Microsoft plays down bug panic

Windows exploit code recently released into the wild is causing confusion in the security world, as it seems to overlap with a critical bug Microsoft patched last month.

Contrary to appearances, however, Microsoft has said the proof of concept code exploits a previously undiscovered flaw that just happens to be in the same component as one patched by last month's MS06-035 update.

That means Microsoft didn't slip up in creating its patch; on the other hand, Microsoft confirmed that the bug is exploitable and has not been patched. The bug affects a fully patched Windows XP with SP2, as well as various versions of Windows Server 2003 and Windows 2000.

A user going by the handle "cocoruder" released the exploit code on the milw0rm.com Web site last month, shortly after MS06-035 was published.

Adrian Stone, a program manager with the Microsoft Security Response Center (MSRC), said the exploit code had triggered speculation that MS06-035 hadn't completely fixed a critical flaw in the Server driver (srv.sys).

"While this appears to have been found after the release of MS06-035, this does not affect the same code path or functionality or vulnerability that was addressed by the update," Stone wrote in an MSRC alert on Friday.

He said that while MS06-035 fixed a serious flaw that could allow an attacker to take over a system, the newer bug is less serious, resulting in a denial of service. "At this time we have not identified any possibilities with this issue that could allow remote code execution," he wrote. Microsoft isn't aware of attacks using the flaw, and is working on a fix, he said.

Researchers from Internet Security Systems X-Force pinpointed the bug after an analysis of the exploit code, the company said in a Friday advisory. "Attackers can reliably cause Microsoft Windows to blue screen," X-Force stated in the advisory. "Users must reboot to recover from the crash. An exploit is available in the wild. As of this writing no patch is available for the vulnerability."

The bug is due to a null pointer dereference in srv.sys. Remote attackers can shut down affected Windows systems by sending a specially crafted network packet, X-Force said.

X-Force and Microsoft agreed that the original workarounds detailed in MS06-035 would also work for the new bug - specifically, restricting traffic to ports 135-139 and 445.

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