Microsoft promises patch for critical Web server bug

Unlikely to have IIS fix ready by next Patch Tuesday, says expert

Microsoft yesterday said it is working on a patch for a bug in its popular Web server software, but it's unlikely the company will field a fix fast enough to make next week's regular release, a security expert predicted.

Late Tuesday, Microsoft issued a formal security advisory for a vulnerability in three older editions of Internet Information Services (IIS) server, just hours after confirming that its security team was poking into exploit code that went public Monday.

"Microsoft is currently working to develop a security update to address this vulnerability," the company said in the advisory.

The bug is in the file transfer protocol (FTP) server included in IIS. The FTP server fails to properly parse specially-crafted directory names, which hackers can leverage to force a stack buffer overflow and then inject their own malicious code onto the Web server.

Exploit code was published on the milw0rm.com site early Monday, something that Microsoft took exception to.

"This vulnerability was not responsibly disclosed to Microsoft and may put computer users at risk," said Alan Wallace, a spokesman for the Microsoft Security Response Center (MSRC). "We believe the commonly accepted practice of reporting vulnerabilities directly to a vendor serves everyone's best interests."

It's almost certain that Microsoft won't have a patch ready by Sept. 8, its next regularly-scheduled security update. For its part, the company used boilerplate language to describe when it would fix the flaw. "Microsoft will release the security update once it has reached an appropriate level of quality for broad distribution," the advisory read.

"I don't expect a fix from Microsoft next week," said Wolfgang Kandek, chief technology officer at security firm Qualys. "That's too soon to expect them to have something ready."

Until a patch is available, Microsoft urged administrators responsible for IIS 5.0, 5.1 and 6.0 Web servers to make one of several possible defensive moves, any one of which will stymie the currently-known exploits.

"The end result of the workarounds is to prevent untrusted users from having write access to the FTP service," Bruce Dang and Jonathan Ness, two members of the MSRC engineering team, said in a entry on a company blog Tuesday night.

"Figure out if you need FTP," said Kandek, echoing one of the Microsoft workarounds. "And if you don't, turn it off. You can also disable write access -- that's a pretty smart way to use the OS to harden FTP. Or see if you can turn off anonymous access to FTP."

Kandek said in-the-wild exploits will quickly make their appearance. "It's so easy," he said of the job hackers have to do to "weaponize" the published exploit code. "All they have to do is put a script around [the exploit code]. It's very, very simple to do."

Although IIS 5.0 on Windows 2000, IIE 5.1 on Windows XP and IIS 6.0 on Windows Server 2003 are all vulnerable to attack, Microsoft said that newer versions of the Web server -- IIS 7.0 on Windows Vista and Windows Server 2008, and IIS 7.5 on Windows 7 and Windows Server 2008 R2 -- are not at risk.

Kandek recommended that users implement one of the workarounds rather than think about upgrading to sidestep the problem. "Upgrading means you'll have to test, and there's always the chance that something may break," he said.

Microsoft's IIS server, currently the world's second-most popular Web server, accounts for about 22 per cent of all Web servers, according to U.K.-based Netcraft. The open-source Apache server software, with a 46 per cent share, remains the top choice.

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Gregg Keizer

Computerworld
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