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MS insists bodged fix didn't spawn Windows crash risk

Teardrop Explodes

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Microsoft has denied claims that an unpatched flaw in a file and printer sharing feature was inadvertently introduced when it fixed an earlier, less severe problem in the software back in December 2007.

Redmond patched a medium severity flaw in the same Server Message Block (SMB) version 2 functionality within Vista nearly two years ago. Unfortunately, in the process, it allegedly created a much more serious flaw that meant systems running Vista, pre-R2 versions of Windows Server 2008, and earlier versions of Windows 7.

Vulnerable systems can be crashed with only a single malformed packet, according to Laurent Gaffié, the security researcher who discovered the so-called Teardrop attack.

Gaffié claims to have tracked back the latter problem to changes Microsoft made in resolving a flaw in SMBv2 signing nearly two years ago (via MS07-063.mspx).

"The only thing I know regarding this 'patching' process is, when they fixed this code, they opened another bigger, worse security issue," Gaffié told Security Focus.

Microsoft denied suggestions that the earlier patch might be responsible for its latest security pickle. "We researched this claim by the researcher and confirmed this vulnerability was not introduced by MS07-063," said Christopher Budd, security response communications lead for Microsoft.

Redmond earlier acknowledged that, whatever its origins, the SMB flaw is real. As well as posing a system crashing risk, the flaw also poses a code injection danger. Security researchers separately warn that the flaw could lend itself to the creation of network worms.

Pending the availability of a fix from Microsoft, users are advised to apply either apply firewall defences (Block TCP ports 139 and 445) or disable SMBv2 as possible workarounds, as explained in an advisory by the software giant here. ®

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