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“MSTIC has found multiple links between DSIRF and the exploits and malware used in these attacks,” Microsoft researchers wrote. “These include command-and-control infrastructure used by the malware directly linking to DSIRF, a DSIRF-associated GitHub account being used in one attack, a code signing certificate issued to DSIRF being used to sign an exploit, and other open source news reports attributing Subzero to DSIRF.”

Referring to DSIRF using the work KNOTWEED, Microsoft researchers wrote: In May 2022, MSTIC found an Adobe Reader remote code execution (RCE) and a 0-day Windows privilege escalation exploit chain being used in an attack that led to the deployment of Subzero. The exploits were packaged into a PDF document that was sent to the victim via email. Microsoft was not able to acquire the PDF or Adobe Reader RCE portion of the exploit chain, but the victim’s Adobe Reader version was released in January 2022, meaning that the exploit used was either a 1-day exploit developed between January and May, or a 0-day exploit. Based on KNOTWEED’s extensive use of other 0-days, we assess with medium confidence that the Adobe Reader RCE is a 0-day exploit. The Windows exploit was analyzed by MSRC, found to be a 0-day exploit, and then patched in July 2022 as CVE-2022-22047. Interestingly, there were indications in the Windows exploit code that it was also designed to be used from Chromium-based browsers, although we’ve seen no evidence of browser-based attacks.

The CVE-2022-22047 vulnerability is related to an issue with activation context caching in the Client Server Run-Time Subsystem (CSRSS) on Windows. At a high level, the vulnerability could enable an attacker to provide a crafted assembly manifest, which would create a malicious activation context in the activation context cache, for an arbitrary process. This cached context is used the next time the process spawned.

CVE-2022-22047 was used in KNOTWEED related attacks for privilege escalation. The vulnerability also provided the ability to escape sandboxes (with some caveats, as discussed below) and achieve system-level code execution. The exploit chain starts with writing a malicious DLL to disk from the sandboxed Adobe Reader renderer process. The CVE-2022-22047 exploit was then used to target a system process by providing an application manifest with an undocumented attribute that specified the path of the malicious DLL. Then, when the system process next spawned, the attribute in the malicious activation context was used, the malicious DLL was loaded from the given path, and system-level code execution was achieved. Microsoft recommends a number of security considerations to help mitigate this attack, including patching CVE-2022-22047, updating Microsoft Defender Antivirus to update 1.371.503.0 or later, and enabling multifactor authentication (MFA).