It's been a rough two weeks for IIS security. On 1 May it was our solemn duty to report the IIS .printer ISAPI vulnerability; on 8 May we reported the sadmind/IIS worm; and today we have to inform you of a brand new stuff-up affecting IIS 4.0 and 5.0, handily exploited with a simple Unicode trick.
An easily malformed file name can be used to load an executable CGI program through a double-decoding glitch in IIS.
When an obfuscated file name passes the first decoding -- which, among other things, searches for .com and .exe extensions -- a second, superfluous decoding restores the name and grants access to the executable file, handily enabling an attacker to carry out a directory traversal and run arbitrary code outside the Web directory.
The vulnerability enables the execution of arbitrary code, denial of service attacks, and data disclosure -- which is a total drag if you have a file full of credit card details somewhere on your server.
The hole was originally discovered by nsfocus on 27 March. After several delays requested by Microsoft, the announcement was finally made on 15 May.
It's similar to the hole exploited by the sadmind/IIS worm, but potentially more malicious. Lord knows it won't be long before we see this one similarly automated.
Best get those patches in place straight away. ®