IE 7 bug reopens debate over patch responsibilities

Researchers argue over who to blame; Microsoft again denies there's a bug

Security researchers are again arguing over who is responsible -- Microsoft or third-party developers -- for protocol-handling bugs after a researcher said late last week that Internet Explorer 7 can be used to trick users into launching malware.

Posting to the Full Disclosure mailing list, Juergen Schmidt, a researcher at Heise Security, blamed IE 7 for passing invalid Uniform Resource Identifiers (URI) to Windows XP. Specifically, said Schmidt, IE 7 accepts URLs from other applications that include the "%" [percent] character, which can launch software or scripts on users' machines if they click on a malformed link.

According to Schmidt and others, the earlier IE 6 doesn't have the bug, indicating that something broke between versions. "Post-IE7 has a flaw/threat/vulnerability it hasn't had pre-IE7," said Thierry Zoller, a penetration tester at German security firm n.runs.

Windows' URI protocol handling, the technology that lets browsers run other programs via commands in the URL, has been criticized since July, when Norwegian researcher Thor Larholm demonstrated how IE and rival Firefox could be used to run malicious code. Even then, researchers feuded over responsibility. Mozilla patched Firefox several days later, but Microsoft declined to fix IE, saying that it didn't consider the issue a vulnerability in its software.

Schmidt identified several applications, including Adobe System's Acrobat Reader, the Netscape browser and Miranda, an IM client, that he said improperly handle URIs with the percent symbol, and he hinted that there were plenty more.

His post drew reaction on Full Disclosure. "The applications are accepting arbitrary input and not validating correctly," said Roger Grimes, a security consultant who said he works at Microsoft. "How is that a Microsoft or Windows problem? How could Microsoft determine ahead of time what is and isn't [a] legitimate character to pass to applications they don't own?"

"How is that _not_ a Windows Problem?" replied Zoller. "It's not that they should decide what to pass or not to pass on, the problem in the example Juergen sent is [that] they pass internally, not to third-party applications."

"If the application is what exposes the URI-handling routine to untrusted code from the Internet, then it's the application's job to make sure that code is trusted before exposing system components to its commands, no?" asked another user who went by the name "Geo."

Microsoft denied responsibility for any vulnerability in July and repeated that to Schmidt after he asked if the company's security center would address the problem. "After its thorough investigation, Microsoft has revealed that this is not a vulnerability in a Microsoft product." The company was not available early this week to confirm that its previous comment remains its official position.

Last summer, however, an IE program manager said it would be "very difficult" to retroactively add checks for possibly invalid URIs and, citing the "limitless variety of applications and their unique capabilities," pointed to the those applications as the real source of the problem. "It is the responsibility of the receiving (called) application to make sure it can safely process the incoming parameters," said Markellos Diorinos.

Some security researchers don't see it the same way. For example, Symantec issued an alert to customers of its DeepSight threat system, warning them of the bug and putting the onus on Microsoft. "This issue is due to a flaw in Microsoft Windows when it attempts to determine which application should be utilized when interpreting protocol-handlers such as 'mailto:', 'http:', and others," the alert read.

"The fundamental flaw here is that Windows' built-in URI handler doesn't invoke external programs correctly, resulting in a shell-injection attack," argued Glynn Clements on the Full Disclosure thread begun by Schmidt. "Modifying individual programs to protect against a shell-injection bug in Windows' URI handler is a work-around, not a fix."

According to a notice it gave last week, Microsoft will patch Internet Explorer tomorrow to fix what it has described as a "critical" vulnerability. Although the expected security update will include changes to IE 7, Microsoft does not specify the vulnerabilities it will address before it posts its patches.

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Gregg Keizer

Computerworld
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