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Microsoft has promised to patch a serious flaw in newer versions of its Windows operating system after hackers released exploit code that allows them to take complete control of the underlying machines.

The flaw, which affects various versions of Windows Vista, 2008, and the release candidate version of Windows 7, resides in the implementation of a network file sharing technology known as SMB, or server message block. The bug, which fails to adequately parse network negotiation requests, was previously believed only to generate a debilitating blue screen of death, but on Tuesday, Microsoft confirmed in some cases it could also be used to remotely execute malicious code on vulnerable machines.

"This is a common practice at Microsoft of discovering critical software vulnerabilities in the latest releases and never back porting them to older (still supported) versions [and] therefore leaving customers hung out to dry," said Marc Maiffret, who as a 20-something year-old hacker, first spotted the vulnerability that led to the devastating Code Red Worm in 2001. He is now director of professional services at The DigiTrust Group.

"Also it is interesting that the vulnerability affects SMB as that was new to Vista and we can therefore assume had been through most of their strict code auditing standards yet we see again things are going to be missed, even extremely critical ones," he added.

To be fair, most attempts to exploit the bug will result in a simple crash of the machine, according to an advisory Microsoft published on Tuesday. What's more, the invulnerability of Windows 7 and Server 2008 R2 suggests Microsoft's security team is at least partially on top of the bug.

Still, the advisory means that at present there are at least two zero-day vulnerabilities in Microsoft products that are relied on by large business customers. In an updated advisory published Friday, company researchers said they were seeing "limited attacks" targeting a file transfer protocol component in the Internet Information Services webserver. For the most part, the attacks only cause vulnerable machines to crash, but an older version, IIS5, can be exploited to remotely execute malicious code.

Microsoft's Patch Tuesday came and went this week without a fix for either vulnerability. With exploit code released for both, don't be surprised if Microsoft issues an unscheduled update in the next couple of weeks.

In the meantime, admins should prevent attacks targeting SMB by disabling the service. If that's not possible, the two TCP ports used by the service, 139 and 445, should be blocked at the firewall. IIS users should protect themselves by turning off FTP if it's not needed, or at the very least, blocking FTP access to unauthenticated users. ®

This article was updated to correct the name of the vulnerable service. It's SMB, or server message block.

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