A security researcher has published over the weekend proof-of-concept exploit code for a wormable Windows IIS server vulnerability.

Tracked as CVE-2021-31166, the vulnerability was discovered internally by Microsoft’s staff and patched last week in the May 2021 Patch Tuesday.

Several security researchers and security firms who reviewed last week’s security updates considered the bug the most dangerous vulnerability Microsoft fixed in this month’s patch cycle.

The bug, which received a severity rating of 9.8 out of 10 on the CVSSv3 scale, is a memory corruption vulnerability in the HTTP protocol stack included with recent Windows versions.

This stack is used by the Windows built-in IIS server. If this server is enabled, Microsoft says that an attacker can send a malformed packet and execute malicious code right on the operating system kernel.

In a security advisory, Microsoft said the bug could be used to create network worms that jump from server to server and recommended “prioritizing the patching of affected servers.”

But while the bug sounds extremely dangerous, there are also a few mitigation factors. The first is that only recent versions of Windows are impacted.

This includes Windows 10 2004 and 20H2, and Windows Server 2004 and 20H2, which basically includes the Windows 10 and Windows Server OS versions released last year, which are very unlikely to have been broadly deployed in production environments.

On Sunday, former Microsoft engineer and current security researcher Axel Souchet released proof-of-concept code for exploiting CVE-2021-31166. The code does not include worming capabilities but only crashes an unpatched Windows system running an IIS server.

Nevertheless, the availability of proof-of-concept code is usually the first step towards attackers experimenting with this attack.

Even if the number of vulnerable Windows IIS servers might be small, this will not dissuade attackers; which usually take whatever they can get.

Microsoft would like to see customers patch their systems. All in all, Microsoft itself is very sensitive to these types of vulnerabilities, especially. In June 2019, a threat actor weaponized an Exim vulnerability to create a worm that spread through the company’s Linux-based Azure cloud servers. While Microsoft has most likely patched IIS servers on its Azure infrastructure, there are still other cloud providers and corporate networks where such servers might still be running.