Critical Vulnerability Discovered in IIS 6.0 Web Services

IIS 6, the version that runs on Windows 2003 Server, was revealed this week to have a serious vulnerability that could allow an attacker to run malicious code on the server. The vulnerability has apparently been known to some malicious groups for some time, as attacks exploiting this vulnerability have been observed as far back as summer of 2016. But last week a proof-of-concept exploit for the vulnerability was posted to GitHub, bringing public attention to the problems and providing potential attackers with a head start on developing their own exploit code.  That is likely to take this from a secretive exploit used by a few actors to one that will be widely used by many attackers, meaning anyone running a vulnerable server is a likely victim.

Vulnerability announcements are common, but this one is especially problematic for several reasons:
  • IIS 6.0 is a part of the Windows 2003 Server operating system, which aged out of support from Microsoft almost two years ago.
  • There are anywhere from 300,000-600,000 web servers currently exposed on the Internet that are running this service.
  • There is no reason to expect any patch from Microsoft to fix this problem. Ever.
It was over three years ago that we were warning clients that they needed to be planning when and how to replace their Windows 2003 servers, as the operating system would be unsupported after July 2015. However, we still see these servers from time to time in organizations, and even being used as web servers. ​A quick search on Shodan shows over 600,000 servers running on IIS 6.0, though this may include a lot of duplicates. https://www.shodan.io/search?query=iis+6.0
​
One bright spot in this story is that the vulnerability is in the WebDAV component, which is commonly enabled without really being needed. Because of that, disabling WebDAV in IIS should prevent exploitation. The vulnerability is exploited when a "PROPFIND" request contains an overly-large "IF" header -- the excess data can overflow the buffer and result in arbitrary code execution. See Microsoft TechNet for more info on managing WebDAV in IIS 6.0, but it is important to ensure that it's not critical to your application before disabling it. Of course, leaving it on may expose you to attacks, so upgrading from IIS 6.0 to a supported version is the best solution. For more info on https://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/844f5e01-4b9e-4dac-897e-2a0bb33f28af.mspx?mfr=true
WebDAV in IIS 6.0 from Microsoft TechNet:

Key strategies to avoid falling victim to this type of exploit include:
  • Keep servers and services up-to-date.
  • Employ a web application firewall or reverse-proxy for web servers, whether integrated into an overall enterprise firewall or provided by a separate, dedicate device.
  • Ensure the intrusion detection/prevention systems and web application firewalls are receiving updated signatures on a regular basis.
Further information:

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