Microsoft Downplays Concerns Of IIS Zero-Day Bug

Microsoft is downplaying talk of a zero-day bug in Internet Information Services, after reports of the security vulnerability began circulating just before Christmas.

The issue was due to the way IIS 6.0 handles semicolons in URLs. However Microsoft contends that because IIS must be in an insecure configuration for the attack to work, the handling of semicolons is essentially besides the point.

“The key in this is … for the scenario to work, the IIS server must already be configured to allow both ‘write’ and ‘execute’ privileges on the same directory,” blogged Christopher Budd, communications lead for Microsoft Security Response Center. “This is not the default configuration for IIS and is contrary to all of our published best practices. Quite simply, an IIS server configured in this manner is inherently vulnerable to attack.”

Still, Budd said, “the IIS folks are evaluating a change to bring the behaviour of IIS 6.0 in line with other versions.”

The incident was brought to light by security researcher Soroush Dalili, who posted information about the situation on his website on 25 December. According to a 24 December Secunia advisory, the situation is the result of a web server “incorrectly executing e.g. ASP [Active Server Pages] code included in a file having multiple extensions separated by ‘;’, only one internal extension being equal to ‘.asp’ (e.g. ‘file.asp;.jpg’). This can be exploited to potentially upload and execute arbitrary ASP code via a third-party application using file extensions to restrict uploaded file types.”

If exploited, Dalili said, the issue could allow an attacker to circumvent content filtering software and upload malicious code to an IIS server.

However, customers using IIS 6.0 in the default configuration or following Microsoft’s recommended best practices don’t need to worry about this issue, Budd wrote.

“If, however, you are running IIS in a configuration that allows both ‘write’ and ‘execute’ privileges on the same directory like this scenario requires, you should review our best practices and make changes to better secure your system from the threats that configuration can enable,” he advised.