Security News > 2020 > May > Improper Microsoft Patch for Reverse RDP Attacks Leaves 3rd-Party RDP Clients Vulnerable

Remember the Reverse RDP Attack-wherein a client system vulnerable to a path traversal vulnerability could get compromised when remotely accessing a server over Microsoft's Remote Desktop Protocol?

Though Microsoft had patched the vulnerability as part of its July 2019 Patch Tuesday update, it turns out researchers were able to bypass the patch just by replacing the backward slashes in paths with forward slashes.

Microsoft acknowledged the improper fix and re-patched the flaw in its February 2020 Patch Tuesday update earlier this year, now tracked as CVE-2020-0655.

Apparently, the workaround works fine for the built-in RDP client in Windows operating systems, but the patch is not fool-proof enough to protect other third-party RDP clients against the same attack that relies on the vulnerable sanitization function developed by Microsoft.

According to researchers, the July patch can be bypassed because of a problem that lies in its path canonicalization function "PathCchCanonicalize," which is used to sanitize file paths, thus allowing a bad actor to exploit the clipboard synchronization between a client and a server to drop arbitrary files in arbitrary paths on the client machine.

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