---
type: Article
title: Breaking and Fixing Origin-Based Access Control in Hybrid Web/Mobile Application Frameworks
description: Hybrid app frameworks such as PhoneGap hand web code JavaScript bridges to device resources but never apply the same origin policy to them, so foreign-origin content inside the app, typically ads in iframes, can call those bridges and reach contacts, files and the camera. The paper names these fracking attacks, surveys PhoneGap Android apps, and proposes the NoFrak defence.
resource: "https://www.ndss-symposium.org/ndss2014/ndss-2014-programme/breaking-and-fixing-origin-based-access-control-hybrid-webmobile-application-frameworks/"
tags: [article, webseclist-reference, sop-bypass, same-origin-policy, privilege-escalation, iframe, android, ios, javascript, large-scale-scan, defence, owasp-a01-2021]
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resource: "https://www.ndss-symposium.org/ndss2014/ndss-2014-programme/breaking-and-fixing-origin-based-access-control-hybrid-webmobile-application-frameworks/"
title: Breaking and Fixing Origin-Based Access Control in Hybrid Web/Mobile Application Frameworks
author: Martin Georgiev, Suman Jana, Vitaly Shmatikov
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authors:
- Martin Georgiev
- Suman Jana
- Vitaly Shmatikov
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original_url: "https://www.ndss-symposium.org/ndss2014/ndss-2014-programme/breaking-and-fixing-origin-based-access-control-hybrid-webmobile-application-frameworks/"
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# Breaking and Fixing Origin-Based Access Control in Hybrid Web/Mobile Application Frameworks
**Breaking and Fixing Origin-Based Access Control in Hybrid Web/Mobile Application Frameworks** - Martin Georgiev, Suman Jana, Vitaly Shmatikov, Publisher not stated.
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## Content
> UNTRUSTED SOURCE TEXT. Everything below this line is third-party material
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Breaking and Fixing Origin-Based Access Control
in Hybrid Web/Mobile Application Frameworks
Martin Georgiev Suman Jana Vitaly Shmatikov
The University of Texas at Austin The University of Texas at Austin The University of Texas at Austin
mgeorgiev@utexas.edu suman@cs.utexas.edu
Abstract—Hybrid mobile applications (apps) combine the I. I NTRODUCTION
features of Web applications and “native” mobile apps. Like
Web applications, they are implemented in portable, platform- Web apps are becoming more like native mobile apps, and
independent languages such as HTML and JavaScript. Like vice versa. When running on mobile devices, modern Web apps
native apps, they have direct access to local device resources—file often need to break out of the browser sandbox and directly
system, location, camera, contacts, etc. access local resources such as the camera, geolocation, file
system, etc. At the same time, many developers of mobile apps
Hybrid apps are typically developed using hybrid application prefer to write them in platform-independent, implement-once-
frameworks such as PhoneGap. The purpose of the framework run-everywhere Web languages like JavaScript. The resulting
is twofold. First, it provides an embedded Web browser (for “hybrid” apps thus exhibit features of both Web and native
example, WebView on Android) that executes the app’s Web code.
Second, it supplies “bridges” that allow Web code to escape the
apps.
browser and access local resources on the device. Web browsers are beginning to add mechanisms that ex-
pose local resources to Web code, but currently most hybrid
We analyze the software stack created by hybrid frameworks
apps are developed using hybrid application frameworks such
and demonstrate that it does not properly compose the access-
control policies governing Web code and local code, respectively. as PhoneGap. The primary purpose of these frameworks is
Web code is governed by the same origin policy, whereas local to supply bridges that provide Web code with direct access to
code is governed by the access-control policy of the operating local resources on the machine. These frameworks thus support
system (for example, user-granted permissions in Android). The the development of portable mobile apps and the conversion
bridges added by the framework to the browser have the of existing Web apps into mobile apps. Their target platforms
same local access rights as the entire application, but are not include mobile-phone operating systems (OS) such as Android,
correctly protected by the same origin policy. This opens the iOS, Windows Phone, and BlackBerry, as well as desktop OSes
door to fracking attacks, which allow foreign-origin Web content such as MacOS.
included into a hybrid app (e.g., ads confined in iframes) to drill
malicious
through the layers and directly access device resources. Fracking hybrid applica6on
adver6ser
(web code) iframe
vulnerabilities are generic: they affect all hybrid frameworks,
all embedded Web browsers, all bridge mechanisms, and all adver6sing
platforms on which these frameworks are deployed. broker
We study the prevalence of fracking vulnerabilities in free
embedded
Android apps based on the PhoneGap framework. Each vul- web browser
nerability exposes sensitive local resources—the ability to read
and write contacts list, local files, etc.—to dozens of potentially b b
r hybrid r
malicious Web domains. We also analyze the defenses deployed by i framework i
hybrid frameworks to prevent resource access by foreign-origin d d
Web content and explain why they are ineffectual. g
e
g
e
We then present N O F RAK, a capability-based defense against …
fracking attacks. N O F RAK is platform-independent, compatible
device resources
with any framework and embedded browser, requires no changes
to the code of the existing hybrid apps, and does not break their
advertising-supported business model. Fig. 1: Hybrid software stack
The software stack created by hybrid frameworks is
Permission to freely reproduce all or part of this paper for noncommercial
purposes is granted provided that copies bear this notice and the full citation
sketched in Fig. 1. A hybrid framework comprises two halves.
on the first page. Reproduction for commercial purposes is strictly prohibited The local half is implemented in a platform-specific language
without the prior written consent of the Internet Society, the first-named author like Java, C#, or Objective-C. It runs as a local process in the
(for reproduction of an entire paper only), and the author’s employer if the host machine’s OS and performs actual device access: reading
paper was prepared within the scope of employment. and writing the file system, working with the camera, etc. To
NDSS ’14, 23-26 February 2014, San Diego, CA, USA
Copyright 2014 Internet Society, ISBN 1-891562-35-5 execute the app, the local half creates an instance of a platform-
http://dx.doi.org/10.14722/ndss.2014.23323 specific embedded Web browser—for example, WebView in
Android, UIWebView in iOS, or WebBrowser in Windows ads or, in general, any third-party content. Furthermore, even
Phone—and runs the app’s code within this browser. The Web this property is enforced incorrectly.
half of the framework is a JavaScript library. The app’s code
includes this JavaScript library and uses its API to access local Our third contribution is a large-scale survey of fracking
resources on the device. vulnerabilities, focusing on free, PhoneGap-based Android
apps. We chose PhoneGap for our survey because it is currently
The access-control model of the software stack shown in the most popular hybrid framework and Android because its
Fig. 1 is quite complex. Web content is governed by the same current market share is over 80%.1
origin policy, while local resource accesses are governed by Our fourth contribution is a simple capability-based de-
the OS’s discretionary access control policy. In contrast to fense called N O F RAK. We implemented a prototype of
advertising-supported native apps, untrusted content in hybrid N O F RAK as a patch to PhoneGap on Android, but conceptu-
apps is not included via a separate local library. Instead, it ally N O F RAK is compatible with any hybrid framework, any
is composed with the app’s own content like a Web mashup, embedded browser, and any platform. It enforces the exact
with different origins isolated in separate iframes, all executing security property that hybrid frameworks promise and app
together in the same Web browser. Since the hybrid frame- developers already rely on. N O F RAK requires no changes to
work provides Web content with the ability to access local the code of the existing hybrid apps and can be deployed
resources, it must correctly propagate origin information and transparently, by modifying the framework but keeping the
apply origin-based access control to resources outside the Web same JavaScript API. Unlike prior defenses, N O F RAK is fine-
browser. In particular, it must ensure that untrusted, foreign- grained and compatible with advertising-supported apps: an
origin Web content included into the hybrid app (for example, app can load third-party content, but this content is blocked
ads confined in iframes) cannot access local resources that are from accessing local resources on the device. If necessary,
available to the app itself. access to specific resources can be granted on a domain-by-
domain basis.
Our contributions. Our first contribution is to demonstrate that
hybrid application frameworks do not correctly compose the II. H YBRID APPS AND FRAMEWORKS
same origin policy and the local access-control policy. We will
use the term fracking for the generic class of vulnerabilities Hybrid applications (apps) are a new type of software that
that allow untrusted Web content to drill through the layers is implemented in conventional Web code but also includes a
of the stack shown in Fig. 1 and reach local resources on local component intended to execute outside the Web browser
the device, thus gaining the ability to steal the user’s contacts, on a mobile device or desktop computer (we will refer to the
write into the device’s external storage, manipulate the camera, local OS as the platform).
etc. The technical details differ from framework to framework Hybrid apps are usually based on one of the hybrid
and from bridge to bridge, but fracking vulnerabilities affect frameworks listed in Section II-A. The main reason for the
all hybrid frameworks on all mobile and desktop platforms, popularity of hybrid frameworks is the support they provide for
and consequently all hybrid apps based on any of these cross-platform, implement-once-deploy-on-any-OS app devel-
frameworks. opment. Hybrid frameworks enable developers to write their
apps in portable Web languages such as HTML, JavaScript,
Our second contribution is a critical analysis of the de-
and CSS. The framework supplies the app’s local component
fenses that hybrid frameworks deploy against fracking attacks.
as well as the bridges (see Section IV) that allow the app’s Web
We focus in particular on PhoneGap’s origin checks, which
code to escape the browser sandbox and access local resources
are intended to ensure that Web content from untrusted origins
on the machine, such as the geolocation, camera, contacts,
cannot invoke certain bridges to local resources. We demon-
etc., thus relieving developers of the need to write their own
strate that these defenses are ineffectual because of conceptual
device-based code in platform-specific languages such as Java
mistakes, implementation bugs, and architectural issues in
or Objective-C.
embedded browsers that limit the visibility of the framework’s
local half into Web code inside the browser. Furthermore, The hybrid software stack is opaque and poorly understood
all hybrid frameworks are vulnerable to the chosen-bridge by many developers. Hybrid apps delegate security enforce-
attack. Even if the origin check for a particular kind of bridge ment—in particular, ensuring that foreign-origin Web content
were implemented correctly, malicious Web content can simply included into the app cannot gain access to local resources—to
choose another, unprotected bridge and bypass the defense. the underlying hybrid framework that creates and manages this
stack. Therefore, hybrid apps inherit all of the frameworks’
We also show that the security properties that hybrid security flaws and vulnerabilities.
frameworks aim (but fail) to enforce are inconsistent. We call
the correct property NoBridge: Web content from untrusted
A. Hybrid application frameworks
origins cannot access the bridge. This is the property that
hybrid frameworks promise and that the developers of hybrid PhoneGap. PhoneGap is a free, open-source framework
apps expect and rely upon. Unfortunately, while promising currently used by over 400,000 developers worldwide. It is
NoBridge to the developers, some frameworks instead enforce the most popular hybrid framework at the time of this writing.
a different property we call NoLoad: the hybrid app cannot PhoneGap supports app development on nine mobile and
load any Web content from untrusted origins. NoLoad is much
cruder than NoBridge and breaks the business model of many 1 http://techcrunch.com/2013/11/12/windows-phone-android-gain-market-
free apps because it prevents them from displaying third-party share-while-apple-slips-despite-growth-in-iphone-shipments/
2
desktop platforms, including Android, iOS, Windows Phone, WebWorks allows hybrid apps to access local resources
Windows 8, MacOS, and Blackberry. PhoneGap development such as the camera, microphone, accelerometer, file system,
tools were acquired by Adobe in 2011. Adobe’s cloud-based etc. Third-party extensions can expose other local functionali-
PhoneGap Build enables “develop-once-deploy-everywhere,” ties.3 Uniquely among hybrid frameworks, WebWorks supports
but if an app is maintained by the developer locally, a separate fine-grained, domain-specific access control for local resources
project is required for each target platform. (see Section VII-E).
By default, PhoneGap-based Android apps request from the Other hybrid development frameworks include RhoMobile,
user and expose to the app’s Web code 16 resources, including AppCelerator Titanium, Appspresso, and CocoonJS.
camera, fine location, audio and video recording, reading and
writing contacts, writing external storage, etc. Individual apps
may choose not to request permissions to some or all of these
resources. Additional resources can be exposed by third-party embedded web browser
PhoneGap plugins.2
hybrid applica5on JS code
The latest versions of PhoneGap allow the developer JS API calls
to whitelist authorized Web domains. PhoneGap then tries hybrid framework JS library
to ensure that Web content from non-whitelisted domains b b
cannot access the local resources available to the app. In r r
i hybrid i
Section VII-B, we explain why this defense is ineffectual. d framework d
g g
e e
MoSync. MoSync is a free, open-source framework that pro-
vides its own implementation of PhoneGap’s JavaScript API. device resources
Therefore, hybrid apps designed to work with PhoneGap’s
API will work with MoSync. MoSync supports “develop-once-
deploy-everywhere” without requiring that the app be built in
the cloud. Fig. 2: Components of hybrid frameworks
By default, MoSync exposes access to Internet and file
storage, but apps can request more permissions if needed. B. Hybrid software stack
Unlike PhoneGap, MoSync does not appear to provide any The software stack created by hybrid application frame-
way for the app’s developer to specify that only certain Web works is shown in Figs. 1 and 2. This hybrid software stack is
domains may access local resources. generic and essentially platform-independent, except for minor
details (e.g., which embedded Web browser is used on a given
Web Marmalade. Web Marmalade is a proprietary framework OS). The security flaws of this stack, which are analyzed in the
currently used by over 50,000 developers on Android and iOS. rest of this paper, are not specific to a particular browser (e.g.,
It provides its own implementation of PhoneGap’s JavaScript WebView) or a particular way of constructing a bridge. These
API and supports “develop-once-deploy-everywhere” via a flaws are generic, too, and affect all platforms, browsers, and
local IDE, as opposed to cloud build. It does not appear to frameworks.
provide any way for the app’s developer to specify that only
certain Web domains may access local resources. The hybrid software stack is a recent entry into the Web
and mobile software landscape, and many developers may not
appMobi. appMobi is a free framework. In February 2013, fully understand the security implications of combining mul-
appMobi development tools were acquired by Intel. Several tiple layers with very different security models (explained in
parts of the software stack were available as open source in Section III). From the security perspective, the key components
2011, but as of April 2013, appMobi’s git repository is empty. of any hybrid framework are the bridges between its Web-
appMobi supports “develop-once-deploy-everywhere” on eight facing JavaScript half and its device-based local half, as shown
platforms and allows local and cloud build via Intel’s XDK. in Fig. 2.
appMobi provides its own implementation of PhoneGap’s At the bottom of the stack is the OS (e.g., Android, iOS,
JavaScript API and uses a fork of PhoneGap on the local side. MacOS, etc.) that manages local device resources such as the
An app can specify a domain whitelist via a JavaScript call. camera, file system, location, etc. The local half of the hybrid
framework is implemented in Java (Android and BlackBerry),
BlackBerry WebWorks. WebWorks is an open-source hybrid Objective-C (iOS), or C# (Windows Phone). It is incorporated
app development framework [4] for BlackBerry platforms, into the hybrid app and runs as a local application from
including BlackBerry 5.0, BlackBerry PlayBook, and Black- the viewpoint of the OS. When installed on the machine, it
Berry 10. Unlike other frameworks, WebWorks was developed obtains access to device resources using one of the mechanisms
by a platform provider and integrated with a custom embedded described in Section III-B.
Web browser. In contrast, other frameworks employ the plat- The framework’s local half includes an embedded Web
form’s default embedded browser, such as WebView on An- browser. The browser is platform-specific, e.g., WebView in
droid, WebBrowser on Windows Phone, etc. (see Section II-B). Android, UIWebView in iOS, WebBrowser in Windows Phone,
2 https://github.com/phonegap/phonegap-plugins/tree/master/Android 3 https://github.com/blackberry/WebWorks-Community-APIs
3
or WebWorks in BlackBerry. The framework executes the indistinguishable from the app’s own code as far as the SOP
app’s own code within this browser, yet enables this code to is concerned.
access resources as if it were running as a local process on
the device. To this end, the framework supplies one or more
bridges that bypass the browser sandbox. All bridges include B. Local security
(1) a local component with access to device resources, and The local half of the hybrid app is supplied by the hybrid
(2) a mechanism for the JavaScript code in the browser to framework. Its security model is determined by the OS, which
invoke the bridge and pass calls and their arguments to the mediates access to device resources, and is very different from
local component. The technical implementation details vary the Web security model described in Section III-A.
from bridge to bridge (see Section IV).
Android uses static permissions (130 as of this writing).4
The hybrid app itself is implemented as Web content in An app requests them at the time of installation and the user
HTML and JavaScript. It executes in the embedded browser can either grant all of them, or decline to install the app.
but, unlike conventional Web code, can access local resources Windows Phone 7.1 has 28 static install-time permissions [30].
via bridges added by the framework. Our survey in Section VI BlackBerry OS 5 and 6 have 20 permissions in three cate-
shows that hybrid apps often include foreign-origin content, gories: connections (6), interactions (10), and user data (4) [3].
such as ads fetched over HTTP or, rarely, HTTPS. Unlike other mobile OSes, BlackBerry OS lets the user grant
only a subset of permissions requested by an app.
III. S ECURITY MODELS
Apple’s iOS uses dynamic run-time access control. When
The primary focus of this paper is on Web attackers. A an app accesses a resource for the first time, iOS prompts
Web attacker controls one or more domains and may host the user to grant the permission. Unlike Android, iOS 6 has
malicious JavaScript there. He does not, however, control the only a few permissions. They control access to location, social
network and cannot tamper with or observe the communica- networks, address book, calendar, Bluetooth, camera, etc.
tions between users and other domains. We assume that the
attacker cannot execute his native code on the user’s device. Fracking vulnerabilities affect all platforms regardless of
Consequently, mobile malware is outside our scope. their access-control model, including desktop platforms like
MacOS that do not rely on user-granted permissions. Frack-
Web content served by a hybrid app may include iframes ing is more dangerous when permissions are coarse-grained
with ads or other content from third parties. These ads are because a single vulnerability exposes more native resources.
supplied by an ad broker and the app’s owner often has no For example, Windows Phone 7.1 has a single permission for
control over their source or content. Therefore, a Web attacker all sensors (ID CAP SENSORS). If a hybrid app requests this
may trick a legitimate app into including malicious content via permission and exposes it to untrusted Web content, the latter
syndicated advertising. In general, this is not a security hole by will be able to access any sensor on the device.
itself since foreign-origin content is confined by the browser’s
same origin policy (see Section III-A).
C. Hybrid security
A stronger threat model is the network attacker who can
act as a man-in-the-middle on the network between the device Security of hybrid apps depends on very subtle trust
and the origins of its Web content. Any content fetched over relationships. For example, on a permission-based platform
HTTP is insecure against the network attacker because it can like Android, the app requests access to device resources from
inject arbitrary malicious scripts into any HTTP origin. the user. The user grants these permissions to the app—but
obviously not to the foreign-origin content (such as ads)
included in the app. The app owner trusts the ad broker and
A. Web security
includes the broker’s scripts into its own origin. These scripts
The Web content of a hybrid app is governed by the same create iframes and display ads in them. Nobody—neither the
origin policy (SOP). The origin of Web content is defined by user, nor the app, nor the ad broker—trusts the advertisers and
its protocol (HTTP or HTTPS), domain, and port number [2]. their content, which is why ads, as well as other foreign-origin
SOP is enforced by all embedded Web browsers used in hybrid content, are isolated in iframes, so that the SOP can block them
frameworks (see Section II-B). For example, if an app’s Web from accessing other parties’ resources.
content includes an iframe with an ad, SOP prevents scripts
in this ad from reading or modifying any non-trivial DOM Hybrid frameworks must guarantee the following security
attributes of the app’s own content. property: Web content from untrusted domains included
into a hybrid app should not be able to access device
Scripts from ad brokers, analytics services, social networks, resources available to the app. To achieve this, hybrid
etc. are usually included directly into the app’s Web content frameworks must carefully “glue together” two very different
and thus run in the app’s origin. For example, an app may security policies: the same origin policy governing the app’s
include a script from an ad syndicator, which creates an Web content, and the OS access-control policy governing the
iframe and fetches an ad to display within this iframe. The framework’s local half. In particular, hybrid frameworks must
syndicator’s script runs in the app’s origin, but the actual ad correctly propagate the same origin policy to local objects
runs in its own origin, isolated from the rest of the app’s outside the Web browser when these objects are accessible
content by the SOP. Therefore, in our threat model we consider from inside the browser via the framework’s bridges.
malicious advertisers, but not malicious ad brokers. The latter
are trusted by app developers and, critically, their Web code is 4 http://developer.android.com/reference/android/Manifest.permission.html
4
In practice, hybrid frameworks attempt to enforce one of ‘prompt’, ’alert’, and ’confirm’ events by overriding the
the following two properties: Web content from unauthorized ‘onJsPrompt’, ‘onJsAlert’, and ‘onJsConfirm’ functions, re-
domains cannot access the bridges created by the framework spectively. This bridge mechanism is used by PhoneGap on
(we call this property NoBridge), or the app cannot load Android 2.3 because of a bug5 that precludes the use of an
any content from untrusted domains (we call this property interface-based bridge described above. On the local side,
NoLoad). Both properties prevent undesired access to re- PhoneGap implements a custom ‘onJsPrompt’ handler. On the
sources, but NoLoad breaks the business model of many free Web side, JavaScript makes a ‘prompt’ call to invoke this
apps because it prevents them from displaying foreign-origin handler; the name and the arguments of the local object to
ads. In Section VII, we show that the frameworks’ defenses be called are serialized and sent to the local side by encoding
are inconsistent and apply different policies in different parts them as prompt messages. MoSync on Android uses a similar
of the same code, often incorrectly. mechanism.
IV. B RIDGES Similarly, WebBrowser, the embedded browser class in
Windows Phone, allows local C# code to install custom
The main purpose of any hybrid framework is to supply handlers for ‘ScriptNotify’ events. JavaScript on the Web
bridges that enable the app’s Web content to access local side uses ‘window.external.Notify’ to trigger these events and
device resources. The app, however, does not invoke these invoke local functions. This technique is used by PhoneGap
bridges directly. Instead, it uses the framework’s JavaScript on Windows Phone.
library, which presents a structured, platform-independent API
for resource access. This library invokes the bridges internally, Event-based bridges can be synchronous (JavaScript pauses
thus the mechanics of different bridge architectures are trans- until the call returns) or asynchronous (the call returns a
parent to the app and the app’s code does not directly depend placeholder or null, but the actual data is returned later via
on the specific bridges used by the framework. a local-to-Web bridge).
To enable asynchronous resource access, frameworks often
provide separate Web-to-local bridges (for invoking a local URL loading interposition. Embedded browsers typically al-
function) and local-to-Web bridges (for receiving the result). low local code to interpose on and handle URL loading within
With this separation, JavaScript executing in the embedded the browser. This feature is intended to support implementation
browser does not block while the device is accessing the of custom protocols, but hybrid frameworks overload it so that
resource. the Web half of the framework can pass arbitrary messages
to the local half by encoding them in the URL, thereby
constructing a Web-to-local bridge.
A. Web-to-local bridges
URLs intended for interposition cannot be loaded in the
Interfaces. Several embedded browsers allow local code to
main browser frame lest the entire app blocks. Instead, hybrid
expose arbitrary functions to JavaScript running within the
frameworks use one of the following two methods.
browser. For example, ‘addJavascriptInterface’ in Android’s
WebView makes local Java objects visible to JavaScript. JavaScript on the Web side can create an invisible iframe
Other platforms have similar functionalities, for example, ‘win- and set its source to an arbitrary URL. The loading of this
dowScriptObject’ in MacOS and ‘ScriptEngine.addExtension’ URL is intercepted by a handler supplied by the framework’s
in BlackBerry WebWorks. local half, without affecting the execution of the main frame
This technique is used, among others, by PhoneGap, Ap- on the Web side. The handler decodes the URL and passes
pCelerator Titanium, and Sencha Touch to enable direct access the message to the local half. For example, PhoneGap on iOS
to local device resources by Web content. creates its own subclass of NSURLProtocol named CDVURL-
Protocol and calls registerClass function to intercept URLs
On Android prior to API level 17, these interfaces are loaded in the UIWebView embedded browser. PhoneGap’s
generically insecure. Malicious JavaScript executing inside JavaScript library creates an iframe whose URL starts with
WebView can use the Java reflection API to invoke any method gap://, PhoneGap’s local half intercepts the loading of this
of any Java object exposed via ‘addJavascriptInterface’ and URL and calls the corresponding local function. In Web Mar-
take control over the local side of the application [1, 18, 23]. malade,6 the framework’s JavaScript library creates an iframe
Starting from Android API level 17, only the methods explic- with the src attribute s3ebridge://queued. A shared JavaScript
itly annotated with @JavascriptInterface are visible in the Java object, s3e.queue, in this iframe is used for communication
objects exposed to WebView via ‘addJavascriptInterface’. between the Web half and local half.
Events. Another common mechanism for the local half of As an alternative to iframe-based communication,
the framework to receive messages from the Web half is via JavaScript on the Web side can make an asynchronous
various JavaScript events. To use this mechanism, the local XMLHttpRequest to a URL which is intercepted and decoded
half must override the event handlers in the embedded browser. by the framework’s local handler. PhoneGap uses this
JavaScript on the Web side triggers events and encodes mes- technique on iOS in addition to the iframe URL interception.
sages in arbitrary strings, the handlers intercept these events
and decode the messages.
5 http://code.google.com/p/android/issues/detail?id=12987
For example, WebView, Android’s embedded browser 6 https://github.com/marmalade/Web-Marmalade/blob/master/wmClipboard/
class, allows local Java code to customize the handling of data/webassets/wm.js
5
Cookies (obsolete). The framework’s Web half can set cookies to the iframe that needs it.8 9 10 These custom, hand-coded
that are readable by the local half, and vice versa. PhoneGap hacks open holes in the same origin policy and can introduce
used this technique on older platforms like BlackBerry 4.x, serious security vulnerabilities, including cross-site scripting
(e.g., see [25]). While interesting, this class of vulnerabilities
is not specific to hybrid apps and we leave its detailed analysis
B. Local-to-Web bridges to future work.
Local-to-Web bridges are used by the framework’s local
V. F RACKING
half to return data to the framework’s JavaScript library on
the Web side. Some data such as device ID can be returned As explained in Section III-C, the hybrid framework must
synchronously via the function’s return value, but usually the guarantee that untrusted foreign-origin content included into
results of device access (e.g., contact list, local files, etc.) are the app—for example, ads—not be able to access local device
returned asynchronously, to avoid blocking Web code while resources such as the file system, contacts, SMS, etc. via
access is performed. the bridges provided by the framework. Unauthorized access
can be exploited for privacy violations (e.g., steal the user’s
Multiplexing a Web-to-local bridge. Messages from the contacts list), security breaches (e.g., use personal data for
local half to the Web half can be sent over the same bridge targeted phishing), monetary gains (e.g., send SMS to premium
as the Web-to-local messages. There are two asynchronous numbers), or denial of service (e.g., cause the phone to vibrate
mechanisms for doing this. or beep incessantly).
Local code can trigger events to notify the JavaScript To prevent unauthorized resource access by foreign-origin
library that there is data waiting for it. JavaScript then reads the Web content, hybrid frameworks rely on several layers of
data via one of the Web-to-local bridges from Section IV-A. defense. The first layer is the same origin policy (SOP) within
This mechanism, using online/offline events,7 is employed by the embedded browser (see Section III-A): for example, ads
PhoneGap on Android. are confined within iframes and thus prevented by the SOP
from accessing bridges in other frames. The second layer is
On many platforms, event-based synchronization is not the bridge mechanism itself which may be difficult to invoke
available if the framework’s JavaScript is running inside an from inside an iframe. The third layer is the origin checks
iframe. Many embedded browsers—including WebView on used by the framework’s local half to block bridge access
Android, UIWebView on iOS, and WebBrowser on Windows initiated by unauthorized Web content (see Section VII-B).
Phone—incorrectly deliver some events to the main frame even The fourth layer is access control within the operating system
if the handler is registered inside an iframe. This issue is (see Section III-B).
sometimes referred to as “frame confusion” [17].
A. Conceptual overview
The alternative is to have JavaScript poll the Web-to-local
bridge at periodic intervals to check whether data is ready. We use the term fracking for any attack that allows
PhoneGap on Android supports this technique, too. malicious foreign-origin JavaScript to “drill” through the de-
fense layers and gain unauthorized access to device resources.
Interfaces. Many embedded browsers have helper func- Several features distinguish fracking vulnerabilities from other
tions that let local code execute arbitrary JavaScript in- security holes in hybrid and native mobile apps.
side the browser. For example, Android’s WebView has a
First, fracking attacks exploit the mismatches and semantic
private reflection API, as well as the ‘loadUrl’ function
gaps between the SOP-based security policy governing the
that allows Java code to load a javascript: URL inside the
Web half of hybrid apps and the OS access-control policy gov-
browser. PhoneGap on Android uses both. PhoneGap on
erning their local half—in particular, the hybrid frameworks’
iOS, BlackBerry, and Windows Phone uses ‘stringByEvalu-
failure to correctly apply the SOP to bridges. Consequently,
atingJavaScriptFromString’, ‘ScriptEngine.executeScript’, and
fracking vulnerabilities are generic and affect all bridge mech-
‘WebBrowser.InvokeScript’ functions, respectively, to execute
anisms in all hybrid frameworks, all embedded browsers, and
JavaScript inside the corresponding embedded browsers.
all mobile and desktop platforms.
Second, fracking vulnerabilities are not caused by the well-
C. Custom cross-origin communication known weaknesses of access control based on user-granted
permissions, such as the tendency of apps to request too
As mentioned above, many local-to-Web bridges often do many permissions or the failure of users to understand the
not work if the framework’s JavaScript library is running inside consequences of indiscriminately granting permission requests.
an iframe. For example, on iOS and Windows Phone 7, bugs in Fracking affects hybrid frameworks even on desktop platforms
embedded browsers cause the data to be returned to the main such as MacOS where access control is not based on user-
frame rather than the iframe that invoked the bridge. granted permissions. Even on Android, the problem is not
This forces app developers to implement “home-brewed” simply that the app requests too many permissions (in fact,
cross-origin mechanisms to deliver the data from the device 8 http://comments.gmane.org/gmane.comp.handhelds.phonegap/16406
9 http://stackoverflow.com/questions/5875083/possible-to-access-phonegap-
7 These events are sent by HTML5 browsers to Web pages when the browser api-within-a-iframe
detects the Internet connection to be enabled/disabled; PhoneGap re-purposes 10 http://hackerluddite.wordpress.com/2012/04/15/getting-access-to-a-
them for local-to-Web notifications. phones-camera-from-a-web-page/
6
many legitimate hybrid apps do need these permissions in ‘WebBrowser.InvokeScript’ functions, used by the
order to function correctly), but that these permissions get framework’s local half on iOS and Windows Phone,
exposed to untrusted Web content included in the app. respectively, to inject JavaScript into browsers, execute it
in the main frame, not the iframe that invoked the bridge.
Third, fracking is not an instance of Android’s permission Therefore, malicious JavaScript inside an iframe cannot see
re-delegation problem [7, 9]. Fracking vulnerabilities occur data returned from the device, but can still cause malicious
at a different layer of the software stack, where permission- side effects through Web-to-local bridges without seeing the
based local access control meets origin-based Web access return values, e.g., create or delete contacts, send SMS to
control. Furthermore, hybrid frameworks such as PhoneGap premium numbers, etc.
do not blindly delegate their permissions to untrusted Web
content. They deploy several defenses to prevent “confused Exploiting event-based bridges. Event-based local-to-Web
deputy” [14] attacks and to ensure that local permissions bridges are difficult to use if the framework’s JavaScript library
are only used from the origin to which they were delegated. is running inside an iframe. The events that synchronize the
Unfortunately, in Section VII we show that these defenses are framework’s Web and local halves are always delivered to the
largely ineffectual. main frame, even if the handler had been registered from an
Fourth, there is an important distinction between conven- iframe, thus preventing the script in the iframe from learning
tional and embedded Web browsers that makes defending that the local half is ready with the results (see Section IV-B).
against fracking difficult. Conventional browsers interact with Furthermore, some of the utility JavaScript objects created
users. For example, Chrome’s implementation of the WebRTC by the framework are not accessible to JavaScript inside
API [29], which allows Web content to access local resources iframes. Because of this, some analyses mistakenly concluded
such as camera and microphone, pops a user dialog box every that event-based bridges cannot be exploited by malicious
time an access is attempted. This dialog shows the name of JavaScript [19].
the requesting domain and asks the user for permission. In This conclusion is false. Modified, malicious clones of the
contrast, hybrid frameworks use embedded browsers so that framework’s JavaScript library can access local resources via
hybrid apps can automatically access local device resources. event-based bridges even when confined in an iframe.
They cannot afford to ask the user for permission on every
access and must rely on programmatic defenses. First, if the malicious script inside an iframe cannot receive
synchronization events from the framework’s local half, it can
simply block for a predefined interval until the local half
B. Technical details
is ready. In event-based bridges on Android, the Java side
A fracking attack is performed by a malicious script con- transfers data to JavaScript through the return values of the
fined within an iframe in the embedded browser. SOP prevents ‘OnJsPrompt’ handler. Unlike synchronization events, these
it from accessing objects in other frames, but does not stop it values are correctly delivered by the embedded browser to
from including the hybrid framework’s JavaScript library or malicious JavaScript code inside the iframe.
even its own, arbitrarily modified clone of this library, crafted
Second, even if the framework’s utility objects are not
to maliciously access the framework’s bridges.
visible from an iframe, the main JavaScript objects implement-
Chosen-bridge attacks are devastating in this setting. ing the bridge are available, and malicious code can access
Frameworks like PhoneGap support multiple bridges to and them directly. For instance, if malicious JavaScript wants to
from local resources. Furthermore, they allow JavaScript on the access the contact list on an Android device via a PhoneGap
Web side to choose a bridge via ‘setNativeToJsBridgeMode’ bridge, it can (1) call cordova.require(’cordova/exec’) to obtain
and ‘setJsToNativeBridgeMode’ functions. These functions are a reference to the exec function that invokes the bridge, and
not intended to be called directly by hybrid apps, since apps are (2) call cordova.require(’cordova/plugin/ContactFindOptions’)
supposed to access the framework only through the public API, to obtain a reference to the contacts search filter. The rest of
but they are not protected by the SOP. Therefore, a malicious the code can be cloned from the framework’s own JavaScript
script is free to invoke them in order to select a vulnerable library and will work without modifications.
bridge. Consequently, even if some bridges are secure, a single
vulnerable bridge is sufficient to bypass all of the framework’s Exploiting URL interposition-based bridges. Both meth-
defenses. Availability of bridges varies from version to version ods for asynchronous URL loading—fetching an invisible
even within the same framework, but malicious JavaScript can iframe whose source URL encodes the message or issuing an
determine the version (e.g., via ‘device.cordova’ in PhoneGap XMLHttpRequest to an encoded URL—work from inside any
and MoSync) and choose the attack adaptively. iframe. Malicious JavaScript confined in an iframe can use
either bridge to access the framework’s local half.
Exploiting interface-based bridges. Any JavaScript object
added to the embedded browser by the framework’s local half VI. F RACKING IN THE WILD
via functions such as ‘addJavascriptInterface’ in Android’s
WebView or ‘ScriptEngine.addExtension’ in BlackBerry is To estimate the prevalence of fracking vulnerabilities in
available by default to JavaScript in any iframe, regardless of real-world hybrid applications, we analyzed 7,167 free An-
its origin. droid apps built using PhoneGap, currently the most popular
hybrid framework. These apps were identified in our dataset of
Frame confusion complicates the exploitation of 128,000 free apps collected from Google Play between January
interface-based local-to-Web bridges on some platforms. 18 and March 18, 2013, by the presence of “cordovaVersion”
The ‘stringByEvaluatingJavaScriptFromString’ and or “phonegapVersion” in the dexdump of their APK (file
7
Fig. 3: Read Contacts Fig. 4: Write Contacts Fig. 5: Write External Storage
format for Android apps) and the presence of PhoneGap into one or more foreign origins included into the app, and
plugins in “plugins.xml”, “cordova.xml”, or “phonegap.xml”. verifying that this JavaScript can successfully access local
device resources.
We implemented a tool called GapFinder to automatically
extract from each PhoneGap app its (1) local permissions, Figs. 7 and 8 estimate the extent of exposure, i.e., how
(2) a subset of the Web domains whose content is included many Web domains can access device resources by fracking
in the app, and (3) the domain whitelist, if any. To extract a hybrid app. Fig. 7 shows that more than 150 apps expose
permissions, GapFinder uses Android’s aapt (Android Asset their resources to at least 50 Web domains each. Fig. 8 shows
Packaging Tool). To find domains contributing Web content that most external domains have access to between 4 and 6
to the app, GapFinder uses apktool to reverse-engineer the resources, but some have access to all 16 resources available
APK, extracts HTML files used by the app, and automatically in default PhoneGap.
crawls each HTML file to depth 1 using Selenium with a
Google Chrome driver and the browser string overwritten to Fig. 9 demonstrates that many apps use third-party plugins.
mimic a Nexus 4 device. The resulting set of domains is a These plugins expose many more resources than default Phone-
conservative underestimate because the app may fetch content Gap, including fine-grained social-media permissions such as
from additional domains reached only by browsing to depth access to the user’s Facebook and Twitter. More than half of
2 or deeper. Also, with syndicated advertising, the actual the apps in our survey incorporate at least 10 different plugins.
domains may change from execution to execution. Examples of vulnerable PhoneGap apps include ForzeAr-
3,794 apps do not include any external content (such as mate, an app for Italian servicemen which exposes the ability
iframes, scripts, etc.) in the HTML files extracted by our tool. to write into external storage (among other permissions) to any
45 apps include only HTTPS content. The remaining 3,328 domain advertising through Google syndication; the Edinburgh
apps include content from at least one external domain via by Bus app, which exposes external storage to a large number
HTTP. Their device resources are thus potentially vulnerable of obscure Eastern European dating sites; and DrinkedIn
to both Web attackers (hosted at any of these domains) and BarFinder, which exposes fine geolocation to domains such as
network attackers. freelifetimecheating.com, www.babesroulette.com, and many
adult sites. Furthermore, content from all of these domains is
loaded over HTTP and thus vulnerable to network attackers,
who automatically gain access to the app’s local permissions.
VII. E XISTING DEFENSES
A. Relying on the browser
Several hybrid frameworks, including MoSync and Web
Marmalade, rely on the embedded browser to prevent untrusted
Web content from accessing bridges to local resources. Be-
cause the bridges are added to the browser by local code,
Fig. 6: Access Fine Location they have no Web origin as far as the browser is concerned.
Therefore, malicious Web content from any origin can directly
Figs. 3 through 6 estimate the exposure of specific sensitive invoke the bridges, as explained in Section V-B.
resources to Web attackers. For example, Fig. 3 shows that 20
PhoneGap apps expose the ability to read the contacts list on All hybrid apps based on any of these frameworks are
the device to 20 or more Web domains each. Fig. 4 shows that generically vulnerable to fracking.
19 apps expose the ability to write the contacts list to 20 or
more domains each. Fig. 5 shows that each of 81 apps allows B. Whitelisting authorized domains
at least 20 domains to write into the device’s external storage.
Some hybrid frameworks, including PhoneGap and Black-
Fig. 6 shows that 407 apps expose fine-grained location data
Berry WebWorks, implement defenses specifically to prevent
to at least 20 domains each.
foreign-origin content from accessing bridges (i.e., fracking
All vulnerabilities were empirically confirmed by randomly attacks). The app creator supplies a whitelist of authorized do-
sampling the set of vulnerable apps, injecting attack JavaScript mains, e.g., the app’s own domain. In PhoneGap, the whitelist
8
Fig. 8: Exposure of device resources to for-
Fig. 7: Foreign-origin content Fig. 9: Hybrid apps with plugins
eign origins
is specified via a platform-specific configuration file such as origin (e.g., via hiframei tags). The resulting policy is thus
‘cordova.xml’ or ‘config.xml’. stricter than the standard same origin policy! The hybrid app
is not only prevented from loading foreign-origin content, but
Correctly implementing this defense is subtle and error- it cannot even include an external image or JavaScript library,
prone. The right property is NoBridge (Section III-C): Web which is a very common practice.
content loaded by the hybrid app from a non-whitelisted origin
should not be able to access the bridge. Instead, the properties All tested versions of PhoneGap for Android, including
enforced by the actual defenses differ from framework to 2.6, incorrectly match intercepted URLs against the whitelist.
framework, platform to platform, and even from bridge to PhoneGap uses Java’s regular expression engine and anchors
bridge within the same framework. For example, some Phone- the expression for each whitelisted domain only at the begin-
Gap bridges aim to enforce NoBridge, while other parts of the ning, but not the end:
same framework aim to enforce a property we call NoLoad: the this.whiteList.add(Pattern.compile("ˆhttps?://(.*\\.)?" +
hybrid app should not be able to load any Web content from a origin));
non-whitelisted origin. Obviously, NoLoad implies NoBridge,
but it is much coarser and breaks both the user interface and For example, if foo.com is whitelisted, PhoneGap allows
the business model of many free apps. content to be loaded from foo.com.evil.com, violating the
desired property. A similar observation was made in [19].
C. Enforcing NoLoad
HTTP/HTTPS is ignored when checking URLs against the
PhoneGap on Android, iOS, and Windows Phone attempts white list. A network attacker can thus downgrade connec-
to enforce the NoLoad property: if the app fetches an HTML tions from HTTPS to HTTP and inject malicious scripts into
element (e.g., an iframe) from a non-whitelisted domain, the el- whitelisted origins.
ement is simply not loaded. Main frames from non-whitelisted
domains are opened in the default system browser which iOS. PhoneGap creates a subclass of NSURLProtocol named
does not have any bridges. Interestingly, event-based bridges CDVURLProtocol to intercept URLs loaded in UIWebView
in PhoneGap on Android attempt to enforce the NoBridge and check whether the URL is whitelisted. UIWebView suffers
property (see Section VII-E), too, even though NoLoad implies from the same problem as WebView and the resulting policy
NoBridge and, had it been implemented correctly, would have is stricter than the same origin policy.
rendered the NoBridge checks in the same code superfluous. PhoneGap on iOS only allows domain names to be speci-
Implementations of the NoLoad defense are rife with fied in the whitelist file, but not HTTP/HTTPS schemes. This
errors of two major types: incorrect URL interception and prevents the app creator from specifying that certain domains
incorrect URL matching against the domain whitelist. should be loaded only over HTTPS, which is a very important
property (see Section VIII-A), and opens the door to HTTPS
Android (other than event-based bridges). Before version downgrade attacks.
2.6, PhoneGap on Android used a WebView callback ‘shoul-
dOverrideUrlLoading’ to intercept the loading of foreign- Windows Phone 7 and 8. PhoneGap installs a handler for
origin content. This callback is not invoked for iframe fetches the browser’s navigation event and checks the whitelist before
or XMLHttpRequests. Therefore, this defense cannot prevent a allowing navigation. This event is not triggered for iframe
hybrid app from loading unauthorized content as, for example, fetches and XMLHttpRequests. Therefore, this defense fails
an ad in an iframe. PhoneGap 2.6, released on April 9, 2013, to guarantee NoLoad.
uses the ‘shouldInterceptRequest’ callback which correctly in-
tercepts the loading of iframes. This callback is only supported D. Inadequacy of NoLoad
by Android API 11 or later.
In addition to the fact that virtually all implementations of
Unfortunately, this implementation intercepts and blocks NoLoad are incorrect, the property itself is too coarse. It does
the loading of any content from non-whitelisted domains. The not allow the hybrid app to include content from third parties
problem is that URL interception in WebView does not provide whose domains are not known at the time of compilation.
any way to distinguish between URLs loaded in the same Of course, the entire business model of free, ad-supported
origin (e.g., via hscripti tags) and URLs loaded in a foreign hybrid apps relies on their ability to load content, such as
9
Web advertising, from origins that are determined at runtime. the content that invoked the bridge, in order to match it against
The ad broker’s origin may be known statically, but only the the whitelist. Unfortunately, many embedded browsers do not
script creating the iframe comes from that origin. The actual support this. For example, if an interface is added to WebView
content inside the iframe comes from an advertiser and its via ‘addJavascriptInterface’, the local half cannot determine the
origin is often determined dynamically, e.g., via a real-time origin of the script that called this interface [17].
auction conducted by the ad broker. Even if implemented
correctly, the NoLoad defense is incompatible with the N O F RAK, our capability-based defense described in Sec-
business model of most free apps. tion VIII, enforces NoBridge without relying on the embedded
browser to transmit correct origin information to the frame-
Suppressing foreign-origin content may also have a nega- work’s local half.
tive effect on the app’s look-and-feel, creating unsightly white
spots where ads and other foreign-origin content would have Android (event-based bridges). For event-based bridges only,
been loaded (see Fig. 10). PhoneGap on Android attempts to enforce the NoBridge prop-
erty. This is possible because, unlike interface-based bridges,
In practice, this defense requires the app creator to make a event-based bridges preserve the origin of the request. For
binary decision whether to whitelist all foreign origins—and example, when the bridge is called via the prompt() method
thus expose the app to fracking attacks—or not use foreign- (Section IV-A), PhoneGap applies Config.isUrlWhiteListed()
origin content at all and thus give up on any revenues from Web to the origin of the script that triggered the prompt.
advertising and, in the case of PhoneGap on iOS and Android
(after version 2.6), even analytics and revenue optimization Unfortunately, the actual check is incorrect because of
services. Not surprisingly, out of 7,167 hybrid PhoneGap apps the anchoring bug described in Section VII-C. If foo.com is
in our study, 2,124 whitelist all domains and would have been whitelisted, malicious JavaScript hosted at any domain starting
vulnerable to fracking even if PhoneGap’s implementation of with foo.com, such as foo.com.evil.com, is permitted to access
whitelisting had been correct. the bridge.
BlackBerry WebWorks. BlackBerry WebWorks incorporates
a custom, WebKit-based embedded browser, which correctly
intercepts URLs of HTML elements and XMLHttpRequests.
WebWorks is unique in that it can restrict access to specific
resources on a domain-by-domain basis.11 For example, an app
creator can use the following whitelist to allow any domain to
be loaded within the browser, but only permit ‘mydomain.com’
to access the user’s contact information:
Fig. 10: NY Times with only nytimes.com whitelisted
(NoLoad policy)
Unfortunately, PhoneGap on BlackBerry does not take
advantage of this facility and enforces NoLoad rather than
NoBridge [22].
Because the enforcement of NoBridge in WebWorks relies
on a customized browser, it is not portable. In contrast, our
defense, N O F RAK, is platform-independent and compatible
with any common embedded browser, including WebView and
UIWebView.
Fig. 11: NY Times with only nytimes.com whitelisted F. Relying on local privilege separation
(NoBridge policy)
Privilege separation has been proposed in the research
literature as a solution to the problem of protecting device
E. Enforcing NoBridge resources from untrusted Web content (see Section IX). Priv-
Unlike NoLoad, NoBridge allows the app to load foreign- ilege separation is straightforward to apply to “pure” mobile
origin content, but ensures that only the content from apps that incorporate ads via local libraries such as AdMob.
whitelisted domains can access the bridge. NoBridge is com- The library and its ads are confined into a separate browser
patible with the advertising-supported business model of free instance, while resources are exposed only to a different
apps and allows them to preserve their look-and-feel (Fig. 11). instance containing the app’s own code [26].
Implementing NoBridge critically depends on the ability of 11 https://developer.blackberry.com/html5/documentation/Access element
the framework’s local half to correctly determine the origin of 834677 11.html
10
Local process separation is non-trivial for hybrid apps. is invisible to the apps. Because the extra argument is added
Unlike mobile apps that display their content and foreign- before the arguments are marshalled for the bridge call, the
origin ads in separate browser instances, hybrid apps “mash implementation of N O F RAK does not depend on the specifics
up” content from multiple origins and render it within a single of the bridge architecture, which resources are accessed via
browser instance created by the hybrid framework. Because the bridge, etc. On the local side of the hybrid framework,
the entire app acts as a single local process on the device, N O F RAK makes a minor modification to check the capability
there is no natural way to extract foreign-origin content and token before permitting resource access.
display it in a separate browser.
The design of N O F RAK is compatible with every existing
PhoneGap’s loading of non-whitelisted domains in the de- platform, hybrid framework, and bridge architecture. As men-
fault system browser is a form of privilege separation since the tioned above, N O F RAK does not require any changes to apps’
default browser does not have the bridges added by PhoneGap. code, but apps must be recompiled because N O F RAK changes
It is applied only to main frames, however. Applying this the local half of the framework, which serves as the local side
defense to HTML elements like iframes would have required of each hybrid app.
the framework to correctly compose multiple browser instances
in order to keep the app’s user interface intact. To the best of Whitelist policies. For transparent compatibility with the
our knowledge, this complex functionality is not supported by existing hybrid frameworks such as PhoneGap, N O F RAK uses
any hybrid framework. the same interface for specifying which origins are authorized
to access local resources: a domain whitelist provided by the
Applying privilege separation along the lines of [24, 26] app developer.
requires either re-factoring the entire application, or significant
modifications to the existing browsers so that they spawn a In PhoneGap, these whitelists are coarse-grained. For ex-
separate browser instance for each occurrence of foreign-origin ample, there is no way for a PhoneGap app to say that content
content. The latter is not feasible. The former is incompatible from a particular domain is allowed to access geolocation
with the raison d’être of hybrid development frameworks. only. This is a limitation of all hybrid frameworks except
They are popular precisely because they allow developers to BlackBerry WebWorks (see Section VII-E) and has nothing
easily convert Web apps into mobile apps and to add native to do with N O F RAK per se. If needed, N O F RAK can also
access to Web apps with minimal modifications to the app’s support finer-grained policies and grant access to specific
code and without requiring the creator to completely re-factor resources on a domain-by-domain basis, as opposed to the
her app. blanket authorization for all whitelisted domains to access
any resource available to the app. For example, all domains
VIII. N O F RAK can be authorized to access geolocation, but only the app’s
own domain is authorized to access the camera. This requires
A. Design changes to PhoneGap’s whitelisting language. Since thousands
N O F RAK is a generic defense against fracking attacks. Its of apps already rely on the current language, this is not a
main design principle is to extend origin-based access control backward-compatible modification.
to local resources outside the Web browser. To achieve this, Unlike PhoneGap, N O F RAK by default does not allow
N O F RAK enforces the NoBridge property (Section VII-E): a “*” whitelists, but, in contrast to PhoneGap, this does not
hybrid app can freely include Web content from any origin, but prevent the app from displaying content from any origin.
unauthorized origins cannot access device resources available Most hybrid apps in our survey access device resources only
to the app. This is the exact security property that hybrid from their local HTML files (those shipped with the app), not
frameworks promise to app developers and that all existing from dynamically loaded Web content. This policy change is
hybrid apps already expect and rely upon. Furthermore, it is transparent to them.
compatible with the business model of advertising-supported
hybrid apps. It permits them to show foreign-origin ads, yet Some app developers may wish to express policies like
ensures that these ads cannot get unauthorized access to the “ads are allowed to access geolocation, but not any other
device. local resource” without explicitly enumerating all domains
that may host advertising. Such policies cannot be enforced
The key idea behind N O F RAK is that all accesses to bridges by any existing Web browser. All access-control decisions for
from the Web side must be authenticated by unforgeable Web content are based on its origin, defined by the protocol,
capability tokens. Each token is unique to a particular Web domain, and port number (Section III-A). Because the purpose
origin and kept in this origin’s localStorage. N O F RAK thus of N O F RAK is to extend origin-based access control to local
leverages the browser’s same origin policy to ensure that resources, any policy enforced by N O F RAK must be based
content from other origins cannot read the token and thus on the origin of the Web content that attempts to access the
cannot access the bridge. resource. Without a fundamental re-engineering of the entire
N O F RAK does not change the JavaScript API that hybrid Web security model, it is not possible to restrict the access
apps use for resource access. Therefore, it is fully transparent rights of Web content based on what it does (e.g., advertising)
to all existing hybrid apps, requiring no changes to their code. vs. where it comes from (i.e., its origin).
On the Web side of the hybrid framework, N O F RAK makes Preventing network attacks. The same origin policy cannot
a minor modification to the framework’s JavaScript library to protect a hybrid app from network attacks. If any content
(1) read the capability token from localStorage, and (2) pass from a whitelisted origin is retrieved over HTTP, a man-in-
it as an extra argument to every bridge call. This modification the-middle attacker—for example, a malicious Wi-Fi access
11
Fig. 13: N O F RAK: Retrieving the result
Fig. 12: N O F RAK: Invoking a bridge (e.g., accesses a device resource). The N O F RAK Store does not
have a JavaScript interface and cannot be written to from the
point—can inject an attack script into it. This script will be Web side, thus N O F RAK is immune to localStorage poisoning.
treated by the browser as if it came from the whitelisted origin
and can thus read this origin’s localStorage and obtain the Preventing reflection attacks. As mentioned in Section IV-A,
capability token. prior to Android API level 17 all bridges based on ‘add-
To prevent network attacks, the app must whitelist only JavascriptInterface’ were vulnerable to reflection attackss [1,
HTTPS origins. N O F RAK then inserts the corresponding to- 18, 23]. Our prototype implementation of N O F RAK is built as
kens into localStorage, and the browser enforces that these a patch to PhoneGap 2.9, which is designed for Android API
tokens can be read only by scripts fetched over HTTPS from level 17 and thus immune to reflection attacks.
the correct origin.
To enable N O F RAK-based apps to run on earlier versions
of Android, N O F RAK makes two small changes to the local
B. Implementation PhoneGap code. First, it sets the default bridge mechanism to
As a proof of concept, we added N O F RAK to PhoneGap events rather than interfaces. Second, it modifies the existing
on Android. We chose PhoneGap because it is open source code in PhoneGap’s exposeJsInterface() so that it does not
and by far the most popular hybrid framework. Our prototype add JavaScript interfaces to WebView if the API level is less
implementation is available for download at https://github.com/ than 17 (PhoneGap’s current code does not add JavaScript
georgiev-martin/NoFrak interfaces if the API level is less than 9 or equal to 11). This
change is backward-compatible and transparent to all benign
Our prototype adds 393 lines of code to PhoneGap’s Java apps because they only access bridges through the PhoneGap
code and modifies 6 lines of code in PhoneGap’s JavaScript JavaScript API, which remains unchanged. The framework’s
library. Any other hybrid framework can be modified in the JavaScript library simply “re-routes” the calls to a different,
same way. secure bridge.
Initialization. Like stock PhoneGap, N O F RAK accepts a To prevent malicious JavaScript from crashing the lo-
whitelist of authorized domains from the app’s developer. The cal side of the app by switching to a non-existing inter-
capability for each whitelisted domain is a 9-digit pseudoran- face bridge, N O F RAK also modifies the setNativeToJsBridge-
dom token, generated by calling SecureRandom when the app’s Mode() method in the local half to deny the request if the API
local half is initialized. It is kept in the N O F RAK Store on the level is less than 17.
local side. Before the embedded browser instance is created,
each token is injected into the browser’s localStorage for the
Local-to-Web bridge. The local-to-Web bridge can be syn-
corresponding domain.
chronous or asynchronous. Synchronous bridges are used for
Web-to-local bridges. The architecture of N O F RAK Web- local accesses that can be answered immediately, for example,
to-local bridges is shown in Fig. 12. Just like with stock device or network info. These can reuse the already authenti-
PhoneGap, the app’s Web code has to include the N O F RAK cated Web-to-local bridge, with the response passed to the Web
JavaScript library. The API of this library is the same as side simply as the return value of the call. Local accesses that
PhoneGap, thus the app’s own code need not be modified. require more work, such as camera and media, need to be
asynchronous to avoid freezing the app’s Web content while
Internally, the library uses the capability token when ac- the device is being accessed.
cessing any of the available bridges. First, it reads the token via
window.localStorage.getItem(‘‘SecureToken’’). Scripts from Because of the bugs in embedded browsers, events and
other origins cannot read it because of the same origin policy. scripts injected by the local code into the browser can execute
To access any bridge, the library calls exec(service, action, in the wrong origin (see Section IV-B). Instead, both asyn-
callbackId, args, localStorage.getItem("SecureToken"));. The chronous local-to-Web mechanisms supported by N O F RAK re-
local half of N O F RAK receives the call, matches the token use the Web-to-local bridge for retrieving the results of bridge
against the N O F RAK Store, and, if found, executes the request invocation.
12
The first mechanism is a modification to pending message Device Resource # of public methods
notification used by PhoneGap. When the response is ready, the Accelerometer 3
local half of N O F RAK sends an unauthenticated notification to Camera 3
JavaScript on the Web side. The message does not contain any Capture 4
data and is safe even if the browser mistakenly delivers it to Compass 3
the wrong origin. Upon receiving the notification, the N O F RAK Contacts 5
JavaScript library retrieves the data via a Web-to-local bridge File 36
authenticated by the token. Geolocation 3
Globalization 12
The other mechanism is active polling. After submitting
InAppBrowser 6
a local request, the N O F RAK JavaScript library keeps polling
the local half for a response. Each query is authenticated by Media 10
the token. Notification 5
Splashscreen 2
Designing the local-to-Web bridge would have been easier Storage 8
if the local half of N O F RAK could write its responses directly
into the localStorage associated with the origin that made TABLE I: Number of public methods for accessing
the request. Unfortunately, existing embedded browsers do different device resources in PhoneGap
not have an API for doing this securely, and redesigning PhoneGap N O F RAK Overhead
the browser would have greatly impaired the portability of Sync 1.7713 ms 1.7755 ms 1.0024x
N O F RAK. Async 0.1244 ms 0.1317 ms 1.0586x
In addition to authenticating bridge invocations, N O F RAK TABLE II: Performance overhead of N O F RAK
authenticates requests to change the bridge mode made from this particular vulnerability. Luo et al. studied two applications
the Web side in exactly the same way. Therefore, N O F RAK but did not find any actual vulnerabilities since the applications
supports future additions to the set of available bridges and in question do not expose any sensitive resources through
prevents chosen-bridge attacks by foreign-origin content. ‘addJavascriptInterface’.
C. Evaluation In this paper, we carry out a comprehensive security
analysis of the hybrid software stack and demonstrate that
To demonstrate that our implementation of N O F RAK is fracking vulnerabilities are generic and affect all bridge mech-
transparent to all legitimate hybrid apps, it is sufficient to verify anisms, all embedded browsers, all hybrid frameworks, and all
that the API of N O F RAK-enhanced PhoneGap is indistinguish- platforms. Many of these vulnerabilities (e.g., those in event-
able from the API of stock PhoneGap. Because legitimate based bridges, which do preserve the origin of the call) not
apps interact with PhoneGap only via this API, this criterion caused by frame confusion and thus different in nature from
obviates the need to test individual apps. the ‘addJavascriptInterface’ vulnerability.
To this purpose, we considered all public API functions Luo et al. briefly dismiss authentication with pseudo-
of PhoneGap. As of this writing, the PhoneGap API has 16 random tokens by arguing that sensitive data returned by
categories: Accelerometer, Camera, Capture, Compass, Con- the framework’s local half may be mistakenly delivered to a
nection, Contacts, Device, Events, File, Geolocation, Global- malicious main frame. We are not aware of any hybrid app
ization, InAppBrowser, Media, Notification, Splashscreen, and whose main frame has a different Web origin than the app
Storage. Table I shows the number of public API methods for itself. Typically, it is either a local file, or has the same origin
each category. Connection, Events, and Device do not have as the app. In any case, our N O F RAK defense ensures that
any public methods. Connection uses 1 public property to only the authorized origins can access the returned data even
retrieve the connection type, Device uses 6 public properties if the main frame is malicious.
to fetch device information, and Events has 15 event handlers.
We developed a JavaScript test suite and verified that in 100% Some mobile advertising libraries on Android expose de-
of the tests, PhoneGap returns exactly the same results with vice resources via ‘addJavascriptInterface’ to JavaScript ads
and without N O F RAK. running inside WebView [11, 26]. Stevens et al. [26] also found
that some of these libraries fetch content over HTTP and are
To measure the performance overhead of N O F RAK, thus vulnerable to network attacks. Their proposed solution is
we benchmarked N O F RAK-enhanced PhoneGap against privilege separation between the browser instance running the
stock PhoneGap. Averaged over 10 independent executions, advertising library and the actual mobile app. As explained in
N O F RAK adds approximately 0.24% overhead to synchronous Section VII-F, local privilege separation is much more difficult
calls and 5.86% overhead to asynchronous calls as shown in to apply to hybrid apps because—like conventional Web apps
in Table II. on which they are based—they “mash up” content from trusted
and untrusted Web origins within the same browser instance.
Besides exposing device resources, mobile advertising li-
IX. R ELATED WORK
braries can cause over-privileging of mobile apps. Pearce et
Luo et al. [17] observed that interfaces added to Android’s al. [21] added advertising API support and corresponding
WebView via ‘addJavascriptInterface’ can be accessed by any advertising permissions as part of the Android platform instead
script regardless of its origin. PhoneGap contains defenses of running the entire advertising library as part of the applica-
against this attack and also implements other bridges without tion. AdSplit [24] enforces privilege separation by running the
13
library and applications as two separate processes with overlaid X. C ONCLUSIONS
displays.
Hybrid applications—and, in general, various mechanisms
With additional system support, privilege separation can for opening the browser sandbox and adding native access
also help prevent click frauds. Quire [8] uses call chains and to Web code—are here to stay. Today, hybrid applications
digital signatures to establish the provenance of IPC calls made are typically built using one of the application development
by Android applications. It can be used to verify that apps frameworks analyzed in this paper, but similar functionality
correctly display ads and users indeed click on them. is being added to conventional Web browsers, too. Chrome
and the nightly versions of Firefox support the WebRTC
Wang et al. [27] analyzed cross-origin vulnerabilities in standard that enables Web content to access local audio and
inter-application communication channels such as intents, video resources [29]. Chrome also supports “packaged apps”12
schemes, and Web-access utility classes on mobile platforms. with native access capabilities, Intel’s MobiUS app browser13
Their threat model involves users installing malicious apps or supports access to device resources and the user’s social media,
clicking on malicious Web links. Morbs, the defense proposed etc.
in [27], is based on labeling all inter-application messages with Hybrid software will continue to present security chal-
their respective origins and enforcing origin-based security lenges. Existing app stores rely on semi-automated static
policies. In contrast, this paper focuses on a different part of audit to filter out malicious apps, but hybrid apps compose
the software stack, namely, device access by untrusted Web their content dynamically. This was less of a problem in
content running in embedded Web browsers, and demonstrates conventional Web applications because they did not have any
the need to apply origin-based access control there, too. local access, but hybrid apps do. App platforms must develop
dynamic, runtime mechanisms for recognizing and blocking
Davi et al. [7] analyzed permission re-delegation attacks on malicious behavior.
Android applications. Permission re-delegation is an instance
of the confused deputy problem [14] where a privileged Hybrid apps contain complex interactions between Web
application exposes some operations as a service that non- code and local code. Unfortunately, the Web security model
privileged applications can invoke, yet does not correctly check and the local security model are not coherent, and the loss
the credentials of the invoking application. Felt et al. [9] found of origin when Web content accesses local resources can be
that many Android applications suffer from permission re- devastating. Furthermore, even minor bugs in either the hybrid
delegation attacks. Many defenses against re-delegation attacks code, or the embedded browser open the door to cross-site
have been proposed in the literature [6, 8, 9, 12, 16]. scripting attacks.14
Domain whitelisting is now done opaquely by app creators.
Fracking vulnerabilities described in this paper can be
Showing the whitelists to the user may help the user make
viewed as an instance of the confused deputy problem, but they
more educated decisions about (not) installing certain apps.
occur at a different level of the software stack than the permis-
sion re-delegation attacks. Fracking vulnerabilities result from Security of the hybrid software stack is a complex, poorly
a mismatch between the security models of the app’s Web understood topic that will only grow in importance. We view
code (governed by the same origin policy) and the framework’s this paper as a step towards better understanding of the issues
local code (governed by the platform’s access control policy). and designing robust defenses.
Fracking vulnerabilities are not specific to Android and apply
to all platforms and all mechanisms currently used in hybrid Acknowledgments. This work was partially supported by the
frameworks to expose local resources to Web code. NSF grants CNS-0746888, CNS-0905602, and CNS-1223396,
a Google research award, the MURI program under AFOSR
Chen et al. [5] proposed using permission event graphs to Grant No. FA9550-08-1-0352, NIH grant R01 LM011028-
prevent malicious applications from misusing their privileges 01 from the National Library of Medicine, and Google PhD
by enforcing OS-context-specific policies on them. Fellowship to Suman Jana.
Proposals for finer-grained access control than the current
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