Microsoft might not be able to fix a zero-day vulnerability in an older version of its Internet Information Services web server that attackers targeted July and August of last year. The exploit lets attackers execute malicious code on Windows servers that run IIS 6.0 while user privileges run the application. A proof-of-concept exploit for the vulnerability in IIS 6.0 is now available to view on GitHub and while IIS 6.0 is no longer supported, it remains widely used even today. Support for this version of IIS stopped in July of last year along with support for the Windows Server 2003, its parent product.
The news raises concern among security professionals as web server surveys indicate that IIS 6.0 is still being used by millions of public websites. Also, it is possible that a large number of companies could still be running web applications on Windows Server 2003 and IIS 6.0 inside their organization. Attackers could, therefore, use the flaw to perform lateral movements if they gain access to corporate networks.
Prior to its publication on GitHub, only a few attackers were aware of the vulnerability — until recently. Now, there is evidence that many attackers now have access to the unpatched flaw. Security vendor Trend Micro offers the following explanation for the vulnerability:
A remote attacker could exploit this vulnerability in the IIS WebDAV Component with a crafted request using PROPFIND method. Successful exploitation could result in denial of service condition or arbitrary code execution in the context of the user running the application. According to the researchers who found this flaw, this vulnerability was exploited in the wild in July or August 2016. It was disclosed to the public on March 27. Other threat actors are now in the stages of creating malicious code based on the original proof-of-concept (PoC) code.
Trend Micro noted that Web Distributed Authoring and Versioning (WebDAV) is an extension of the standard Hypertext Transfer Protocol that lets users create, change and move documents on a server. The extension provides support for several request methods such as PROPFIND. The company recommends disabling the WebDAV service on IIS 6.0 installations to help mitigate the issue.