Google has confirmed -- and I've verified -- that a fix for GMail has been distributed to block a flaw that allows hackers to hijack e-mail messages.
The cross-site request forgery exploit, discovered and partially disclosed by GNUCitizen's Petko D. Petkov, gave attackers an easy way to plant GMail filters to forward incoming mail to a third-party (hacker-controlled) e-mail address.
Even after Google's fix, GMail users are strongly encouraged to check their filter lists because the patch does not remove the rigged filter.
Remember, GMail filters are not the same as labels, which appear on the left pane of the GMail interface.
First, click on Settings in the top-right corner and then the Forwarding and POP tab to ensure that e-mail forwarding is either disabled or not hijacked to send your e-mail elsewhere.
Then, in the Filters tab, look carefully for any strange filter that may compromise your mailbox.