iBackDoor: High-Risk Code Hits iOS Apps
Introduction
FireEye mobile researchers recently discovered potentially
“backdoored” versions of an ad library embedded in thousands of iOS
apps originally published in the Apple App Store. The affected
versions of this library embedded functionality in iOS apps that used
the library to display ads, allowing for potential malicious access to
sensitive user data and device functionality. NOTE: Apple has worked
with us on the issue and has since removed the affected apps.
These potential backdoors could have been controlled remotely by
loading JavaScript code from a remote server to perform the following
actions on an iOS device:
- 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 contained identifying data suggesting that
it is a version of the mobiSage SDK [1]. We found 17 distinct versions
of the potentially backdoored ad library: version codes 5.3.3 to
6.4.4. However, in the latest mobiSage SDK publicly released by adSage
[2] – version 7.0.5 – the potential backdoors are not present. It is
unclear whether the potentially backdoored versions of the ad library
were released by adSage or if they were created and/or compromised by
a malicious third party.
As of November 4, we have identified 2,846 iOS apps containing the
potentially backdoored versions of mobiSage SDK. Among these, we
observed more than 900 attempts to contact an ad adSage server capable
of delivering JavaScript code to control the backdoors. We notified
Apple of the complete list of affected apps and technical details on
October 21, 2015.
While we have not observed the ad server deliver any malicious
commands intended to trigger the most sensitive capabilities such as
recording audio or stealing sensitive data, affected apps periodically
contact the server to check for new JavaScript code. In the wrong
hands, malicious JavaScript code that triggers the potential backdoors
could be posted to eventually be downloaded and executed by affected apps.
Technical Details
As shown in Figure 1, the affected mobiSage library included 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 potential backdoors and
exposed 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 potential backdoors by
invoking the interfaces exposed by msageCore. Each component has its
own separate version number.
src="https://www.fireeye.com/content/dam/fireeye-www/blog/images/ibackdoor/ibackdoorfig1.png"
width="233" height="269" />
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 class="code">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
At-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 potential backdoors in the library. They expose the potential
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 potentially exposed
users to additional risks by including logic to promote and install
“enpublic” apps as shown in Figure 4. As we have highlighted in
previous blogs [footnotes 3, 4, 5, 6, 7], enpublic apps can introduce
additional security risks by using private APIs in certain versions of
iOS. These private APIs potentially allow for background monitoring of
SMS or phone calls, breaking the app sandbox, stealing email messages,
and demolishing arbitrary app installations. Apple has addressed a
number of issues related to enpublic apps that we have brought to
their attention.
Figure 4: Installing “enpublic” apps to bypass
Apple App Store review
We can see how this ad library functions 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 class="code">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 remote
server and submitting commands to msageCore. The file layout of
msageJS is shown in Figure 7. Inside class="code">sdkjs.js, we find a wrapper object called class="code">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 class="code">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 class="code">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 class="code">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.
Figure 9: Server response to msageJS update
request via HTTP POST
Enterprise Protection
To ensure the protection of our customers, FireEye has deployed
detection rules in its href="/content/fireeye-www/en_US/products-delete/nx-network-security-products.html">Network
Security (NX) and Mobile Threat Prevention (MTP) products to
identify the affected apps and their network activities.
For FireEye NX customers, alerts will be generated if an employee
uses an infected app while their iOS device is connected to the
corporate network. FireEye MTP management customers have full
visibility into high-risk apps installed on mobile devices in their
deployment base. End users will receive on-device notifications of the
risky app and IT administrators receive email alerts.
Conclusion
In this blog, we described an ad library that affected thousands of
iOS apps with potential backdoor functionality. We revealed the
internals of backdoors which could be used to trigger audio recording,
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 potential backdoors in
ad libraries could be controlled remotely by JavaScript code should
their ad servers fall under malicious actors’ control.
[2] http://www.adsage.cn/
[3] href="/content/fireeye-www/en_US/blog/threat-research/2015/08/ios_masque_attackwe.html">https://www.fireeye.com/blog/threat-research/2015/08/ios_masque_attackwe.html
[4] href="/content/fireeye-www/en_US/blog/threat-research/2015/02/ios_masque_attackre.html">https://www.fireeye.com/blog/threat-research/2015/02/ios_masque_attackre.html
[5] href="/content/fireeye-www/en_US/blog/threat-research/2014/11/masque-attack-all-your-ios-apps-belong-to-us.html">https://www.fireeye.com/blog/threat-research/2014/11/masque-attack-all-your-ios-apps-belong-to-us.html
[6] href="/content/fireeye-www/en_US/blog/threat-research/2015/06/three_new_masqueatt.html">https://www.fireeye.com/blog/threat-research/2015/06/three_new_masqueatt.html
[7] https://www.virusbtn.com/virusbulletin/archive/2014/11/vb201411-Apple-without-shell
Source:
iBackDoor: High-Risk Code Hits iOS Apps
