Update: Microsoft to patch Windows Uniform Resource Identifier bug

Computerworld - After several months of claiming that fixing a critical flaw was not its job, Microsoft Corp. reversed direction yesterday and acknowledged a bug in Windows XP and Windows Server 2003 that could let attackers hijack machines.

In a security advisory issued yesterday, Microsoft conceded that Windows XP and Server 2003 running Internet Explorer 7 (IE7) mishandle some Uniform Resource Identifiers (URI). Hackers can craft malformed URIs that, when clicked, launch malicious code or script. Only PCs with IE7 are at risk, Microsoft maintained, and Vista is invulnerable. Ironically, just last week, Microsoft stripped an antipiracy requirement from IE7 downloads, a move seen by some as an attempt to get that version on more machines.

Although the advisory didn't promise a patch, a posting to the Microsoft Security Response Center blog did. "We're working on a security update," the MSRC's Jonathan Ness said in a post yesterday afternoon. "Our plan is to revise our URI handling code [within Windows]."

Microsoft did the turnabout because it believed that recent public discussions about the vulnerability had increased the chance that the bug would be exploited by attackers, said Ness. Those discussions, led by Juergen Schmidt of Heise Security on a security mailing list, reopened debate about whether Microsoft was responsible for ongoing problems Windows has when handling protocols; Schmidt claimed that IE7 had broken Windows XP's ability to sanitize and reject invalid URIs associated with the mailto: protocol.

In a long and sometimes confusing post, Ness described how in an attempt to make IE7 more secure, Microsoft instead made it possible for the browser to hand off an invalid URI to ShellExecute, the Windows function that runs code. "IE7 began to do more validation up front to reject malformed URIs," Ness said. "When this malformed URI with a % was rejected by IE7, ShellExecute() tries to 'fix up' the URI to be usable. During this process, the URI is not safely handled."

According to Ness, Microsoft will update Windows XP and Server 2003 so that the URI handling within ShellExecute is "more strict."

Even so, Microsoft isn't ready to let third-party developers off the hook. "While our update will help protect all applications from malformed URIs, application vendors that handle URIs can also do stricter validation themselves to prevent malicious URIs from being passed to ShellExecute()," he said, noting that several vendors have done just that.

Mozilla Corp. and Skype, for instance, updated their best-known applications, Firefox and the namesake VoIP software, respectively, in July. At the time, however, both companies complained that the vulnerabilities they fixed were not completely their fault and said Microsoft should share some of the blame. More recently, Adobe Systems Inc. said last week that it will patch its Reader and Acrobat products, and offered a workaround defense users can apply until then.