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The zero-day exploits of Operation WizardOpium
#1
Bug 
Quote:Contents Google Chrome remote code execution exploit

In the [Only registered and activated users can see links Click here to register] we described the exploit loader responsible for initial validation of the target and execution of the next stage JavaScript code containing the full browser exploit. The exploit is huge because, besides code, it contains byte arrays with shellcode, a Portable Executable (PE) file and WebAssembly (WASM) module used in the later stages of exploitation. The exploit abused a vulnerability in the WebAudio OfflineAudioContext interface and was targeting two release builds of Google Chrome 76.0.3809.87 and 77.0.3865.75. However, the vulnerability was introduced long before that and much earlier releases with a WebAudio component are also vulnerable. At the time of our discovery the current version of Google Chrome was 78, and while this version was also affected, the exploit did not support it and had a number of checks to ensure that it would only be executed on affected versions to prevent crashes. After our report, the vulnerability was assigned CVE-2019-13720 and was fixed in version 78.0.3904.87 with the following [Only registered and activated users can see links Click here to register]. A use-after-free (UAF) vulnerability, it could be triggered due to a race condition between the Render and Audio threads.

As you can see, when the audio buffer is set to null in ConvolverNode and an active buffer already exists within the Reverb object, the function SetBuffer() can destroy reverb_ and shared_buffer_ objects.

These objects might still be in use by the Render thread because there is no proper synchronization between the two threads in the code. A patch added two missing locks (graph lock and process lock) for when the buffer is nullified.

The exploit code was obfuscated, but we were able to fully reverse engineer it and reveal all the small details. By looking at the code, we can see the author of the exploit has excellent knowledge of the internals of specific Google Chrome components, especially the [Only registered and activated users can see links Click here to register] memory allocator. This can clearly be seen from the snippets of reverse engineered code below. These functions are used in the exploit to retrieve useful information from internal structures of the allocator, including: SuperPage address, PartitionPage address by index inside the SuperPage, the index of the used PartitionPage and the address of PartitionPage metadata. All constants are taken from [Only registered and activated users can see links Click here to register].
 
It’s interesting that the exploit also uses the relatively new built-in [Only registered and activated users can see links Click here to register] class to handle 64-bit values; authors usually use their own primitives in exploits.

At first, the code initiates OfflineAudioContext and creates a huge number of IIRFilterNode objects that are initialized via two float arrays.

After that, the exploit begins the initial stage of exploitation and tries to trigger a UAF bug. For that to work the exploit creates the objects that are needed for the Reverb component. It creates another huge OfflineAudioContext object and two ConvolverNode objects – ScriptProcessorNode to start audio processing and AudioBuffer for the audio channel.

This function is executed recursively. It fills the audio channel buffer with zeros, starts rendering offline and at the same time runs a loop that nullifies and resets the channel buffer of the ConvolverNode object and tries to trigger a bug. The exploit uses the later() function to simulate the Sleep function, suspend the current thread and let the Render and Audio threads finish execution right on time.

During execution the exploit checks if the audio channel buffer contains any data that differs from the previously set zeroes. The existence of such data would mean the UAF was triggered successfully and at this stage the audio channel buffer should contain a leaked pointer.

The PartitionAlloc memory allocator has a special exploit mitigation that works as follows: when the memory region is freed, it byteswaps the address of the pointer and after that the byteswapped address is added to the FreeList structure. This complicates exploitation because the attempt to dereference such a pointer will crash the process. To bypass this technique the exploit uses the following primitive that simply swaps the pointer back:
...
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