Microsoft is investigating a new zero-day flaw in Internet Explorer that could put fully-patched Windows systems at risk of takeover. Less serious bugs have also been reported in the Firefox and Safari browsers.
The IE bug, discovered by Michael Zalewski and posted on the full-disclosure mailing list on Sunday, could allow attackers to take over a system, Zalewski said.
"Perhaps not surprisingly, there appears to be a vulnerability in how Microsoft Internet Explorer handles (or fails to handle) certain combinations of nested OBJECT tags," Zalewski wrote. "At first sight, this vulnerability may offer a remote compromise vector, although not necessarily a reliable one... As such, panic, but only slightly."
Danish security firm Secunia, which maintains a vulnerability database, flagged the bug as "highly critical", noting that "successful exploitation allows execution of arbitrary code". The bug has been confirmed on fully patched Windows systems with IE 6 and Windows XP SP2, but other versions may be affected, Secunia said.
France's FrSIRT also gave the bug a "critical" rating. "This flaw is due to a memory corruption error when processing a specially crafted HTML script that contains malformed "object" tags, which could be exploited by attackers to remotely take complete control of an affected system by convincing a user to visit a specially crafted Web page," the company said.
Microsoft said it is investigating the report, but downplayed its significance, saying its investigations so far have shown that the bug is likely only to cause IE to crash.
No proof of concept code has been published to date, according to researchers.
The bug in Safari, also non-critical, is due to a problem in processing some "td" HTML tags, Secunia said. "This can be exploited to consume a large amount of CPU and memory resources on a vulnerable system by tricking a user into visiting a malicious website," Secunia said in an advisory.
Both the Firefox and Safari bugs are as yet unpatched.