Hackers can exploit an unpatched flaw in Microsoft's Internet Explorer browser to access Gmail accounts, according to security firm Cenzic.

Cenzic has warned Internet Explorer users than the browser contains an unspecified cached files bug that, when combined with a cross-site request forgery flaw in Gmail, exposes the webmail account sign-ons and lets others access those accounts and any messages or file attachments there.

Although not a bug that can be exploited remotely - an attacker must have local, physical access to the PC - as Cenzic pointed out, there are scenarios where that's not a limitation. "These vulnerabilities could be exploited such that all users of a shared computer, who use Internet Explorer and share a user account - a common practice at computer kiosks in a library or internet cafe - could be vulnerable," said Cenzic.

Gmail contributes to the overall vulnerability because its URLs display attachments when viewed using the 'View Source' command, the warning added. Internet Explorer, however, sports "improper use of caching directives [and] incorrect access checks on cached Internet Explorer files".

Together, the bugs could conceivably let someone at a public PC hijack any Gmail log-on credentials that had been entered on the machine since the Internet Explorer cache had last been purged. Internet Explorer deletes the contents of its cache only as new files are added - and the oldest are deleted - or when the user explicitly instructs the browser to clear the cache using the 'Delete Browsing History' command.

However, Microsoft denied that Internet Explorer even has a bug. "Microsoft has thoroughly investigated the claim and found that this is not a product vulnerability," said a company spokesman. "In the scenario in question, an attacker would need authenticated access to the system in order to modify files located in the cache. With that level of access, an attacker could install malicious programs that would have more impact than the scenarios described."

While true, that did not address Cenzic's scenario; by design, public PCs such as those in libraries, schools or web cafes do not require authentication to access. Microsoft was not available for clarification.

Google did not immediately respond to questions about Gmail's part in the threat.

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