeout, leading to essarily complete in generating all possible HTML forms inconsistent behavior and, thus, a false report. In practice, these for executing JS snippets. Therefore, if there exist unknown FP cases can be scheduled again for further testing with an HTML forms of executing JS snippets, DiffCSP may miss CSP extended timeout to remove false positives. enforcement bugs, producing false negatives. However, note that, among 47 reported CSP bugs involving JS execution since F. Performance 2010, including 18 ones that we found, DiffCSP generated HTML instances that triggered 46 bugs (97%). A total of 200 hours with 88 cores and 153 hours with 192 core CPUs were consumed to find CSP enforcement bugs We emphasize that the grammar rules that we proposed for desktop and mobile browsers, respectively. In addition, include not only the general rules corresponding to the HTML two researchers spent two days analyzing the decision trees and ECMAScript specifications but also the adversarial gen- to identify the root causes of the detected 37 bugs. eration rules corresponding to the 28 known CSP bugs. We 12 observed that these adversarial payloads were applied as-is to set up a CSP or give up entirely. They nevertheless provided the browser’s regression test set, without any combination or clear evidence of the uptake in CSP deployment, necessitating mutation. These adversarial grammar rules helped DiffCSP to a thorough analysis of the enforcement in browsers. Eriksson identify 10 more bugs, which would not be discovered by only and Sabelfeld [50] then analyzed the not-yet implemented using the general grammar rules and the known payloads. navigate-to directive and proposed to automate the pro- cess of curating policies with the directive. DiffCSP supports extending the current HTML grammar by adding new derivation rules. Security researchers and All of this research has demonstrated that curating a practitioners are thus able to add their own derivation rules functional and secure CSP is a challenging task. However, to attempt to find more bugs. Specifically, they can extend none of the works have attempted to systematically analyze the grammar by symbolizing and classifying new HTML or to what extent CSPs are properly implemented in browsers, in JS snippets with reference to the grammar in Table II. The particular for edge cases. In contrast, with DiffCSP, our work testing CSPs can also be extended by adding new directives aims to understand enforcement bugs in browsers, which may or values to Table I. even result in the bypassing of seemingly secure CSPs. We adopt the interpretation of the computed decision trees For their 2016 paper, Calzavara et al. [43] also evaluated to group test instances that share the same decision paths, browser supports for CSPs. Contrary to DiffCSP using differ- thereby avoiding to analyze each inconsistent execution result ential testing, they manually composed tests and modeled their and its corresponding test input. However, to identify common expected behaviors, leaving many corner cases unexplored. causes of these bugs, we still manually analyzed 525 and 581 In particular, they utilized visual cues to model the expected paths for the desktop and mobile browsers, respectively. This behavior corresponding to each page and manually accessed step required two authors to spend two days identifying the the test page from each browser to examine whether the causes of these paths. visual cues were well represented (e.g., JS should be executed We note that the conditions that appear in the decision and an alert should appear). Due to this manual approach, tree paths help distinguish bugs due to unsupported features they modeled 15 tests, contributing to finding one CSP bug from bugs caused by developer mistakes. For example, five involving JS execution. inconsistencies due to unsupported specific directives and By contrast, DiffCSP conducts scalable and systematic directive values are distinguishable from other inconsistencies CSP enforcement testing by leveraging various types of ad- because those unsupported features appeared in the conditions versarial HTML instances and differential testing, enabling in the paths leading to the groups having those bugs. These to avoid a manual analysis to identify correct behaviors for conditions can help browser vendors quickly disregard such each generated test, thereby helping to find a total of 37 cases. bugs. By conducting differential testing, we narrow down the scope of promising tests that invoke potential bugs and then VII. R ELATED WORK sample representative tests using the decision trees to manually Content Security Policy. Prior work has largely focused on investigate. studying the prevalence and struggles with CSPs [44, 49, 51, Browser security policy testing and analysis. There has 56, 57, 71, 72], finding that building applications that are been a surge in research in the study of web security poli- compliant with a safe CSP is a challenging task that few sites cies provided by browsers [47, 54, 55, 67]. In particular, master. Follow-up work has then attempted to aid developers several works have focused on testing SSL/TLS implemen- in the process of building CSPs. Pan et al. [56] proposed tation [42, 48], same origin policy [61, 63], HTTPOnly CSPAutoGen, which automatically composes declarations for cookies [73], HSTS [53], and clickjacking protection [46] enforcing a CSP in real-time through both analyses of existing Calzavara et al. [47] found that the inconsistent adoption resources and rewriting to enable compliance. Doupé et al. [49] of security mechanisms across different pages within the introduced an automatic code rewriting technique to extract same origin can express conflicting security requirements. trusted inline scripts from web applications and use these Roth et al. [59] discovered that client-side policies, including extracted scripts in the script-src directive in generated CSPs, X-Frame-Options, HSTS, and security cookies, were ap- CSPs. plied differently when accessing the same site through different Another recent line of research focuses on analyzing the settings. Recently, there have been several studies on security trend of insecurity regarding CSPs deployed on the Web [43, policies in mobile browsers [52, 54, 55]. Luo et al. [54] 45, 50, 58, 60, 64, 65, 66, 68, 70]. Weichselbaum et al. [70] ex- investigated the browser supports for eight different secu- amined the effectiveness of deployed CSPs in 1,680,000 hosts rity mechanisms, including CSP, HSTS, and referrer header, on the Internet. They also demonstrated that approximately across 351 unique browser versions. Kondracki et al. [52] 95% of the collected policies offered little security protection demonstrated that enabling data-saving functionality in mobile against XSS attacks due to the usage of unsafe-inline browsers poses security threats, including TLS man-in-the- and unsafe endpoints. Calzavara et al. [43, 45] examined the middle attacks and HSTS deactivation. updates in CSPs and demonstrated that the CSPs were not fre- quently updated to mitigate insecure practices. Roth et al. [58] VIII. CONCLUSION performed a historical analysis of how CSP adoption has evolved from 2012 to 2018. They found that many CSPs can With CSP’s adoption rates rising and more sites to mitigate be bypassed through expired domains or domains with typos, the impact of cross-site scripting (XSS) flaws every day, it is but more importantly, developers often struggle for years to imperative that enforcement of these policies is consistent and 13 secure across all browsers. While prior work had found indi- [19] “Firefox bugzilla - CSP script-src with hashes allow inline event vidual bugs in CSP, our community lacked a comprehensive handlers to match the hash (even if ‘unsafe-hashes’ is not present),” and systematic way of testing CSP implementations. https://bugzilla.mozilla.org/show_bug.cgi?id=1683506. [20] “Firefox bugzilla - implement CSP ‘script-src-elem’ and ‘script-src-attr’ To close this research gap, we proposed DiffCSP, the directives,” https://bugzilla.mozilla.org/show_bug.cgi?id=1529337. first differential testing framework designed to identify CSP [21] “Get fetch directive fallback list,” https://www.w3.org/TR/CSP3/ enforcement bugs regarding JS execution. Our key contribu- #directive-fallback-list. tions are (1) to propose an HTML grammar enumerating all [22] “GitHub robots.txt,” https://github.com/robots.txt. known HTML instances that execute simple JS snippets, (2) to [23] “Global market share held by leading desktop internet browsers from conduct differential testing to identify the correct behavior for january 2015 to december 2021,” https://www.statista.com/statistics/ 544400/market-share-of-internet-browsers-desktop/. each generated CSP and HTML instance, and (3) to analyze [24] “HTML5 security cheatsheet,” https://html5sec.org/. a large volume of execution inconsistencies by leveraging [25] “Informational 1xx,” https://www.rfc-editor.org/rfc/rfc9110.html# decision trees. Our testing uncovered critical flaws in major section-15.2. browsers, including Chrome, Firefox, and Safari, which allow [26] “Internationalized domain names for applications (IDNA): Definitions an XSS attacker to fully bypass CSPs. We found 29 security and document framework,” https://datatracker.ietf.org/doc/html/rfc5890. bugs and eight functional bugs, demonstrating the effectiveness [27] “Issue 882270: Security: url spoofing using 304 status code,” https: of DiffCSP in finding CSP enforcement bugs. //bugs.chromium.org/p/chromium/issues/detail?id=882270. [28] “Page.navigate doesn’t fail for status code other than 200 (eg. 204 - no content),” https://bugzilla.mozilla.org/show_bug.cgi?id=1618863. ACKNOWLEDGMENTS [29] “Parse a serialized CSP,” https://www.w3.org/TR/CSP3/ #parse-serialized-policy. The authors would like to thank the anonymous reviewers [30] “Playwright enables reliable end-to-end testing for modern web apps.” for their concrete feedback. This work was supported by Insti- https://playwright.dev/. tute of Information & Communications Technology Planning [31] “scheme-part matching,” https://www.w3.org/TR/CSP3/ & Evaluation (IITP) grant funded by the Korea government #match-schemes. (MSIT), No.2020-0-00209 and Korea Internet & Security [32] “Script directives pre-request check,” https://www.w3.org/TR/CSP3/ Agency (KISA) grant funded by the Korea government (PIPC) #script-pre-request. (No.1781000003, Development of a Personal Information Pro- [33] “script-src,” https://www.w3.org/TR/CSP3/#directive-script-src. tection Framework for Identifying and Blocking Trackers). [34] “script-src-attr,” https://www.w3.org/TR/CSP3/ #directive-script-src-attr. [35] “script-src-elem,” https://www.w3.org/TR/CSP3/ R EFERENCES #directive-script-src-elem. [1] “Android debug bridge (ADB),” https://developer.android.com/studio/ [36] “Source lists,” https://www.w3.org/TR/CSP3/ command-line/adb. #framework-directive-source-list. [2] “The Chromium projects - Blink (rendering engine),” https://www. [37] “Twitter robots.txt,” https://twitter.com/robots.txt. chromium.org/blink/. 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[67] B. Stock, M. Johns, M. Steffens, and M. Backes, “How the web tangled itself: Uncovering the history of client-side web (in) security,” in Proceedings of the USENIX Security Symposium, 2017, pp. 971–987. 15 IX. A PPENDIX Note that the 1xx class of status code indicates that a server has received a request and continues to generate its A. Additional Root Causes response [25]. According to the WHATWG fetch standard, a Cause #6: Auto-enabling directive values by default. response with the status code 100 or 102 should be ignored, We observed that Firefox enabled unsafe-hashes in the and status code 101 should have a body set to null [40]. script-src directive by default. We argue that this default We observed that Safari and Firefox correctly followed this behavior deviates from the CSP specification and poses a standard. However, Chromium treated the content of these security threat. responses normally as if they had the status code of 200, while ignoring the headers of these responses. Therefore, when a 1 CSP: script-src-elem 'sha256-Mg0QnPgA...'; naive developer writes a web page with any of the response 2 regardless of the CSP in this webpage. Chromium developers acknowledged and patched our re- Consider a webpage with the CSP of script-src porting of this bug and stated that this bug contributes to by- ‘sha256-Mg0QnPgA...’, as shown above. Note that passing not only CSP but also other security policies, including the inline script in Line 3 must be blocked because HTTP Strict Transport Security (HSTS) and X-Frame-Options. unsafe-hashes is not specified in the CSP. However, Firefox allows the execution of the inline script in the onload Cause #9: Incorrect handling of malformed CSPs. We handler due to this bug. In this case, the adversary is able observed that Chromium and Safari ignored an entire given di- to reuse the existing event handler to execute an injected rective when the directive contains an invalid value. In contrast, script [68]; they are able to inject the arbitrary tag embedding Firefox only drops an invalid value in the directive. For in- the exploiting payload in the data attribute, thus allowing the stance, consider the CSP of script-src http://a.com XSS attack. This bug had been reported to Firefox by others http://.com. Chromium and Sa- over three years ago, but Mozilla hasn’t patched it [19]. After fari ignore the script-src directive, allowing the execution we found this bug via DiffCSP, we mentioned to the vendor of any scripts. Conversely, Firefox still honors the directive by that this bug hadn’t been fixed yet, and recently, in November allowing the execution of scripts from http://a.com and 2022, it was finally patched. blocking the execution of other inline and external scripts. We also found that Safari enables * in the Interestingly, the CSP specification describes the script-src-elem directive by default. This default Chromium and Safari behavior as valid [29]. The reason behavior bug imposes a security threat; when a site is to prevent the worst scenario that the page itself does operator attempts to block all script requests by specifying not work due to a malformed CSP. Assume that we specify script-src-elem ‘none’, Safari allows fetching default-src http://.com. scripts from arbitrary endpoints, contrary to the operator’s If a browser only drops the invalid value, the CSP becomes expectations. We reported this bug to the WebKit team, and default-src, which is semantically equivalent to the vendor patched the bug. default-src ‘none’, which blocks all resources. On the other hand, if a browser drops the entire directive in Cause #7: Auto-enabling directive values on spe- compliance with the CSP specification, it blocks no resources cific conditions. We have found several bugs in Sa- on the page. Although the handling of malformed CSPs fari in which specific directive values are automati- adopted by the current specification provides convenience to cally enabled under certain conditions. For example, Sa- end users, it poses a security threat. fari automatically activates unsafe-inline for the script-src-elem directive when strict-dynamic Cause #10: Allowing out-going requests. According is present. Also, when a hash-source is specified in the to the script directives pre-request check in the CSP script-src-elem or script-src-attr directive, we specification [32], when there is strict-dynamic in observed that unsafe-inline is auto-enabled for both the script-src directive, the request from the parser- directives. Unfortunately, inconsistencies #20–#23 could be inserted script (e.g., regular script tags) must be blocked. triggered even with an arbitrary hash value rather than the However, Firefox does not follow this specification and sends hash of an inline script in a testing webpage. These bugs are outgoing requests for thus able to enable unsafe-inline in certain CSPs, thus even though CSP is script-src ‘nonce-123’ allowing an XSS attacker to inject inline scripts. ‘strict-dynamic’. In Chromium-based browsers, the same bug occurs when a page is , and its CSP identified them. is script-src ‘nonce-123’ ‘strict-dynamic’ [URL]. Cause #8: Non-supporting CSP for specific status code. We observed that Chromium ignored a CSP when the HTML re- Since script execution is still blocked, the impact of the sponse came with the status code of 100. We further confirmed problem for XSS attacks is limited. However, such scripts can that this behavior also occurred when the status code is 101 be used for the exfiltration of sensitive data. We have reported or 102. This means that Chromium-based browsers ignore any these bugs; however, the vendors have not responded yet. CSPs when a webpage is fetched with one of these response status codes. 16