This month’s Patch Tuesday brings us a relatively small number of CVEs being patched, but an abnormally high percentage of noteworthy critical vulnerabilities.
Vulnerability Analysis: CVE-2021-34535
One such vulnerability is identified as CVE-2021-34535, which is a remote code execution flaw in the Remote Desktop client software, observed in mstscax.dll, which is used by Microsoft’s built-in RDP client (mstsc.exe). The vulnerability is very closely related to a bug released in July of 2020, CVE-2020-1374, which also came through Microsoft’s Patch Tuesday process and had highly similar characteristics. The vulnerability is an integer overflow due to an attacker-controllable payload size field, which ultimately leads to a heap buffer overflow during memory allocation. The vulnerability can be triggered via the RDP Video Redirection Virtual Channel Extension feature [MS-RDPEV], which is typically deployed on port 3389, and is contained inside of compressed UDP payload and encrypted RDP using TLS.
But does this flaw, despite its impressive 9.9 CVSS score, rise to the level of past RDP vulnerabilities, including the infamous BlueKeep (CVE-2019-0708)? Not so fast – there are a few additional factors to take into consideration.
Attack Scenario
First and foremost, this is a client-side vulnerability, meaning there is no real ability for self-propagation, or “wormability” from an Internet perspective. The most likely attack scenario would be to convince a user to authenticate to a malicious RDP server, where the server could trigger the bug on the client side. During reproduction of the issue, we were able to easily trigger the crash and observe a later memcpy using the controlled overflow, which should facilitate exploitation. We think it is likely that exploits will be developed for this vulnerability but the availability of a patch prior to any ..
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