Well, it seems that there’s a stack overflow exploit in the way they’re handled by the OS. In a nutshell, when it copies the data into memory, it doesn’t properly check the size of the memory being copied. The result is that memory is overwritten and the stack overflows.
The Zero-day Emergency Response Team has a pretty good writeup on their site about the exploit as well as a patch to resolve the problem. This is a pretty big security issue, so I recommend at least checking out the info on their site.
This vulnerability affects Windows 98, 2000, XP, Server 2003, and Vista. The Internet Storm Center also warns that other unsupported versions of Windows, probably Windows 95 and ME, are also likely affected. Neither ZERT nor Microsoft are likely to release a patch for Windows 95 or ME. Additionally, they have a nice matrix that explains which mail clients are vulnerable to this as well.
Microsoft has released an out-of-cycle patch for this vulnerability. You can find the relevent files on their advisory page, bulletin MS07-017. Patches for Windows 2000, XP, Server 2003, and Vista are available. If you still use Windows 98, the ZERT patch is your only option.
Update : eEye had released a patch back on March 30th for this vulnerability. However, this patch only ensures that .ANI files are loaded from the SystemRoot and not anywhere else. While this helps prevent most exploits, if an attacker can somehow gain access to the SystemRoot, the system is still vulnerable.
Please take special note : This is being actively exploited in the wild. This is a serious remote access vulnerability which can lead to your computer being compromised. Please make sure you have an anti-virus program installed and up-to-date. And remember, your first line of defense is you. Be responsible, know the risks, install the patches, and keep yourself safe.