MysterySnail attacks with Windows zero-day

Besides finding the zero-day in the wild, we analyzed the malware payload used along with the zero-day exploit, and found that variants of the malware were detected in widespread espionage campaigns against IT companies, military/defense contractors, and diplomatic entities.

They are calling this cluster of activity MysterySnail.
Code similarity and re-use of C2 infrastructure we discovered allowed us to connect these attacks with the actor known as IronHusky and Chinese-speaking APT activity dating back to 2012.

The discovered exploit is written to support the following Windows products:
Microsoft Windows Vista
Microsoft Windows 7
Microsoft Windows 8
Microsoft Windows 8.1
Microsoft Windows Server 2008
Microsoft Windows Server 2008 R2
Microsoft Windows Server 2012
Microsoft Windows Server 2012 R2
Microsoft Windows 10 (build 14393)
Microsoft Windows Server 2016 (build 14393)
Microsoft Windows 10 (build 17763)
Microsoft Windows Server 2019 (build 17763)

The list of supported products and supported Windows 10 build numbers, explicit declaration of server OSs and the fact that exploits were only discovered in attacks on servers, all lead us to believe the exploit was developed and advertised as a solution to elevate privileges on servers.

CVE-2021-40449 is a use-after-free vulnerability in Win32k’s NtGdiResetDC function.
As with many other Win32k vulnerabilities, the root cause of this vulnerability lies in the ability to set user-mode callbacks and execute unexpected API functions during execution of those callbacks.
The CVE-2021-40449 is triggered when the function ResetDC is executed a second time for the same handle during execution of its own callback.

The exploitation process for this vulnerability is as follows:
A user-mode call to ResetDC executes syscall NtGdiResetDC and its inner function GreResetDCInternal.
This function gets a pointer to a PDC object, and then performs a call to function hdcOpenDCW.
Function hdcOpenDCW performs a user-mode callback and it can be used to execute ResetDC for the same handle a second time.
If an exploit executes ResetDC during a callback, NtGdiResetDC and GreResetDCInternal are executed again for the same DC.
If an exploit ignores all the callbacks during the second call to GreResetDCInternal, this function will be executed as intended.
It will create a new DC and get rid of the old one (the PDC object is destroyed).
In the callback, after the second ResetDC call has completed, the exploit can reclaim the freed memory of the PDC object and finish the execution of the callback.
After execution of the callback, function hdcOpenDCW returns to GreResetDCInternal, but the pointer retrieved in step (1) is now a dangling pointer – it points to the memory of the previously destroyed PDC object.
In the late stage of GreResetDCInternal execution, a malformed PDC object can be used to perform a call to an arbitrary kernel function with controlled parameters.

In the discovered exploit attackers are able to achieve the desired state of memory with the use of GDI palette objects and use a single call to a kernel function to build a primitive for reading and writing kernel memory.
This step is easily accomplished, because the exploit process is running with Medium IL and therefore it’s possible to use publicly known techniques to leak kernel addresses of currently loaded drivers/kernel modules.
It would be preferable if the Medium IL processes had limited access to such functions as NtQuerySystemInformation or EnumDeviceDrivers.

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