This story was updated on 28th September to report that the vulnerability has been patched.

Yesterday, we reported on an unholy trinity of Google vulnerabilities that put emails, private photos and website security at risk. Today came word of a new weakness that makes it easy for bad guys to silently put a backdoor in Gmail accounts.

The technique comes courtesy of Petko D. Petkov, a researcher at GNU Citizen, who writes in a blog post that the backdoor is installed simply by luring a victim to a specially crafted website while logged in to Gmail. The naughty site uses a sleight of hand known as a multipart/form-data POST, which writes a filter to Gmail that causes all email with attachments to be forwarded to collect@evil.com.

Petkov didn't provide a proof of concept or detailed documentation, but Ryan Naraine of the Zero Day blog writes here that the exploit was demonstrated for him. The bug "is particularly nasty because of the way the exploit works without any user action and the fact that it’s difficult for the average Gmail user to know that emails are being stolen," he writes.

Users aren't likely to notice a filter has been added unless they think to check the "Filters" section of their Gmail Settings.

A Google spokesman on Thursday said company engineers had repaired the problem.

"We worked quickly to address the recently reported vulnerability, and we rolled out a fix," he said. "We have not received any reports of this vulnerability being exploited."

Petkov's discovery is just the latest way people can be burned when entrusting their personal data to Google. Yesterday, various researchers showed how a vulnerability in the Google search appliance, which the company sells to webmasters, can be used to inject code and overwrite content on the pages of third-party sites. Another flaw made it possible to steal photos designated as private on the Picasa application.

Google also suffered from a vulnerability that resided in the so-called polls application, a part of Google Groups, that made it possible to steal contacts and messages from Gmail accounts. A Google spokesman on Monday afternoon said the flaw had been fixed. ®