The latest news breaking over the Christmas period is that of a fairly serious bug in IIS that allows local file inclusion (LFI) of any filetype due a bug in the way IIS filters handle semicolons (;).
Secunia has confirmed the vulnerability “on a fully patched Windows Server 2003 R2 SP2 running Microsoft IIS version 6. Other versions may also be affected”.
Although oddly it only classifies the bug as “Less critical” – basically a 2/5 on their threat scale.
A researcher has identified a vulnerability in the most recent version of Microsoft’s Internet Information Services that allows attackers to execute malicious code on machines running the popular webserver.
The bug stems from the way IIS parses file names with colons or semicolons in them, according to researcher Soroush Dalili. Many web applications are configured to reject uploads that contain executable files, such as active server pages, which often carry the extension “.asp.” By appending “;.jpg” or other benign file extensions to a malicious file, attackers can bypass such filters and potentially trick a server into running the malware.
There appears to be some disagreement over the severity of the bug, which Dalili said affects all versions of IIS. While he rated it “highly critical,” vulnerability tracker Secunia classified it as “less critical,” which is only the second notch on its five-tier severity rating scale.
It’s a pretty nasty bug if you ask me, it means any CMS, forum software or gallery page where users are allowed to upload files (running on IIS) can be owned by a webshell without any effort at all.
Even if an app doesn’t allow native uploading, LFI can now be executed using another exploit and it will bypass any filtering IIS provides against executable files such as .asp scripts.
I don’t really see how this bug is “Less critical” – I’d imagine there’s some mass pwnage going around the World right now.
“Impact of this vulnerability is absolutely high as an attacker can bypass file extension protections by using a semicolon after an executable extension such as ‘.asp,’ ‘.cer,’ ‘.asa’ and so on,” Dalili wrote. “Many web applications are vulnerable against file uploading attacks because of this weakness of IIS.”
In an email to El Reg, Dalili offered the following attack scenario:
“Assume a website which only accepts JPG files as the users’ avatars. And the users can upload their avatars on the server. Now an attacker tries to upload “Avatar.asp;.jpg” on the server. Web application considers this file as a JPG file. So, this file has the permission to be uploaded on the server. But when the attacker opens the uploaded file, IIS considers this file as an ASP file and tries to execute it by ‘asp.dll.’
“So, the attacker can upload a web-shell on the server by using this method. Most of the uploaders only control the last part of the files as their extensions, and by using this method, their protection will be bypassed.”
Microsoft as per usual is ‘looking into it’ – I would guess within a week or so users will be screaming for a patch in the next round of updates planned for January if not sooner.
Although if you are using IIS, I wouldn’t hold your breath for an out of schedule patch – we all know what Microsoft thinks of those.
Source: The Register