---
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title: Cross-Origin Web Attacks via HTTP/2 Server Push and Signed HTTP Exchange
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resource: "https://www.ndss-symposium.org/ndss-paper/cross-origin-web-attacks-via-http-2-server-push-and-signed-http-exchange/"
title: Cross-Origin Web Attacks via HTTP/2 Server Push and Signed HTTP Exchange
author: Pinji Chen, Jianjun Chen, Mingming Zhang, Qi Wang, Yiming Zhang, Mingwei Xu, Haixin Duan
also_at:
- "https://www.ndss-symposium.org/wp-content/uploads/2025-1086-paper.pdf"
authors:
- Pinji Chen
- Jianjun Chen
- Mingming Zhang
- Qi Wang
- Yiming Zhang
- Mingwei Xu
- Haixin Duan
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---
# Cross-Origin Web Attacks via HTTP/2 Server Push and Signed HTTP Exchange
**Cross-Origin Web Attacks via HTTP/2 Server Push and Signed HTTP Exchange** - Pinji Chen, Jianjun Chen, Mingming Zhang, Qi Wang, Yiming Zhang, Mingwei Xu, Haixin Duan, NDSS Symposium.
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# Cross-Origin Web Attacks via HTTP/2 Server Push and Signed HTTP Exchange
--- page 1 ---
Cross-Origin Web Attacks via HTTP/2 Server Push
and Signed HTTP Exchange
Pinji Chen
, Jianjun Chen
yz
, Mingming Zhang
y
, Qi Wang
, Yiming Zhang
, Mingwei Xu
, Haixin Duan
Tsinghua University,
y
Zhongguancun Laboratory
f
cpj24, qi-wang23
g
@mails.tsinghua.edu.cn,
f
jianjun, zhangyiming, xumw, duanhx
g
@tsinghua.edu.cn,
zhangmm@mail.zgclab.edu.cn
Abstract
—In this paper, we investigate the security implications
of HTTP/2 server push and signed HTTP exchange (SXG)
on the Same-Origin Policy (SOP), a fundamental web security
mechanism designed to prevent cross-origin attacks. We identify
a vulnerability introduced by these features, where the traditional
strict SOP origin based on URI is undermined by a more
permissive HTTP/2 authority based on the SubjectAlternative-
Name (SAN) list in the TLS certicate. This relaxation of
origin constraints, coupled with the prevalent use of shared
certicates among unrelated domains, poses signicant security
risks, allowing attackers to bypass SOP protections. We introduce
two novel attack vectors, CrossPUSH and CrossSXG, which
enable an off-path attacker to execute a wide range of cross-origin
web attacks, including arbitrary cross-site scripting (XSS), cookie
manipulation, and malicious le downloads, across all domains
listed in a shared certicate. Our investigation reveals the prac-
ticality and prevalence of these threats, with our measurements
uncovering vulnerabilities in widely-used web browsers such as
Chrome and Edge, and notable websites including Microsoft. We
responsibly disclose our ndings to affected vendors and receive
acknowledgments from Huawei, Baidu, Microsoft, etc.
I. I
NTRODUCTION
The Same-Origin Policy (SOP) is a cornerstone of web
security, designed to prevent malicious scripts on one website
from accessing data on another, thereby safeguarding user data
against cross-origin attacks. Despite its foundational role, we
nd that recent advancements in HTTP protocol, specically
HTTP/2 server push and signed HTTP exchange (SXG),
pose challenges to this web security paradigm. These new
HTTP features introduce novel security concerns that could
potentially allow the circumvention of SOP, undermining the
security of web content across domains.
At the core of this vulnerability lies two critical issues.
First, the notion of “authority” in HTTP/2 and HTTP/3 is
looser compared to the “origin” denition of the browser's
SOP. Browser's SOP denes the origin of a resource as
the URI scheme/host/port tuple [1], while RFC 9110 [2]
states that in HTTP/2 and HTTP/3, any domain listed inz
Corresponding author.
the SubjectAlternativeName (SAN) of a TLS certicate is
recognized as the authority. Second, HTTP/2 server push and
SXG both allow to specify the origin of resources that they
deliver. Combining these two characteristics, a domain can
spoof the origin of another domain within the SAN list of
TLS certicates to deliver resources and browsers will accept
them as legitimate same-origin resources. This shift from a
traditional strict URI-based origin to a more permissive SAN
list-based origin could weaken the traditional boundaries of
browser origin security.
Even worse, the situation is exacerbated when this permis-
sive boundary meets the problematic ecosystem of the shared
certicate, whose SAN list contains multiple domains. Previ-
ous work has shown that multiple-domain shared certicates
are common and these domains are often managed by disparate
entities [3]. It is common on today's web to connect to a
website whose certicate is shared with an attacker-controlled
domain [4]. Such shared certicates create an avenue for
attackers to exploit these features for malicious purposes.
In this paper, we propose two novel cross-origin web attacks
via server push and SXG (CrossPUSH and CrossSXG). These
techniques enable an off-path attacker with access to a shared
certicate to execute a range of web attacks, such as arbitrary
XSS, cookie manipulation, and malicious le downloads,
across all domains listed in the certicate. These attacks are
particularly severe because they remain effective even against
victim websites that adhere to HTTPS best practices and
implement strict SOP security measures like Content Security
Policy (CSP).
We further introduce several strategies to demonstrate the
practicality and impact of CrossPUSH and CrossSXG attacks.
First, we demonstrate that acquiring the attack condition (i.e.,
shared certicates) is feasible in practice via simple vulnerabil-
ities, such as by domain reselling or domain takeover. Second,
we explore practical exploitation using validation reuse to
extend attack duration and the Add Grace Period (AGP) to
reduce attack cost. Furthermore, we introduce a technique
to make illegitimately acquired shared certicates difcult to
revoke by legitimate domain owners, ensuring the persistence
of the attack.
To evaluate the real-world impact of these two attacks, we
conduct both client-side and server-side measurement studies.
Our client-side measurement results show that: (1) 11 out
of 14 popular browsers, including Chrome and Edge, are
Network and Distributed System Security (NDSS) Symposium 2025
24-28 February 2025, San Diego, CA, USA
ISBN 979-8-9894372-8-3
https://dx.doi.org/10.14722/ndss.2025.231086
www.ndss-symposium.org
--- page 2 ---
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--- page 3 ---
vulnerable to at least one of our attacks; (2) Numerous popular
mobile applications, including Instagram and WeChat, are also
affected by our attacks; (3) Some vulnerable libraries, such as
Chrome-Net, can expose browsers to our attacks. Our server-
side measurement results show that: (1) More than 10K Tranco
Top 1M domains, including many corporate websites, have
been resold and potentially exposed to our attacks; (2) About
5K dangling domains, including a subdomain of
windowsup-
date.com
, can be exploited for our cross-origin web attacks;
(3) About 85% of Tranco Top 1K domains are included
in the shared certicates of domains ranked out of Tranco
Top 1M. That risky dependency allows attackers to attack
the top websites via less secure sites. We have responsibly
reported our ndings to the affected providers and received
acknowledgments from Huawei, Baidu, Microsoft, etc.
Contributions.
In this paper, we make the following con-
tributions.
We present CrossPUSH and CrossSXG attacks that
broadly threaten web security. We demonstrate this threat
is practical and severe, enabling off-path attackers to con-
duct a range of web attacks by exploiting HTTP/2 server
push and SXG, thus circumventing SOP protections.
We conduct comprehensive measurements to evaluate the
real-world impact of the attacks, revealing vulnerabilities
across numerous popular browsers and websites.
We propose four approaches to mitigate CrossPUSH and
CrossSXG attacks. We have also responsibly disclosed
issues to affected vendors and received positive feedback.
II. B
ACKGROUND
A. HTTP/2 Server Push
Server push is one of the new characteristics of HTTP/2,
which was rst dened in RFC 7540 [5]. Without server push,
traditional website access follows the request-then-response
pattern that a client initially requests for an HTML document,
and then the web server replies with the assets. Afterward,
the browser parses the HTML and discovers other referenced
assets, such as scripts, images, and style sheets. Due to the
discoveries, the browser needs to request these assets again,
which takes at least two round-trip times (RTT) to get critical
resources. Before the browser gets all of the resources, the user
can merely see a blank page on his screen. Server push solves
this problem more efciently. When the server responds with
HTML, it can also proactively push what the users are going
to need for the requested pages. Consequently, pushed assets
prevent the browser from requesting resources again, the whole
process only costs one RTT to load the contents if the server
preemptively sends all critical assets, which greatly improves
the performance of website access. Specied in HTTP/2 by
the IETF working group in 2015, server push has been going
on ever since and continued to be kept in HTTP/3 [6] in 2022.
HTTP/2 server authority.
In HTTP/2 and HTTP/3, a
client attributes authority to a server if the URI origin's host
matches any of the hosts present in the server's certicate
[2]. Although server authority can be lots of origins in
the certicate, the client always attributes server authority
to the current requesting URI tuple. However, the HTTP/2
server push is unique here. It can indicate the authority of
a push stream by specifying
:authority
pseudo-header.
Moreover, the browser can accept the push stream as long
as its
:authority
matches any origins in the certicate
according to the specication of RFC 9113 [7].
B. Signed HTTP Exchange
Signed HTTP exchange (SXG) is a delivery mechanism
that makes the browser authenticate the origin and integrity
of a resource without considering how it was delivered [8].
With the SXG, a publisher can safely make their content
portable, i.e., available for redistribution by third parties,
while still keeping the content's integrity and attribution. The
most common practice of SXG is to prefetch and serve the
contents signed by the publisher through a cache. This boosts
cross-origin navigations from referer sites while also ensuring
the untampered contents as well as the proper attribution.
For instance, Google Search crawls and caches SXGs when
available and prefetches SXG that the user is likely to visit,
e.g., the rst search result, to provide users with a faster page
load experience from the search results page. In 2018, Google
announced the support of SXG in Chrome 73 and later [9].
Since then, many other Chromium-based browsers have begun
to be available for SXG as well, e.g., Edge 79 and Opera
64 [10]. Some Content Delivery Network (CDN) vendors can
also congure SXG service [11].
Origin of SXG.
Browser attributes the origin of an SXG
to
request-url
header as long as this URL is same-origin
with
validity-url
and the certicate in
cert-url
can
authenticate
validity-url
. Similar to server push, SXG
providers can also indicate the origin of a resource to any
domain in the certicate by specifying these headers.
C. Shared Certicate
SSL/TLS certicate is the digital identity card of a website.
In an HTTPS connection, one party can ensure the identity of
another party via the certicate issued by the trusted certicate
authorities (CAs), reducing the risks of man-in-the-middle
attacks. Nevertheless, it is inconvenient if one certicate can
merely authenticate one domain. Some organizations have
plenty of domains, which is a heavy burden to issue certicates
for each one. To issue, manage, and revoke an SSL/TLS
certicate more efciently, the certicate can be shared by
multiple subjects. Specically, a shared certicate uses the
SAN extension to include multiple alternate domains, and
each of these domains can be authenticated by the certicate.
For example, a certicate with a SAN list [
*.google.com
,
android.com
] would be accepted by both
www.google.com
and
android.com
.
Shared certicate issuance.
The most crucial step in issu-
ing a certicate is domain ownership validation (DOV). When
issuing a shared certicate with multiple domains, CAs need
to verify the ownership of every domain. There are three ways
to perform DOV: (1) DNS mode. Add appointed DNS records
2
--- page 4 ---
to the authoritative nameservers. This is the only case to issue
the certicate of wildcard domains; (2) HTTP mode. Write
a given le to the root directory of the website, e.g.,
/.well-
known/pki/
; (3) Email mode. Click the received verication
link in the website administrator's email. Besides, issuing a
certicate that can sign an SXG additionally needs the CA
to support the CanSignHttpExchanges extension. To the best
of our knowledge, only two CAs (
Digicert
and
Google
) are
now available for the SXG certicate and ask the domains to
congure the DNS CAA records
1
.
Certicate expiration.
The expiration time is important for
both attackers and owners because it is the nal backstop of
the compromised certicates. Although the CA/Browser forum
suggests that setting the lifespans of certicates to no more
than 398 days offers several benets [12], the specic validity
period depends on issuers and the implementations of CAs. In
addition, the draft of SXG limits the lifespan of the certicate
with CanSignHttpExchanges extension to 90 days [13].
III. C
ROSS
PUSH
AND
C
ROSS
SXG A
TTACK
This paper introduces a novel vulnerability that adversaries
can utilize shared TLS certicates in server delivery processes
to bypass SOP. The threat is brought by two server delivery
mechanisms, HTTP/2 server push and SXG, and we refer to
them as
CrossPUSH
and
CrossSXG
attacks, respectively. In
this section, we rst present the threat model (Section III-A)
and concept (Section III-B). Then, we delve into the novel
security implications (Section III-C) and methodology (Sec-
tion III-D) of these attacks.
A. Threat Model
In this paper, we assume an off-path adversary with two
capabilities:
First, an attacker can acquire a TLS certicate
shared with a victim's websites.
This requirement is feasible
in practice via common vulnerabilities such as unsecured
le uploads in the “
/.well-known
” directory [14] or an email
provider's oversight in protecting the domain's administrative
email addresses [15]. Besides, we propose two practical meth-
ods for attackers without any intrusion abilities to acquire
multi-domain certicates shared with victim's websites in our
study: (1) Domain reselling. An attacker can register numerous
domains and issue a shared certicate for these domains, then
resell domains to victims while retaining the certicate (Sec-
tion IV-A Method-1); (2) Domain takeover. An attacker can
take over the dangling domains whose DNS records pointed
to discontinued CDN services, de-provisioned VPS IPs, or
expired domain names, and then issue a shared certicate
(Section IV-A Method-2).
Second, the attacker can lure the
victim to visit the attacker's website.
This can be accomplished
through methods such as phishing or embedding an iframe on
a popular site. Notably, we do not require the attacker to be
in-path or on-path to intercept and sniff encrypted trafc.
For the victims, all users visiting attackers' websites with
vulnerable browsers (Chrome, Edge, and others in Sec-
tion VI-A3) could be affected. They will be susceptible to1
https://github.com/google/webpackager/wiki/Certicate-Authorities
Fig. 1: Attack concept of CrossPUSH and CrossSXG.
arbitrary XSS, cookie manipulation, and other attacks in the
context of the websites that are included in the attacker's
shared certicate.
B. Attack Concept
CrossPUSH and CrossSXG attacks take advantage of three
key observations to bypass browser SOP protections, thus
enabling various cross-origin web attacks.
Observation-1: HTTP/2 server authority is less restrictive
than the browser's same-origin policy.
While both policies
indicate the resource's origin, browsers dene the same origin
based on identical URI tuples, whereas HTTP/2 considers any
domains listed in the SAN of the TLS certicate to be from the
same authority. Therefore, this discrepancy could compromise
the browser's SOP protection when the browser's origin is
reassigned to match the HTTP/2 authority.
Observation-2: Server push and SXG can specify the au-
thority of the resource in HTTP responses.
Typically, the
authority of an HTTP response aligns with the request's URI.
However, server push and SXG, two new features in the server
delivery process, can utilize the
:authority
pseudo-header
and
request-url
to reassign the resource's authority to
any domains listed in the certicate's SAN. Thus, a domain
can spoof the origin of another domain within the SAN list
of shared certicates to deliver resources and browsers will
accept them as legitimate resources under the target domain.
Observation-3: Misalignment between certicate and do-
main ownership exacerbates attacks.
Previous work has shown
that multiple-domain shared certicates are common and these
domains are often managed by disparate entities [3]. In ad-
dition, there is no coercive measure to keep the certicate
and domain owner in line. Attackers can exploit common
web vulnerabilities or scenarios presented in Section IV-A
to acquire a certicate shared with others' domains, thereby
launching our attacks to numerous victim websites.
As illustrated in Figure 1, based on the above observations,
the attack ow for both CrossPUSH and CrossSXG can
be summarized into four steps: (1) the attacker acquires a
shared certicate containing both attacker-controlled domains
(e.g.,
attacker.com
) and victim domains (e.g.,
victim.com
), as
detailed in Section IV-A; (2) the attacker can set up an HTTP/2
server or an SXG server on
attacker.com
using the shared
certicate, and lure users to visit; (3) the attacker delivers
a malicious cross-origin resource (e.g., a malicious script)
via server push or SXG, specifying the origin as
victim.com
;
3
--- page 5 ---
(4) browsers check the authority of the attacker's scripts and
recognize them as resources from
victim.com
. Consequently,
when users revisit
victim.com
, their browsers execute the
malicious scripts within the context of the
victim.com
.
C. Novel Security Implication
Notably, our attacks introduce novel security concerns com-
pared to previous works, as outlined below.
Implication-1: Enabling more practical web attacks with il-
legitimate certicates.
Previously, acquiring a certicate would
only facilitate man-in-the-middle (MitM) attacks, which re-
quires the attacker to be on-path, but now it is straightforward
for an off-path attacker to execute web attacks via server push
and SXG, making it much easier to launch an actual attack
against users.
Implication-2: Increasing the attack surface of high-prole
sites.
For instance, if an adversary wants to attack
google.com
,
he might typically look for vulnerabilities on this strictly pro-
tected site. However, our attacks expose more attack surfaces
by enabling the adversary to execute various web attacks
against
google.com
by compromising another domain, such
as
admob-cn.com
or a subdomain, that shares certicates with
google.com
. Many of these domains are low-ranked websites,
which, as previous studies [16] have shown, respond more
slowly to security events like Heartbleed, thereby exposing
more risks of certicate breaches to enable our attacks.
Implication-3: Extending the attack duration time of some
short-lived vulnerabilities.
Previous vulnerabilities like dan-
gling domain takeover will be invalid after the victim deletes
the stale DNS record (from hours to days). In contrast, our
attacks extend the attack duration to the certicate expira-
tion (796 days after removing the dangling records as we
demonstrate in Section IV-B). An attacker can issue a shared
certicate of dangling domains, and then leverage CrossPUSH
to continuously attack the dangling domains for more than two
years. Further, we also nd a technique (Section IV-D) to make
victims difcult to revoke the acquired certicate themselves
which results in a persistent attack.
D. Attack Methodology
CrossPUSH and CrossSXG attacks exhibit fundamental
similarities but differ in execution details. Below, we elaborate
on the specic methodologies of each attack. Our threat model
assumes that an attacker has already obtained a certicate
sharing between
attacker.com
and
victim.com
and we would
demonstrate how an attacker can acquire the shared certicate
in Section IV-A.
CrossPUSH attack.
Typically, browsers accept resources
that are pushed from the same origin as the requests. The
authority of the pushing server is determined by verifying
the SAN domains in the SSL/TLS certicates during TLS
connections. Furthermore, browsers assess the
:authority
pseudo-header in HTTP responses to conrm the authenticity
of the resources. However, a malicious server could push
resources for other domain names sharing the same TLS
certicates by crafting a forged
:authority
header.
Key entities.
Figure 2(a) presents the
CrossPUSH
attack
model, which comprises three key components: (1) A browser
equipped with a push cache; (2) An attacker in control of a
website,
attacker.com
, sharing a TLS certicate with victim
websites; (3) A victim website
victim.com
.
Attack details.
To conduct a
CrossPUSH
attack, the attacker
rst tricks users into visiting
https://attacker.com
, possibly via
phishing or embedding an iframe on a popular site. Then,
while sending a standard HTML response, the attacker uses
an HTTP/2 server push to send script.js, claiming its authority
as
victim.com
. This could be done by using Node.js's HTTP/2
framework to directly set the stream's
:authority
[17].
Since the authority of script.js (
victim.com
) aligns with the
origin specied in the shared certicate but does not match
the requested host (
attacker.com
), the browser accepts and
stores it in the push cache but does not apply it to the current
page. Afterward, the attacker can use the HTML
http-equiv
attribute in the response stream to make the
browser request
victim.com
, leading the attack to take effect.
The dotted line in Figure 2(a) indicates that the following
request will not arrive at
victim.com
since the browser will
check the push cache rst and nd that the requesting host
matches the authority of script.js. Consequently, via Cross-
PUSH, the browser loads the script.js from the cache that is
actually sent by
attcker.com
. Although HTTP/2 server push
may be designed to permit cross-origin pushes, this feature
can be exploited by malicious attackers with access to shared
certicates to falsely present trusted authority for harmful
content, such as malicious scripts, thereby circumventing the
browser's SOP protection.
CrossSXG attack.
SXG is designed for publishers to dis-
tribute their web content via third parties or caches, enabling
browsers to securely serve cross-origin resources. Here, we
should claim that rather than discuss the aforementioned
capability of
serving
others' cross-origin content via SXG, we
elaborate on how a website administrator can
sign
the SXGs
of other websites. Our threat model reveals a pressing concern,
akin to the CrossPUSH scenario: an illegal entity could create
and sign malicious web content as SXG, falsely presenting it
as originating from any victim domains in the SAN list of a
shared TLS certicate. This is achieved by manipulating the
request-url
and
validity-url
headers.
Key entities.
Figure 2(b) depicts the
CrossSXG
model,
which also comprises three parts: (1) A browser enabled
with SXG support; (2) A compromised website,
attacker.com
,
serving
victim.com
's SXG and cert.cbor. Importantly, cert.cbor
represents the shared certicate in concise binary object repre-
sentation (CBOR) format [18], which is essential for verifying
the victim's SXG; (3) A victim website
victim.com
.
Attack details.
First, the attacker prepares malicious web
content signed as an SXG for
victim.com
. Then, they entice
users to visit
https://attacker.com
and deliver the SXG to
the browser. The browser then requests the
cert-url
from
the SXG headers for certicate validation, which, controlled
by the attacker, points back to
attacker.com
. This allows
the attacker to respond with cert.cbor containing
victim.com
4
--- page 6 ---
Browser
(
compromised
)
attacker.com
(SAN contains
victim.com
)
req https://attacker.com/push script.js(
victim.com
)victim.com req https://victim.comBrowser’s push cachescript.jshtml
--- page 7 ---
Browser
(
compromised
)attacker.com
(Serve
victim.com’s SXG
&cert.cbor
)req https://attacker.com/
SXG(
victim.com
)
victim.com req “cert-url”cert.cbor(
victim.com
)verify OK, decode SXGverify fail, req “request-uri”
--- page 8 ---
(a) CrossPUSH attack. The attacker forges the authority of the push
stream to victim.com. Since the browser accepts all of the resources
that are authorized in the scope of SAN, the browser loads the script.js
that is actually sent by attacker.com from the push cache when the user
requests victim.com next time.
(b) CrossSXG attack. The attacker forges two headers of the SXG,
”request-url” and ”validity-url” to victim.com. Since the browsers accept
all of the SXGs whose aforementioned headers are same-origin with the
domains in the SAN of the certicate, the browser decodes the SXG as
the victim's web content but is actually crafted by the attacker.
Fig. 2: Details of CrossPUSH attack and CrossSXG attack
in the SAN. The browser checks if this certicate matches
the
validity-url
and whether the
request-url
is the
same origin as the
validity-url
. If veried, the browser
displays
victim.com
in the address bar and loads the attacker's
content. If not, it would typically request
victim.com
. However,
as the attacker can tailor the SXG content, the verication
usually succeeds, rendering the request to the victim's site
impossible, as indicated by a dotted line in the gure. Con-
sequently, the attacker with access to a shared certicate
(cert.cbor) can successfully sign an SXG for
victim.com
and
execute malicious content on the victim website's context.
IV. A
TTACK
P
RACTICALITY
In this section, we introduce a series of techniques to make
these attacks more practical and dangerous in the real world.
We propose (1) two common and exploitable cases to reduce
the demands on the attacker's capability to acquire shared cer-
ticate (Section IV-A); (2) a novel insight on validation reuse
to extend attack duration time (Section IV-B); (3) an abuse of
AGP to reduce attack cost (Section IV-C); (4) a technique to
make illegitimate certicate irrevocable (Section IV-D).
A. Acquiring Shared Certicates without Intrusion
We propose two broadly applicable and exploitable meth-
ods that enable even a basic attacker, lacking any intrusion
capabilities, to meet the shared certicate condition necessary
to initiate our attacks.
Method-1: Domain reselling—issuing a multi-domain cer-
ticate and then reselling part of the included domains.
Due
to the absence of mechanisms aligning certicate ownership
with domain ownership, domain reselling allows attackers to
retain the shared certicate while reselling domains included
in that certicate to another party. Thus, an attacker can obtain
a shared certicate through the following steps:
À
The attacker purchases many domains (e.g., victim.com,
attacker.com) and issues shared certicates for them.
Á
The attacker popularizes domains included in the shared
certicate with many methods to increase the reselling proba-
bilities, such as exploiting blackhat search engine optimization
(SEO) platforms [19], promoting websites via social media
and domain resale marketplaces, or improving rankings in top
domain name lists [20].
Â
The attacker waits for these domains to be bought by
any potential victims. If a victim acquires the resold domains
before the certicate expires, the attacker successfully creates
attack conditions.
While domain reselling has become prevalent
2
, this resale
process not only grants all original domain owners access to
the attack conditions but also exposes anyone purchasing the
resold domains to our described attacks.
Method-2: Domain takeover—taking over dangling do-
mains and issuing shared certicates.
Domain takeovers have
been demonstrated as a potent method for hijacking domain
names [21], [22]. This occurs when domain names point to
inactive cloud services, de-provisioned VPS IPs, or expired
domain names. As these targets are publicly available, attack-
ers can seize control of some victim domain names without
authority. Previous work [23] shows that once a domain
is taken over, attackers can circumvent domain ownership
validation (DOV) via HTTP mode when obtaining SSL/TLS
certicates. This enables them to issue a shared certicate that
includes both the hijacked domain and an attacker-controlled
domain, setting the stage for our attacks. We illustrate attack
requirements for different scenarios to launch domain takeover
in Appendix A Table V. The key steps to obtain a shared
certicate through domain takeover are shown below:2
https://www.name.com/zh-cn/blog/3-easy-ways-to-get-started-reselling-
domains
5
--- page 9 ---
Domain Validation Reuse
Period (
398 days
)Certificate Lifetime (398 days)certificate expirationtWithout
validation
reuseTattack duration time 398 T Certificate Lifetime (398 days)certificate expirationtattacker issued the
shared certificate
Tattack duration time 796
T
last day to issue the
shared certificateWith
validation
reuseattacker issued the
shared certificate
--- page 10 ---
TABLE I: Validation reuse period of celebrated CAs Digicert GlobalSign Entrust Let's Encrypt Google397 397 398 30 901
The certicate with SXG extension has validation reuse too.
Although there is no document to expose how long it is, our
experiment on Google CA shows that the reuse period lasts at
least 30 days.
À
The attacker locates the victims (e.g.,
victim.com
) by scout-
ing for dangling domains whose DNS records are pointing to
inactive cloud services (e.g., CDN services), de-provisioned
VPS IPs, or expired domain names.
Á
For each scenario, the attacker applies for the stale resources
(e.g., CDN-assigned endpoints like victim.cdnservice.com)
that are pointed by dangling DNS records of
victim.com
and
then congures the endpoints to route trafc from
victim.com
to the attacker's server. Consequently, the DNS resolution
would be as follows:
victim.com
)
stale resources
)
attacker's web server
Â
With the dangling DNS records, a CA could connect to the
attacker's server to fetch the challenge token during the DOV
of victim.com in HTTP mode. This enables the attacker to
issue a multi-domain certicate that covers both victim.com
and attacker-controlled domains, thereby successfully creating
the attack condition.
B. Extending Attack Duration via Domain Validation Reuse
Attack duration time signicantly affects the practicality
of our attacks. Although the recommended validity period
for public TLS certicates has been reduced to less than
398 days [24], or in some cases even 90 days [25], our
recent ndings indicate that a user-friendly approach, known
as domain validation reuse [26], can potentially extend the
duration of our attacks to a maximum of 796 days.
Domain validation reuse allows a certicate issuer to by-
pass DOV by reusing previously validated domain ownership
information. This approach, automatically activated after the
issuer has successfully passed validation in the recent past,
aims to ease the issuer's burden and enhance usability. The
CA/Browser Forum has set the maximum period for validation
reuse at 398 days [26]. However, individual certicate author-
ities (CAs) have considerable discretion when implementing
this policy. The specic validation reuse periods for several
well-known CAs are detailed in Table I.
Nevertheless, it has been overlooked that validation reuse
not only extends the certicate's lifespan but also potentially
grants the domain owner extended control over the certicate.
This aspect signicantly increases the potential duration of
our proposed attacks. As shown in Figure 3, we assume
that the attack time span would be 398 - T days in the
absence of validation reuse, where “398” is the certicate
lifespan and “T” is the time an attacker waits for a victim to
purchase or gain control of the domains included in the shared
certicate. However, when employing validation reuse, the
Fig. 3: Extending attack duration time. Validation reuse en-
ables the attacker to prolong the attack window to 796 - T
days, where T is the period that the attacker waits for victims
to get control of the domains in the shared certicate, e.g.,
buying the attacker's resold domains.
attacker can issue shared certicates without DOV for another
398 days (solid red bar in Figure 3) following the initial
issuance. During this period, the shared certicate remains
rmly under the attacker's control. Notably, domain owners
even cannot completely revoke the shared certicate, because
the attacker can continually reissue it as long as their account
remains within the validation reuse period. Moreover, if the
attacker reissues a new shared certicate on the last day of
the validation reuse period, the duration of the attack can
potentially be extended to a maximum of 796 days
3
.
Even worse, there is no established security protocol or joint
effort between domain registrars and certicate authorities
(CAs) in this context. To test its feasibility, we conducted
a real-world experiment. We registered a test domain from
Godaddy
and issued a shared certicate for this domain autho-
rized by
Let's Encrypt
. We then released the domain, making it
available to other customers. When the domain was available
again
4
, we acted as a potential victim and purchased the
domain through
Namecheap
. We then issued certicates using
another account via
ZeroSSL
and
Let's Encrypt
, respectively.
Our ndings are concerning: (1) Even when two accounts from
the same CA (e.g.,
Let's Encrypt
) control the same domain
simultaneously, this does not disrupt the process of validation
reuse; (2) Despite later revoking all issued certicates, the
original account can still bypass DOV to reissue certicates
due to validation reuse.
C. Reducing Attack Cost via AGP Abuse
In a domain reselling scenario (Method-1 in Section IV-A),
an attacker needs to acquire many domain names for attacks.
Therefore, minimizing the cost of these domain purchases
signicantly enhances the attack's viability. We nd that Add
Grace Period (AGP) [27], a user-friendly feature of domain
registration, can be abused by attackers to facilitate this3
This is 120 days for CrossSXG attack because the lifespan (90 days)
and validation reuse period (30 days) of the certicate with CanSignHttpEx-
changes are shorter.
4
The domain releasing time from a registry is tested to be 4 days in our
experiment.
6
--- page 11 ---
process. AGP, supported by ICANN and domain registrars,
provides registrants the opportunity to return domain names
within 5 days without cost. Two typical cases of abuse are
domain tasting [28] and domain kitting [29]. Due to a
rise in abuse, ICANN implemented the AGP Limits Policy in
2008 [30], signicantly curbing such incidents. However, the
rules are not strictly followed, and certain domain registrars,
including Godaddy
5
, Google
6
, and Namecheap
7
, still permit a
limited amount of refunds.
For the rst time, we examine the security risks associated
with AGP abuse, specically in the context of shared certicate
issuance. Attackers can purchase numerous domains from
registrars offering refunds, then issue shared certicates for
these domains. Subsequently, they release the domains during
the AGP and receive refunds. This strategy enables attackers
to acquire a shared certicate with multiple released domains
at no cost.
D. Making Illegitimate Certicate Irrevocable
A fundamental prerequisite for CrossPUSH and CrossSXG
attacks is the use of an illegitimately acquired shared certi-
cate. If such a certicate is identied and subsequently revoked
by the victim, the attack becomes ineffective. However, we
discover that it is possible to hinder the victim's ability to
revoke the certicate without the attacker's consent.
Commonly, when requesting a CA to revoke a shared cer-
ticate, users must either be able to pass DOV for all domains
included in the certicate [31] or possess the private key of
the certicate
8
. If the attacker issues a new shared certicate
that encompasses both the victim's and the attacker's domains,
the victim would be unable to meet either of the revocation
conditions, rendering the illegitimate certicate non-revocable.
We have veried this problem leveraging a trusted CA (i.e.,
ZeroSSL
) and our test domain names. Despite reporting our
security concerns about the existence of spoong certicates
beyond our control through the CA's ofcial problem reporting
platform, the shared certicate could not be revoked. This
demonstrates that due to this technique, even if the victims
identify an illegitimate certicate, they cannot prevent Cross-
PUSH and CrossSXG threats.
V. R
EAL
-W
ORLD
E
XPLOITATION
In this section, we discuss exploiting CrossPUSH and
CrossSXG. Once an attacker has a shared certicate, they
can launch a malicious website to target other domains in
the certicate. Since server push and SXG are rendered in
HTTP responses, the attacker can manipulate both HTTP body
and header to initiate various cross-origin web attacks by
exploiting CrossPUSH and CrossSXG.5
https://www.godaddy.com/en-sg/legal/agreements/refund-policy
6
https://support.google.com/domains/answer/6000754?hl=en
7
https://www.namecheap.com/legal/general/refund-policy/
8
https://letsencrypt.org/docs/revoking/
A. Leveraging HTTP Body
HTTP bodies determine the main contents exhibited on
web pages. By crafting HTTP bodies, attackers could execute
UXSS or conduct phishing attacks.
Exploit-1: Universal cross-site scripting (UXSS) attacks.
Given the ability to forge origins and control front-end web
pages, a straightforward exploitation is the XSS attack. An
attacker can add a script tag into the response body, e.g.,
, to
let the browser run arbitrary JavaScript on the victim's website.
Such exploitation can steal a user's cookie and initiate a money
transfer to an attacker's account. Besides, our attack is the most
harmful UXSS which is independent of the target website.
Regardless of whether the target website has vulnerabilities,
is ofine, or no longer exists, our attack still works. It is
worth noting that security policies like Content Security Policy
cannot prevent such attacks because browsers consider the
scripts from the same origin.
Exploit-2: Website phishing.
The attacker can customize
the whole HTTP body to a phishing HTML, inducing victims
to hand over usernames, passwords, or other secret information
on a familiar website. It should be noted that our phishing is
hard to detect. For instance, if the attacker wants to phish the
users of
example.com
, he can disguise his malicious website
as
example.com
's portal, leading users to click. Then, through
CrossPUSH or CrossSXG, victims will see the address in the
navigation bar rewritten to the
example.com
with the certicate
status showing secure (a lock). In addition, the page can be
spoofed to look exactly like
example.com
with only modifying,
e.g., the address of the submission form to the servers of the
attacker, thus completing the phishing attack imperceptibly.
B. Leveraging HTTP Header
Many HTTP headers carry security properties, with some
having been analyzed in previous works [32]. In our attacks,
exploitable headers should satisfy three basic conditions: 1)
response header, 2) cacheable, and 3) security-related. Upon
thorough inspection, we identied two stateful headers (i.e.,
Set-Cookie
,
Strict-Transport-Security
) as ex-
ploitable, given their ability to modify client states or behav-
iors.
Exploit-3: Setting arbitrary cookies.
The
Set-Cookie
HTTP response header is used to send a cookie from the server
to the user agent, so that the user agent can send it back to
the server later to notify the state of the user, such as login
status or personal prole. This header has many attributes, the
attacker can set
Domain
to the victim domain,
path
to
/
,
and
Expire
to a distant future, to sustainably set the user's
cookie. As the cookie is commonly used in today's web access,
an attacker who is capable of setting an arbitrary cookie to
the victim websites can lead to disastrous impacts like session
xation attack [33].
Exploit-4: Bypassing HTTP Strict Transport Security
(HSTS).
The
Strict-Transport-Security
header in-
forms browsers that the site should only be accessed us-
ing HTTPS, and any future attempts to access it using
7
--- page 12 ---
HTTP should automatically be converted to HTTPS. This
header is a pivotal security strategy that avoids SSL strip
and other types of man-in-the-middle (MITM) attacks, how-
ever, it can be bypassed by our attacks. The adversary
can set
Strict-Transport-Security: max-age=0
to delete the HSTS of the victim domains on browsers.
Consequently, the browser can use HTTP to access the victim's
websites, breaking the security practices of the websites.
Additionally, in the case of SXG, signing a le with the
aforementioned stateful headers is disallowed for security
reasons. However, we discover that the implementation of this
restriction is awed and can be circumvented. Rather than
having browsers check for stateful headers, this verication
is performed by the ofcial SXG web package provided by
WICG
9
. This approach overlooks the possibility of an attacker
not using the ofcial tool but instead developing their own.
Therefore, we modied the web package for SXG and created
an SXG generator that does not check for stateful headers,
enabling us to launch CrossSXG by leveraging HTTP headers.
C. Leveraging Body and Header
Besides exploiting the HTTP body and header respectively,
the attacker can also use HTTP headers to control how the
body is parsed, combining them together to launch an attack.
Exploit-5: Malicious le download.
An attacker can lever-
age the
Content-Disposition
header to indicate if the
content is expected to be displayed inline (as a web page,
default) or as an attachment. Since the attacker can also
control the HTTP body, he is able to arbitrarily specify
the content of the le and let the victim browser download
it as an attachment. More importantly, this header has an
attribute
filename
, which is capable of specifying the
le name as well as the le extension. As a result, the
attacker can use the ransomware or trojan as the HTTP
body and the
Content-Disposition: attachment;
filename="instruction"
as the header to make the
victim's browser automatically download a le named “in-
struction”. From the victim's perspective, this le is down-
loaded from an intended website with a reasonable name,
which seems to be secure. If the victim clicks the le,
his computer will be possibly implanted with a virus and
controlled by the attacker.
D. Attacking Multiple Targets with Single Click
Furthermore, the high concurrency of HTTP can be ex-
ploited to attack multiple targets with a single click. HTTP/2
supports numerous concurrent streams whose maximum num-
ber is limited by
MAXConcurrentStream
(set as 128 in
default). For the CrossPUSH attack, an attacker can initiate
multiple push streams simultaneously, and each authority
header of the streams is forged to be one of the victim websites
in the shared certicate, enabling attacking numerous targets
at once. Regarding CrossSXG, it's easier to achieve this goal
since CrossSXG merely needs the static
request-uri
to indicate9
https://github.com/WICG/webpackage
the target website, rather than the authority of the stream.
Therefore, an attacker can directly sign the SXGs of different
domains in the shared certicate and serve them on distinct
paths. At last, the attacker can use plenty of iframe to let the
browser request all of the SXGs, attacking multiple targets
with a single click.
VI. L
ARGE
-S
CALE
E
VALUATION
In this section, we conduct a large-scale measurement to
evaluate the real-world impact of our attack. The evalua-
tion mainly falls into two categories: one is the client-side
browsers' behaviors for CrossPUSH or CrossSXG, and the
other is the server-side affected websites.
A. Client-side: Vulnerable Browsers
CrossPUSH and CrossSXG attacks are constrained by
browser behavior, as different browsers may react distinctively
to these two attacks. Additionally, the diversity and quantity
of vulnerable browsers greatly affect the impact of the attacks.
To nd out how many client-side browsers are vulnerable, we
conduct a large-scale measurement of browsers' behaviors.
1) Measurement Challenge and Solution:
Considering the
wide variety of browsers used in our daily lives, it is com-
mon to have multiple browsers installed on different devices
such as smartphones, tablets, and computers. Furthermore,
the multitude of browser versions can lead to behavioral
inconsistencies across different versions. Thus, manually col-
lecting all the different browsers is a formidable task, posing
signicant challenges for comprehensively evaluating client-
side browsers. This motivates us to develop
PSChecker
, an
integrated platform to conduct the client-side measurement
and help to analyze the browsers' behaviors on server push
and SXG. By deploying the platform on high-trafc websites,
visitors will obliviously help us to automatically test their
browsers' behaviors, increasing the range and diversity of the
browsers to complement our unmeasured test objectives.
Nevertheless,
PSChecker
may raise ethical concerns if we
directly use it to measure CrossPUSH and CrossSXG attacks
in the wild. Moreover,
PSChecker
only provides coarse-
grained access data, which may require a few supplements.
To address the above issues, we determined to accomplish
client-side measurement through a two-step process. First, we
utilize
PSChecker
to broadly measure the browsers' feature
support for server push and SXG without any cross-origin
attack behaviors. The user's access data will enable us to
discover more testing objectives and give us an overview of
browsers' support for these two features. Second, we launch
a strictly controlled web environment to block other users'
requests and download all suspicious browsers locally to test
their behaviors on CrossPUSH and CrossSXG attacks. We also
manually supplement some data, such as the latest version of
the vulnerable browser, the older version of the invulnerable
browser, and the high-ranked browsers that were not tested,
etc., to enhance the comprehensiveness of the measurement at
a ne-grained level.
8
--- page 13 ---
Fig. 4: Overview of
PSChecker
2) PSChecker and Local Test: PSChecker
takes full consid-
eration of ethics and does not exploit or steal any credentials
from its users. It just uses the legal server push and SXG to
measure the feature support and records
User-Agent
(UA)
to help us identify browser varieties and versions. Figure 4
shows the overview of
PSChecker
. It consists of three parts:
(1) High-trafc website.
PSChecker
needs more visitors to
include as many browsers as possible, so we embedded our test
site as an iframe on a high-trafc website from our industrial
partners; (2) Feature measurement server. An HTTP/2 server
returns scripts via server push and SXG for measuring feature
support when users request it. Besides, it records some basic
information like UA and sends logs to log servers; (3) Log
server. An HTTP server that processes post requests and
records success log, failure log, as well as the full log. These
logs will be used in the following analysis.
PSChecker
workow.
First, users request our industrial
partner's high-trafc website through their browsers. As a
trafc honey pot, the website not only returns the normal
HTML but also adds an iframe within the
tag of the
HTML, with the link source set to the feature measurement
server. The width and height of the iframe are set to 0 to make
it invisible, which aims to avoid affecting users' browsing
experience. Second, the browser receives the response HTML
and then accesses the iframe link, directing the trafc to the
feature measurement server. After receiving the request, the
feature measurement server initiates server push and SXG with
scripts for measuring feature support. If the browser supports
the feature, it will execute the script and send the success log
to the log server. If the browser does not support the feature,
e.g., reject the push stream, the feature measurement server
will receive an error event. We can catch the error and post
the failure log to the log server. Last, since not all failures
result in an error event, for example, browsers without SXG
support do not have a corresponding error event. Therefore,
the feature measurement server will post all user requests to
the log server to maintain a full log for comparison. If the
behavior of a browser is neither in the success log nor in the
failure log, then this browser failed without an error event.
Subsequent local tests.
After we automatically get the
result of browsers' support for server push and SXG from
PSChecker
, we then locally test the behaviors of suspicious
browsers on our attacks in a lab environment. We launch
an attack server (like
attacker.com
in Figure 1) to initiate
CrossPUSH and CrossSXG attacks with an XSS script. The
origin of the XSS script is set to another domain in the shared
certicate. Besides, in CrossPUSH, whether the IP address
of the requesting domain is the same as the authority header
will affect browsers' behaviors. So, we further indicate the
origin of the pushed script to cross-ip domain and same-
ip domain, dividing CrossPUSH into cross-ip and same-
ip CrossPUSH. Moreover, we identify browsers from UAs
in the result of
PSChecker
and supplement uncovered top-
used browsers reported by Statcounter
10
to the testing list.
We download different versions of browsers in the list to
three leading platforms (Windows, iOS, Android) from ofcial
websites. At last, we use browsers to request the attack server
and analyze the behaviors.
3) Client-side Result Analysis:
We deployed
PSChecker
for
30 days, from Nov 15 to Dec 15, 2023. There were 10,072
visits to our
PSChecker
in 30 days. Measurement results
cover 2,873 different user agents and more than 30 different
types of browsers after de-duplication. We tried our best to
gure out the browser type from each UA and supplement
the missing top-used browsers in Statcounter. Finally, we
identied and systematically tested 26 browsers at a granular
level across different software versions and operating systems
in total. They were categorized into three groups: (1) Top-used
browsers, (2) Default browsers on mobile, and (3) Celebrated
apps.
Top-used browsers: 11/14 of them are vulnerable to
at least one of our attacks.
The upper part of Table
II includes top-used browsers reported by Statcounter and
other well-known browsers identied from UA. We evaluated
these browsers' behaviors on three popular platforms, i.e.,
Windows, iOS, and Android. The result shows that the latest
versions of most browsers (11/14) by the time of testing
are vulnerable to at least one of our two attacks on at least
one platform. Browsers' behavior on iOS is the most special
case: except for the UC browser being fully vulnerable to
CrossPUSH (we will analyze this nding later), other browsers
are either partially vulnerable or not vulnerable. That is
because Apple Inc. requests all browsers on iOS to use Webkit
kernel, making browsers on iOS react the same as Safari. In
terms of CrossSXG, the latest versions of most browsers are
vulnerable. Safari, Firefox, and IE are not vulnerable since
they do not support the SXG feature now. Whether this feature
will be developed in the future and suffered from our attack
is unknown. For CrossPUSH, UC, QQ browser (Win), and
Instabridge (Android) are fully vulnerable, while the rest of
the browsers are partially vulnerable. The reason is that most
developers of these browsers use Chromium implementation,
and Google removed the server push feature in Chromium106.
So, the latest version does not support server push, but the
old versions are vulnerable. Webkit-based safari only suffered
from same-IP CrossPUSH until version 17, which required
that the IP address of the victim website and the currently10
https://gs.statcounter.com/browser-market-share
9
--- page 14 ---
TABLE II: Browser behaviors on CrossPUSH and CrossSXG Top-used BrowsersCrossPUSHCrossSXGBrowserWiniOSAndroidWiniOSAndroidChromeG#G#G# # Safari-G#--#-EdgeG#G#G# # Firefox#G#####OperaG#G#G# # UC ###360G#G# # IE#--#--YandexG#G#G# # QQ browser G#G# # CocCocG#-G# - WhaleG#G#G# # Instabridge-G# -# Xunlei G#G# # Default Browsers on MobileSamsung--G#-- Huawei-- -- Oppo-- -- Xiaomi--G#-- Vivo-- -- 1
The red bar symbolizes that the browser's latest version is vulnerable
to at least one of our attacks.
2
denotes that the browser is vulnerable in the latest version;
3
#
denotes that the browser is not vulnerable;
4
G#
denotes that the browser is partially vulnerable, e.g., vulnerable in
the old versions or just vulnerable to same-IP CrossPUSH;
5
- denotes that the browser is not supported on this platform.
connected website should be the same, otherwise, it would
not use cross-origin pushed assets. Besides, iOS also no
longer supports the server push feature in the latest version
17. In summary, invulnerable browsers or partially vulnerable
browsers are caused by the temporary lack of support for SXG
or server push, while supported versions other than Firefox and
IE are all vulnerable, which is alarming.
Default browsers on mobile: ve leading mobile
browsers are vulnerable.
Mobile browsers contribute more
trafc than desktop browsers worldwide
11
, so we systemati-
cally tested the behaviors of default browsers on mobile. We
collected 5 leading mobile phone manufacturers in the world
12
(Apple Safari has been presented in the upper part), as shown
in the lower part of Table II, all of the tested default browsers'
latest versions are vulnerable to at least one of our attacks,
posing a signicant threat to the mobile users. It should be
noted that many mobile applications will open the URL link
in the default browser, which increases the possibility of the
users being attacked.
Celebrated applications: vulnerable Webviews spread
the threat to apps.
Apart from the browser, our
PSChecker11
https://gs.statcounter.com/platform-market-share/desktop-mobile-tablet
12
https://www.globalbrandsmagazine.com/top-10-mobile-brands-in-world/
TABLE III: Celebrated apps CrossPUSHCrossSXGAppAndroidiOSAndroidiOSBaidu #Quark ##InstagramG#G# #WechatG#G# #QQmailG#G# #WeiboG#G# #TiktokG#G# #1
All symbols have the same meaning as Table II.
found that lots of apps also have built-in browsers that are
susceptible to our attacks. Thus, we further tested them at a
ne-grained level. As shown in Table III, while all of them are
vulnerable, Baidu and Quark behave distinctively compared to
other apps, which we will analyze later. Notably, the rest of
the apps are partially vulnerable to CrossPUSH because these
apps' behaviors vary from devices in our test (e.g., succeed
on Huawei Mate 50 but fail on Xiaomi 10 pro). We think the
reason is these apps depend on the Webviews of the user's
devices. Unfortunately, the default browser on mobile reects
the behaviors of Webview in general, considering that many
devices have insecure default browsers, our attacks will affect
a large number of apps.
Software libraries: some libraries and software sup-
ply chains are vulnerable.
We observed that UC, Baidu,
and Quark are vulnerable to the CrossPUSH attack on iOS,
behaving distinctively compared to other browsers or apps,
even though they indeed use the Webkit kernel that is forced
by Apple. We consider the main reason is that the network
requests are taken over from Webkit by the developers. This
viewpoint is veried by the developers of the Baidu App, who
used the Chrome-Net library to take over the network requests.
This critical nding reveals that vulnerable libraries may also
lead browsers to suffer from our attacks, and the software
supply chains of browsers may be vulnerable.
B. Server-side: Affected Websites
1) Measurement Objectives:
In this part, we measure how
many websites are potentially threatened by our attacks. Con-
cretely, within our attacks, the websites that can be included
in the shared certicates of attackers, are the victim websites.
However, given the diverse methods of acquiring shared cer-
ticates, we cannot evaluate all situations. As a result, we
make clear our measurement objectives as the following three
typical categories of affected websites.
Reselling domains.
These affected domains indicate the
domains once resold from attackers to victims. As we describe
in Section IV-A Method-1, an attacker can issue a multiple-
domain shared certicate rst and resell domains to victims.
Subsequently, these resold domains can be directly targeted by
the original domain owners with our attacks until the shared
certicates expire.
10
--- page 15 ---
Dangling domains.
These affected domains indicate the
domains whose DNS records point to applicable public ser-
vices that can be hijacked by any attackers. As shown in
Section IV-A Method-2, an attacker can take over these
dangling domains and issue the shared certicate. Then, these
dangling domains will be under our attacks for more than two
years even after the dangling records are removed.
Cert-sharing domains.
These affected domains indicate the
top websites that are included in the existing certicates' SAN
lists of other peer websites. For instance,
google.com
is a cert-
sharing domain since it is included in the certicate of other
websites like
admob-cn.com
. Because peer websites hold the
shared certicate, these cert-sharing domains can be directly
attacked by peer websites using CrossPUSH and CrossSXG.
More importantly, attackers can compromise a less secure peer
website rst, get the shared certicate, and launch our attacks
against these cert-sharing domains.
2) Measurement Methodology:
We measure three types of
affected websites in different ways, as the following:
Measuring reselling domains.
We use WHOIS history data
to measure how many of the Tranco Top 1M domains have
experienced reselling within our attack window (796 days),
disclosing domains that are subject to potential exploitation
by the original domain owners. Firstly, since we cannot
analyze anonymous WHOIS, we excluded as much anonymous
WHOIS data as possible through keywords [3]. Secondly, we
use three strict conditions to lter out the reselling domains:
(1) Three main registrant's information has been changed; (2)
Levenshtein similarity [34] between old registrant organiza-
tion and new one was less than 0.3, which aims to prevent
some tiny changes in one entity, e.g., from Google to Google
LLC; (3) Two reselling period (createdDate to expiresDate)
does not overlap and the interval between them is less than
our attack window (796 days). Domains that meet these three
strict conditions are considered as reselling domains affected
by our attacks.
Measuring dangling domains.
To identify dangling do-
mains in the wild, we utilized the state-of-the-art tool, Host-
ingChecker [22], for large-scale measurement. This tool
primarily targets public hosting services susceptible to domain
takeovers. However, some of the services (e.g., OSS) are not
exploitable for issuing TLS certicates and launching our
attacks. Through manual analysis, we identify three scenarios
feasible for forging certicates: (1) Dangling DNS records
pointed to CDN services; (2) Dangling
A
records pointed
to applicable virtual private server (VPS) IPs; (3) Dangling
canonical name (
CNAME
) records pointed to unregistered
domains. To this end, we extend HostingChecker to assess
these specic situations. For case (1), we supplement with
more vulnerable CDN services by manually testing all of
the CDN vendors from CDN planet
13
since HostingChecker
misses some vendors and several CDNs like Bunny do not
allow certicate issuance in our tests. By adding the outdated
CDN service ngerprints to the cong of HostingChecker,13
https://www.cdnplanet.com/
TABLE IV: Overall result on server-side affected websites Reselling Dangling Cert-SharingScope Top 1M Top 1M and subdomains Top 1K
Count 11741 4919 829 we can directly measure this case. For case (2), we utilize
HostingChecker to resolve every domain to IP address. Then,
we check whether the IP belongs to three main VPS providers,
i.e., AWS, Azure, and Aliyun. At last, we refer to the liveness
detection method in previous work [23] to nd if the pointed
VPS is applicable. For case (3), we just make a WHOIS
request to every
CNAME
record resolved by HostingChecker
and inspect whether there is an unregistered domain.
Measuring cert-sharing domains.
We scope this measure-
ment to the cert-sharing domains in Tranco Top 1K. At a high
level, we nd out which website's TLS certicate contains
Tranco Top 1K websites and whether there are high-ranked
domains included in the shared certicates of low-ranked ones,
revealing the security degradation of top websites. Given the
extensive domain landscape, acquiring and analyzing all of
the domain's certicates proves inefcient. As a result, we
should quickly locate the relevant domains of the Top 1K
sites and continue the measurements within this pruned range.
Our key insight is that the domains in the SAN list of Top
1K websites' certicates must have a strong association and
be possible to share certicates with Top 1K. So, we take
measurements in the following steps: (1) Collecting “related”
domains in certicates' SAN of Top 1K sites. We rst establish
a connection on port 443 for each Top 1K site and utilize the
Python SSL library to extract the certicates from the context,
capturing the domains in the SAN from each certicate as the
fundamental dataset; (2) Constructing relevant domain dataset.
Secondly, We expand the data in the form of wildcards from
the fundamental dataset by recursively crawling subdomains.
For subdomain crawling, we employ various methods to crawl
as many subdomains as possible, including searching for
subdomains in HTTP response, Certicate Transparency log
(crt.sh), and passive DNS (from our industrial partners). We
use this augmented dataset as the relevant domain dataset;
(3) Constructing the Top 1K dependency dataset. Then, we
use the method of step (1) again to retrieve certicates of
relevant domains and inspect whether their certicates' SAN
lists contain Top 1K websites; (4) Acquiring rank dependency.
Consequently, we search the rank of the domains that share
certicates with the Top 1K websites from both Tranco Top
1M and Tranco Top 1M (with subdomains) to nd the rank
dependency phenomenon. We supplement the Tranco rank
(with subdomains) to ensure the fairness of our evaluation
since lots of domains are not apex domains.
3) Server-Side Overall Results:
Overall results are shown
in Table IV. Specically, at least 11,741 Tranco Top 1M
domains were once resold within our attack window, which
could be attacked by original domain owners. In a month,
11
--- page 16 ---
4,919 dangling domains (including domains from celebrated
companies, like Microsoft as we present in Section VI-C) can
be compromised by us and we can launch CrossPUSH to these
websites even for two years after they are not ”dangling”.
Besides, 829 (82.9%) of Tranco Top 1K domains are cert-
sharing domains that are suffering security downgrades.
4) Result Analysis of Reselling Domains:
Finding-1: Do-
main reselling is a widespread practice, with even many
high-ranking domains having been resold.
By carefully
analyzing the WHOIS history data, we identied 1 domain in
Tranco Top 1K, 59 domains in Top 10k and 11,741 domains in
Top 1M have been resold before. They all have been changed
hands from one person/organization to another within our
attack window (796 days). Notably, domain reselling can also
threaten corporate websites. For example, our measurement
showed that the owner of ftstatic.com (Tranco rank 3895 on
Nov 18, 2023) once changed from foodtraders.com.au Pty Ltd
(an Australian food company) to Flashtalking (an American
advertising agency). Even worse, the community is unaware
of the security risks posed by domain reselling at all, and has
even developed numerous domain reselling platforms
14
. These
discoveries demonstrate that our attacks will pose a novel and
serious threat to the prevalent domain reselling.
5) Result Analysis of Dangling Domains:
Finding-2: The
amount of exploitable dangling domains is large.
We
measured the dangling domains of Tranco Top 1M and its
subdomains 12 times (a month) to reveal how many websites
were threatened by our attacks due to temporary dangling DNS
records. For (1) dangling DNS records pointed to applicable
CDN services, 19 domains can be exploited by us. For (2)
dangling
A
records pointed to applicable VPSs, 3,952 domains
were exploitable, of which 2,513 domains'
A
records were
pointed to AWS VPSs, 788 to Azure, and 651 to Aliyun. For
(3) dangling
CNAME
records pointed to unregistered domains,
we found that 948 domains'
CNAMEs
were pointed to unreg-
istered domains. Attackers can acquire the shared certicate
of these dangling domains by merely applying for the services
or buying the unregistered domains. As a result, dangling
domains are very common, which demonstrates that a large
number of websites are affected by our attacks.
Finding-3: The duration of our attack is commonly
longer than traditional domain takeover.
Moreover, we ex-
plored the existence duration of these exploitable dangling do-
mains. For example, by delving into the 948 dangling domains
whose
CNAME
records pointed to unregistered domains, we
found that they are pointing to 162 unregistered apex domains
after de-duplication. Through analyzing the WHOIS historical
data, we statistically obtained that the median number of days
from the expiry time of each unregistered domain to the
start time of our measurements was 148 days. At the end of
the one-month measurements, only 2 once-dangling domains
were still pointing to unregistered domains, and almost all
other dangling DNS records were removed or the domains
they were pointing to were registered. This implies that most14
https://www.resellerspanel.com/domain-names/
dangling domains cannot be hijacked after six months, which
is also demonstrated by the previous study on dangling domain
lifecycle [22]. Such results prove that our attack poses a
more signicant threat to dangling domains, as we extend the
duration of domain takeover attacks, which usually tend to be
invalid within six months, to up to 796 days.
6) Result Analysis of Cert-Sharing Domains:
The result
painted a bleak picture of certicate sharing and security
dependency. Although these issues cannot let an attacker ob-
tain the shared certicate directly, the analysis of cert-sharing
domains demonstrated how CrossPUSH and CrossSXG can
increase the attack surface of high-prole sites. For example,
instead of looking for vulnerabilities in strictly protected top
sites, the adversary can compromise less secure websites
that share certicates with these sites rst, and launch our
attacks against these high-prole cert-sharing domains with
the acquired certicates. Therefore, we analyze the current
situation of shared certicates below.
Finding-4: 829 Tranco Top 1K websites are included
in the shared certicates of low-ranked websites.
By
crawling the subdomains, we discovered 99,827 fully qualied
domain names (FQDNs) potentially relevant to Tranco Top
1K domains and eventually conrmed that there were 45,930
domains sharing certicates with the top 1K websites. Among
these peer domains, 16,950 domains were the subdomains of
the top 1K domains and the rest were the non-subdomains.
It shows that many high-prole websites not only rely on the
security of their subdomains but also rely on non-subdomains,
which may belong to acquired companies, subsidiaries, or even
distinct organizations. In addition, as shown in Figure 5, most
websites sharing certicates with Tranco Top 1K were out of
Tranco 1M. For instance, 43,594 / 45,930 (94.9%) domains
were out of Tranco (only apex) 1M while this was 39,436 /
45,930 (85.9%) in Tranco (withsub). Actually, 953 and 829
of the Top 1K websites were included in the certicates of
domains ranked out of the Top 1M according to these two
domain rankings.
This existing ecosystem of certicate-sharing raises two
major concerns: (1) Many companies do not always update
their certicates when their acquired companies have changed
ownership, which was conrmed by Baidu. After receiving
our report, they noticed that some of their acquired com-
panies became independent but still held certicates shared
with Baidu's domains, which brought potential threats. (2)
Certicate sharing leads top-ranked sites to suffer security
degradation. Generally, the lower-ranked sites are relatively
less secure, since security practices may act as ranking sig-
nals
15
and previous study [16] demonstrated that low-ranked
websites react more slowly than high-ranked sites to security
events like Heartbleed. Therefore, attackers are more likely to
compromise these less secure sites, acquire certicate private
keys, and launch attacks on high-prole sites.15
https://developers.google.com/search/blog/2014/08/https-as-ranking-
signal
12
--- page 17 ---
Out1M: 39436 (85.9%)
Out1M: 43594 (94.9%)
--- page 18 ---
Fig. 5: Rank distribution of the domains that share certicates
with Top 1K websites. Most of the domains sharing certicates
with Top 1K are ranked out of 1M.
C. Case Study: Attack Microsoft's Domain
To demonstrate the threat of our attacks in the real world,
we analyzed a domain from Microsoft. We found that the
CNAME
record of 14.au.www.download.windowsupdate.com
(a Microsoft's domain) was pointing to an unregistered
domain au.download.windowsupdate.qtlcdnect.com. So, we
bought qtlcdnect.com from Godaddy. Due to ethical con-
siderations, we did not continue the exploitation. If we
were malicious attackers, we could set the
A
record
of au.download.windowsupdate.qtlcdnect.com to our server.
Then, we can issue a certicate of our own domain shared with
14.au.www.download.windowsupdate.com in HTTP mode.
With the shared certicate, we can successfully launch Cross-
PUSH attacks on this domain. As this domain serves as a
download site for Windows update les, we can initialize the
malicious le download attack to let the victims who request
the malicious website (disguised as a Windows update portal)
receive a malicious le. Because the browser shows that the
le is indeed downloaded from Microsoft's ofcial website,
victims are then likely to execute it, thus being implanted with
a trojan horse or virus. Remarkably, compared to traditional
domain takeover, our attacks will remain effective for a
maximum duration of 796 days, even after Microsoft removes
the dangling DNS records.
VII. D
ISCUSSION
A. Mitigation
The root cause of our attacks stems from two primary
factors: (1) Relaxation of the browser's origin in HTTP/2 and
HTTP/3. As server push and SXG can indicate the origin of
the resources to all domains in the SAN list of multi-domain
shared certicates, the boundary of the browser's origin is
changed on these two features. This relaxation can com-
promise the SOP protection, which leads to various attacks.
(2) Misalignment between certicate and domain ownership.
Currently, there are no robust mechanisms to ensure that
certicate ownership aligns with domain ownership. In our
analysis, there are various ways in which an attacker can get
a certicate for a domain that does not belong to him, making
the attack condition of shared certicates can be fullled in
practice. As server push and SXG overly trust the certicate
owner's control of the domains in the certicates, the attacks
can seriously threaten the real world. To defend against our
attacks, we propose the following mitigations:
Enforcing consistent authority in browsers to mitigate
CrossPUSH.
To mitigate CrossPUSH attacks, browsers should
detect and reject server pushes where the authority of the push
stream does not match the origin of the current connection.
This includes preventing such pushes from being cached. Ad-
ditionally, we suggest browsers adhere to the recommendations
of RFC 9110 [2], i.e., perform a secondary DNS query to
check that the origin's host contains the same server IP address
as the established connection in HTTP/2 and HTTP/3.
Enforcing single-domain certicates to mitigate
CrossSXG.
In terms of CrossSXG, we suggest maintaining
its cross-origin delivering ability but deleting its cross-origin
signing ability, i.e., an SXG provider should not use a
multiple-domain shared certicate to generate SXG. Since
verifying the attribution of SXG resources only needs single-
domain certicates, browsers can just reject to decode the
SXG that is signed by a shared certicate.
Inspecting certicate status in domain registration.
To
mitigate the risk of attackers reselling domains while retain-
ing the certicates, domain buyers should inspect Certicate
Transparency (CT) logs when registering new domains. Reg-
istrars should also assist by reminding customers to check the
certicate status of domains being transferred, and potentially
even notifying CAs directly.
Revoking shared certicate upon requests from domain
owners in SAN lists.
Given the potential for attackers to use
tactics (as described in Section IV-D) that prevent certicate
revocation, CAs should facilitate the removal of domains from
shared certicates at the request of domain owners listed in
the SAN. Since revoking the shared certicate may affect the
current use, CAs should also notify the current holders of
the shared certicate and assist them in renewing a certicate
without the removed domains.
B. Ethical Consideration and Responsible Disclosure
Ethical consideration.
We take the utmost care of potential
ethical issues. (1)
In terms of client data collection
, our
client-side measurement is conducted in collaboration with
an industrial partner who deployed a measurement script on
their website. First, the script is designed solely to determine
whether a client supports server push and SXG, and to
log the browser's
User-Agent
string. No other potentially
sensitive client attributes are sent or stored. Second, the partner
company includes a consent mechanism at login and sign-
up, informing users that their browser features would be
measured. Third, the measurement script and collected data
are also rigorously reviewed by our partner company to ensure
it cannot track users or affect user privacy or experience.
Notably, all websites and servers are under the company's
control, and we do not participate in deployments. At last,
the client-side attack tests are locally conducted on our own
websites with web application rewalls to strictly block other
13
--- page 19 ---
trafc, which does not affect any other users or websites; (2)
In terms of server-side measurement
, we limit the scanning
rate to minimize the impact on real-world servers; (3)
In
terms of disclosure process
, companies encourage security
tests through bug bounty programs, and we carefully follow
disclosure guidelines when disclosing issues to vendors and
domain owners.
Responsible disclosure.
We have reported these security
problems to vulnerable browsers and affected websites. Up to
now:
Huawei
acknowledged our report and has conrmed the
vulnerability. They have updated their browser applica-
tion to 14.0.4 to prevent our attacks.
Baidu
was interested in the attacks and had an in-
depth discussion with us about the specics. They have
conrmed the vulnerability and will arrange a x for this
problem in the next version.
Xunlei browser
acknowledged our report and has con-
rmed the vulnerability and will x this problem in the
next version.
Google
acknowledged the vulnerability and is still in
discussion on how to address this issue.
Microsoft
acknowledged our report and replied that they
would remove the dangling DNS record of the affected
websites later.
C. CDN Analysis
Content Delivery Networks (CDNs) might exacerbate our
CrossPUSH attack, making it more severe and widespread.
With CDNs, end-to-end HTTPS connection is split into two
parts: the front-end communication between client and CDN
surrogate servers, and the back-end communication between
CDN surrogate servers and original web servers. Since surro-
gate servers always use shared certicate [35] and some CDNs
even merge different entities' domains into one certicate
[36], these naturally shared certicates of CDNs in front-end
connections potentially create an avenue for attackers to launch
CrossPUSH attacks.
For instance, Fastly CDN uses a wildcard shared certi-
cate to support HTTP/2 in TLS services
16
, as long as the
surrogate servers support forwarding HTTP/2 push stream to
clients, an attacker can push scripts to any other websites
in
*.freetls.fastly.net
as all of them are authorized by the
shared certicate on Fastly surrogate server. To evaluate this
phenomenon, we investigate nine prominent CDNs and nd
that HTTP/2 features have not been well-supported. Speci-
cally, certain CDNs, such as Amazon Cloudfront and CDN77,
do not provide HTTP/2-to-origin support. Some CDNs, like
Cloudare and Fastly, do not forward HTTP/2 push streams.
Moreover, CDNs like Tata Communications have not imple-
mented any form of HTTP/2 support at all. However, there is a
growing trend within the community towards embracing those16
https://docs.fastly.com/en/guides/setting-up-tls-on-a-shared-fastly-
domain#support-for-http2-ipv6-and-tls-12
advanced features, e.g., Go programming language has intro-
duced client-side support for HTTP/2 server push recently
17
.
This trend reveals that the landscape of CDN support for these
features may evolve, possibly inuenced by vulnerabilities
highlighted in our study. Furthermore, we suggest CDN ven-
dors consider our attacks when integrating HTTP/2 features in
the future, and the recommendations outlined in Section VII-A
can assist CDNs in preventing these potential issues.
VIII. R
ELATED
W
ORK
A. CrossPUSH and CrossSXG
Jake's blog [37] rst described that HTTP/2 server push can
push cross-origin resources to other domains in a certicate.
However, he only briey mentioned it and did not conduct an
in-depth study. The real-world security threats, exploitation
methods, and attack conditions of this feature are not yet
known. As for SXG, Andrew Ayer [38] exposed in the draft
discussion stage that this feature will enable the attacker who
has obtained the certicate to launch an off-path attack on
the victim's website and gave several examples of obtaining
illegitimate certicates in the past few years. Nonetheless, this
security issue has not been systematically studied. In contrast,
we provide a detailed and in-depth study of CrossPUSH
and CrossSXG attacks, along with their exploitations and
practicality evaluations in the real world.
B. Other Attacks of Shared Certicate
Previous study shows that it is common for administrators
to deploy shared certicates across multiple services, possibly
without consideration to cross-protocol attacks [39]. For
instance, a MitM attacker can redirect the HTTP request to
an FTP server that has a certicate compatible with the web
server. If the FTP server is error-tolerant, it will reect the em-
bedded JavaScript to the client in an error message, making the
exploitable browser execute it within the context of the original
request. Another work leverages the shared TLS certicate to
launch the HTTPS context confusion attack [32]. By rerouting
encrypted trafc to another awed server that shares the TLS
certicate, attackers can bypass the security practices, hijack
the ongoing HTTPS connections, and subsequently launch
other attacks, including phishing and payment hijacking. We
should note that all of these attacks need the adversary to
have the privilege of rerouting the trafc, and the exploitable
server should have some aws, e.g., not adhering to the best
HTTPS practices. On the contrary, the attacker with a shared
certicate can launch off-path web attacks on other people's
websites via CrossPUSH and CrossSXG, even when the target
website does not have any vulnerability or is not online.
IX. C
ONCLUSION
We propose how an off-path attacker only with the shared
certicate can launch cross-origin web attacks via HTTP/2
server push and SXG, along with a comprehensive study
of their practicality in the real world. On the client side,17
https://go-review.googlesource.com/c/net/+/181497
14
--- page 20 ---
11/14 top-used browsers, including Chrome and Edge, are
vulnerable to at least one of our attacks, and the threat even
spreads to numerous apps. On the server side, we show that
three exploitable cases, reselling domains, dangling domains,
and cert-sharing domains are prevalent. We consider that the
essence of this security problem is server push and SXG, two
emerging features with the ability to indicate server authority,
can leverage the inconsistencies between browser origin and
HTTP/2 authority to break the same origin protection, enabling
an adversary to attack any domains in his illegitimately
acquired shared certicate. We expect our work to raise
awareness of this problem and set off further discussion among
practitioners and researchers. We also envision our work to
motivate the browser vendors, domain registrars, and CAs to
collaborate on operational efforts to address the issues in the
current practices.
A
CKNOWLEDGMENT
We thank the anonymous reviewers and our shepherd for
their insightful feedback that helped improve the quality of
the paper. We thank Hui Jiang, Biao Tian, Hao Mo, and
Hua Lin from Baidu for their constructive comments and
generous provision of high-trafc websites for conducting our
measurement. We also thank our industry partners from Qi-
ANXIN for their support of passive DNS data. We are grateful
to Yihang Wang, Keran Mu, and Run Guo for their peer review
and helpful suggestions. This work was in part supported by
the NSFC #62272265. Any opinions, ndings, and conclusions
or recommendations expressed in this material are those of
the authors and do not necessarily reect the views of their
employers or the funding agencies.
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A
PPENDIX
A. Attack Requirements
TABLE V: Attack requirements for acquiring shared certi-
cates Attack Methods Exploited Resources Attack RequirementsDomain reselling Resold domains No requirementsInactive cloud services
CNAME
/
NS
records of victim.com
pointed to discontinued cloud servicesDomain takeover De-provisioned VPS IPs
A
records of victim.com pointed to
de-provisioned VPS IPsExpired domain names
CNAME
records of victim.com
pointed to unregistered domain names 16