Attackers have begun actively targeting an unpatched hole in Microsoft's Internet Information Services webserver using new exploit code that greatly expands the number of systems that are vulnerable to the bug.
In an updated advisory published Friday, Microsoft researchers said they are seeing "limited attacks" exploiting the vulnerability, which resides in a file transfer protocol component of IIS. Exploit code publicly released in the past 24 hours is now able to cause vulnerable servers to crash even when users don't have the ability to create their own directories.
That means the bug, for which there is no patch, is easier to exploit than previously thought. In an earlier advisory, Microsoft said attacks only worked when untrusted users had write access to directories. For the moment, Microsoft continues to say that IIS5 running on Windows 2000 appears to be the only version that is vulnerable to attacks that can remotely execute malicious code on an underlying server. But it's now clear that hackers can target every version of IIS to cause denial-of-service attacks.
Microsoft said Thursday that it planned to issue five security updates for next week's Patch Tuesday. None of the affected software listed in the limited disclosure included IIS, so users shouldn't expect a fix then. Microsoft has said only that it plans to issue a patch as soon as one is ready.
In the meantime, IIS users should follow workarounds that include turning off FTP if it's not needed (in more recent versions it's disabled by default), or at the very least, blocking FTP access to unauthenticated users.
The bug is exploited by listing directories with specially manipulated names that trigger a
buffer stack overflow in the application. The new exploit code is able to cause IIS6 systems to crash, but Microsoft makes no mention those systems can be further compromised. ®