iBackDoor: High-risk Code Sneaks into the App StoreThe library embeds backdoors in unsuspecting apps that make use of it
to display ads, exposing sensitive data and functionality. The
backdoors can be controlled remotely by loading JavaScript code from
remote servers to perform the following actions:
- Capture audio and screenshots.
- Monitor and upload
device location. - Read/delete/create/modify files in the
app’s data container. - Read/Write/Reset the app’s keychain
(e.g., app password storage). - Post encrypted data to remote
servers. - Open URL schemes to identify and launch other apps
installed on the device. - “Side-load” non-App Store apps by
prompting the user to click an “Install” button.
The offending ad library contains identifying data suggesting that
it is a version of the mobiSage SDK [1]. We found 17 distinct versions
of the backdoored ad library, with version codes between 5.3.3 and
6.4.4. However, in the latest mobiSage SDK publicly released by adSage
[2], identified as version 7.0.5, the backdoors are not present. We
cannot determine with certainty whether the backdoored versions of the
library were actually released by adSage, or whether they were created
and/or compromised by a third party.
As of publication of this blog, we have identified 2846 apps
published in the App Store containing backdoored versions of mobiSage
SDK. Among these 2846 apps, we have observed over 900 attempt to
contact their command and control (C2) server. We have notified Apple
and provided the details to them.
These backdoors can be controlled not only by the original creators
of the ad library, but potentially also by outside threat actors.
While we have not observed commands from the C2 server intended to
trigger the most sensitive capabilities such recording audio or
stealing sensitive data, there are several ways that the backdoors
could be abused by third-party targeted attackers to further
compromise the security and privacy of the device and user:
- An attacker could reverse-engineer the insecure HTTP-based
control protocol between the ad library and its server, and then
hijack the connection to insert commands to trigger the backdoors
and steal sensitive information. - A malicious app developer
can similarly inject commands, utilizing the library’s backdoors to
build their own surveillance app. Since the ad library has passed
the App Store review process in numerous apps, this is an attractive
way to create an app with these hidden behaviors that will pass
under Apple’s radar.
App Store Protections Ineffective
Despite Apple’s reputation for keeping malware out of the App Store
with its strict review process, this case demonstrates that it is
still possible for dangerous code that exposes users to critical
security and privacy risks to sneak into the App Store by piggybacking
on unsuspecting apps. Backdoors that enable silently recording audio
and uploading sensitive data when triggered by downloaded code clearly
violate the requirements of the iOS Developer Program [3]. The
requirements state that apps are not permitted to download code or
scripts, with the exception of scripts that “do not change the
primary purpose of the Application by providing features or
functionality that are inconsistent with the intended and advertised
purpose of the Application as submitted to the App Store.” And,
for apps that can record audio, “a reasonably conspicuous audio,
visual or other indicator must be displayed to the user as part of
the Application to indicate that a Recording is taking place.”
The backdoored versions of mobiSage clearly violate these
requirements, yet thousands of affected apps made it past the App
Store review process.
Technical Details
As shown in Figure 1, the backdoored mobiSage library includes two
key components, separately implemented in Objective-C and JavaScript.
The Objective-C component, which we refer to as msageCore,
implements the underlying functionality of the backdoors and exposes
interfaces to the JavaScript context through a WebView. The JavaScript
component, which we refer to as msageJS, provides high-level
execution logic and can trigger the backdoors by invoking the
interfaces exposed by msageCore. Each component has its own separate
version number.
Figure 1: Key components of backdoored mobiSage SDK
In the remainder of this section, we reveal internal details of
msageCore, including its communication channel and high-risk
interfaces. Then, we describe how msageJS is launched and updated, and
how it can trigger the backdoors.
Backdoors in msageCore
Communication channel
MsageCore implements a general framework to communicate with msageJS
via the ad library’s WebView. Commands and parameters are passed via
specially crafted URLs in the format adsagejs://cmd¶meter. As
shown in the reconstructed code fragment in Figure 2, msageCore
fetches the command and parameters from the JavaScript context and
inserts them in its command queue.
Figure 2: Communication via URL loading in WebView.
To process a command in its queue, msageCore dispatches the command
along with its parameters to a corresponding Objective-C class and
method. Figure 3 shows portions of the reconstructed command
dispatching code.
Figure 3: Command dispatch in msageCore.
High-risk interfaces
Each dispatched command ultimately arrives at an Objective-C class
in msageCore. Table 1 shows a subset of msageCore classes and the
corresponding interfaces that they expose.
msageCore Class Name
|
Interfaces
|
MSageCoreUIManagerPlugin | width="162" valign="top">- captureAudio:
-
captureImage:
- openMail:
- openSMS:
-
openApp:
- openInAppStore:
- openCamera:
-
openImagePicker:
- ...
MSageCoreLocation | | valign="top">- start:
- stop:
- setTimer:
- returnLocationInfo:webViewId:
- ...
MSageCorePluginFileModule |
- createDir -
deleteDir: - deleteFile: - createFile: -
getFileContent: - ... |
MSageCoreKeyChain | | valign="top">- writeKeyValue:
- readValueByKey:
- resetValueByKey:
MSageCorePluginNetWork | width="162" valign="top">- sendHttpGet:
-
sendHttpPost:
- sendHttpUpload:
- ...
MSageCoreEncryptPlugin | width="162" valign="top">- MD5Encrypt:
-
SHA1Encrypt:
- AESEncrypt:
- AESDecrypt:
-
DESEncrypt:
- DESDecrypt:
- XOREncrypt:
-
XORDecrypt:
- RC4Encrypt:
- RC4Decrypt
-
...
Table 1: Selected interfaces exposed by msageCore
The selected interfaces reveal some of the key capabilities exposed
by the backdoors in the library. They expose the ability to capture
audio and screenshots while the affected app is in use, identify and
launch other apps installed on the device, periodically monitor
location, read and write files in the app’s data container, and
read/write/reset “secure” keychain items stored by the app.
Additionally, any data collected via these interfaces can be encrypted
with various encryption schemes and uploaded to a remote server.
Beyond the selected interfaces, the ad library exposes users to
additional risks by including explicit logic to promote and install
“enpublic” apps shown in Figure 4. As we have highlighted in previous
blogs [4, 5, 6, 7, 8], enpublic apps can introduce additional security
risks by using private APIs, which would normally cause an app to be
blocked by the App Store review process. In previous blogs we have
described a number of “Masque” attacks utilizing enpublic apps [5, 6,
7], which affect pre-iOS 9 devices. The attacks include background
monitoring of SMS or phone calls, breaking the app sandbox, stealing
email messages, and demolishing arbitrary app installations.
Figure 4: Installing “enpublic” apps to bypass Apple App Store review
We can observe the functionality of the ad library by examining the
implementations of some of the selected interfaces. Figure 5 shows
reconstructed code snippets for capturing audio. Before storing
recorded audio to a file audio_xxx.wav, the code retrieves two
parameters from the command for recording duration and threshold.
Figure 5: Capturing audio with duration and threshold.
Figure 6 shows a code snippet for initializing the app’s keychain
before reading. The accessed keychain is in the
kSecClassGenericPassword class, which is widely used by apps for
storing secret credentials such as passwords.
Figure 6: Reading the keychain in the kSecClassGenericPassword class.
Remote control in msageJS
msageJS contains JavaScript code for communicating with a C2 server
and submitting commands to msageCore. The file layout of msageJS is
shown in Figure 7. Inside sdkjs.js, we find a wrapper object called
adsage and the JavaScript interface for command execution.
Figure 7: The file layout of msageJS
The command execution interface is constructed as follows:
adsage.exec(className, methodName, argsList, onSuccess, onFailure);
The className and methodName parameters correspond to classes and
methods in msageCore. The argsList parameter can be either a list or
dict, and the exact types and values can be determined by reversing
the methods in msageCore. The final two parameters are function
callbacks invoked when the method exits. For example, the following
invocation starts audio capture:
adsage.exec("MSageCoreUIManager",
"captureAudio", ["Hey", 10, 40], onSuccess, onFailure);
Note that the files comprising msageJS cannot be found by simply
listing the files in an affected app’s IPA. The files themselves are
zipped and encoded in Base64 in the data section of the ad library
binary. After an affected app is launched, msageCore first decodes the
string and extracts msageJS to the app’s data container, setting
index.html shown in Figure 7 as the landing page in the ad library
WebView to launch msageJS.
Figure 8: Base64 encoded JavaScript component in zip format.
When msageJS is launched, it sends a POST request to
hxxp://entry.adsage.com/d/ to check for updates. The server responds
with information about the latest msageJS version, including a
download URL, as shown in Figure 9. Note that since the request uses
HTTP rather than HTTPS, the response can be hijacked easily by a
network attacker, who could replace the download URL with a link to
malicious JavaScript code that triggers the backdoors.
Figure 9: Server response to msageJS update request via HTTP POST
Conclusion
In this blog, we described a high-risk ad library affecting
thousands of iOS apps in the Apple App Store. We revealed the
internals of backdoors which can be used to silently record audio,
capture screenshots, prompt the user to side-load other high-risk
apps, and read sensitive data from the app’s keychain, among other
dubious capabilities. We also showed how these backdoors can be
controlled remotely by JavaScript code fetched from the Internet in an
insecure manner.
FireEye Protection
Immediately after we discovered the high-risk ad library and
affected apps, FireEye updated detection rules in its NX and href="https://mobile.fireeyecloud.com">Mobile Threat Prevention
(MTP) products to detect the affected apps and their network
activities. In addition, FireEye customers can access the full list of
affected apps upon request.
FireEye NX customers are alerted if an employee uses an infected app
while their iOS device is connected to the corporate network. It is
important to note that, even if the servers that the backdoored
mobiSage SDK communicates with do not deliver JavaScript code that
triggers the high-risk backdoors, the affected apps still try to
connect to them using HTTP. This HTTP session is vulnerable to
hijacking by outside attackers.
FireEye MTP management
customers have full visibility into high-risk apps installed on
mobile devices in their deployment base. End users receive on-device
notifications of the detection and IT administrators receive email alerts.
Click here to learn
more about FireEye Mobile Threat Protection product.
[1] http://www.adsage.com/mobisage
[2] http://www.adsage.cn/
[3]
https://developer.apple.com/programs/ios/information/iOS_Program_Information_4_3_15.pdf
[4] https://www.fireeye.com/blog/threat-research/2015/08/ios_masque_attackwe.html
[5] https://www.fireeye.com/blog/threat-research/2015/02/ios_masque_attackre.html
[6] https://www.fireeye.com/blog/threat-research/2014/11/masque-attack-all-your-ios-apps-belong-to-us.html
[7] https://www.fireeye.com/blog/threat-research/2015/06/three_new_masqueatt.html
[8] https://www.virusbtn.com/virusbulletin/archive/2014/11/vb201411-Apple-without-shell
Source:
iBackDoor: High-risk Code Sneaks into the App Store
