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Microsoft's DNS Patch Flawed, Security Official Says

nCircle's Tyler Reguly discovered a vulnerability that renders the Microsoft's DNS patch virtually useless if a server has already been compromised.

Patch Tuesdays usually bring a nightly ritual of "coffee, bad jokes and really bad music" for Tyler Reguly, senior security engineer at San Francisco-based nCircle. However, this time, the routine was interrupted by his company's discovery of a major flaw in a patch for Microsoft's Domain Name System (DNS) and Windows Internet Name Service (WINS) servers.

The patch (MS09-008) had just been released this week as part Microsoft's monthly security rollout. What Reguly discovered was a vulnerability that renders the new patch virtually useless if a server has already been compromised.

The vulnerability allows users to set Web Proxy Automatic Discovery (WPAD) program entries in DNS when "dynamic updates are enabled." Dynamic updates allow a workstation to send messages to the DNS server to provide an IP address. Internet Explorer then attempts to download proxy settings from the server, which if infected, could allow the hacker to jump into the process.

Reguly described this kind of attack as a "man-in-the-middle" hack. He goes into greater technical detail describing the problem in his blog entry here.

"Like any [Patch] Tuesday, we sit and do an all-nighter and analyze the patches," he recalled in a phone interview on Thursday afternoon. "I happened to draw the straw, I guess, and I just noticed that when we were doing testing, we discovered that if we tested against a vulnerable host server, the patch doesn't work as prescribed."

After sending Microsoft an e-mail on Tuesday night, Reguly posted the blog entry to inform people of the issue. On Wednesday, Microsoft replied, saying that the patch achieved its intended functionality.

Reguly and others contend that without a proper response from Microsoft, it becomes race as to whether or not this vulnerability will be patched first or exploited first.

"So I'm the hacker sitting there waiting and everything you do via Internet Explorer is now coming over to my computer and I have full-access to everything you do," Reguly said. "The impact of a successful man-in-the-middle attack can result in stolen passwords in addition to monitoring and redirecting web traffic to sites containing malicious code."

The DNS flaw got attention last summer when Dan Kaminsky, a researcher at security firm IOActive Inc., announced a new-found vulnerability. Problems with WPAD entries go back further still, to 2002.

Two years ago, a hacker named Beau Butler told Australian media that Microsoft did not respond to e-mails regarding the existence of a security bug exploiting Internet Explorer via WPAD programs. However, by that time, the bug had been around for about five years.

Microsoft has yet to comment on Reguly's discovery or admit to flaws in the patch. In Reguly's estimation, Microsoft seems satisfied with the steps it took. The vulnerability exists simply because DNS can't tell the difference between a valid WPAD entry and a malicious one, he added.

With that in mind, Reguly said, "I don't think the response [from Microsoft] was sufficient. I think the ball has been dropped on this one."

About the Author

Jabulani Leffall is an award-winning journalist whose work has appeared in the Financial Times of London, Investor's Business Daily, The Economist and CFO Magazine, among others.

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