Quote:A researcher has released a proof-of-concept (PoC) exploit for CVE-2021-31166, a use-after-free, highly critical vulnerability in the HTTP protocol stack (http.sys) that could lead to wormable remote code execution (RCE).

Microsoft discovered the flaw internally, releasing a patch in its May 11 Patch Tuesday update. This was the most severe bug in that batch: an http.sys issue that requires neither user authentication nor user interaction to exploit. An exploit would allow RCE with kernel privileges or a denial-of-service (DoS) attack.

According to a tweet from Microsoft’s Justin Campbell, the vulnerability was found by @_mxms and @fzzyhd1.
 
http.sys enables Windows and applications to communicate with other devices; it can be run standalone or in conjunction with Internet Information Services (IIS).

“In most situations, an unauthenticated attacker could send a specially crafted packet to a targeted server utilizing the HTTP Protocol Stack (http.sys) to process packets,” Microsoft explained in its advisory. Given that the vulnerability is wormable, Microsoft recommends prioritizing the patching of affected servers.