A serious flaw in the way Firefox handles log-ons could be used by identity thieves to dupe users into disclosing passwords, according to Aviv Raff, an Israeli researcher best known for ferreting out browser flaws.

Raff revealed the Firefox spoofing vulnerability on his personal blog, and posted a demonstration video there. He did not go public with any proof-of-concept code or working exploit, however.

According to Raff, Firefox - Mozilla's most current version - fails to sanitise single quotation marks and spaces in what's called the "Realm" value of an authentication header. Visit Security Advisor for the latest PC security news, reviews, tips and tricks.

"This makes it possible for an attacker to create a specially crafted Realm value which will look as if the authentication dialog came from a trusted site," said Raff.

Raff outlined a pair of possible attack vectors. One would rely on a malicious site that included a link to a trusted site - a well-known bank, say, or a web email service such as Gmail or Hotmail - that when clicked would display its usual log-on dialog.

In the background, however, the attacker would have crafted a script that exploited the Firefox vulnerability to redirect the username and password entered by the user to the hacker's server instead of the real deal.

Alternately, a rigged image could be delivered via email or embedded in a blog or MySpace page that when clicked generated a legitimate-looking log-on dialog.

Raff's video - a lower-resolution version is on YouTube - shows a spoof of Google's Checkout payment system.

"Until Mozilla fixes this vulnerability, I recommend not to provide username and password to websites which show this dialog," said Raff in his blog.

The company last patched Firefox in late November when it updated the browser to Thursday, Mozilla's chief of security, Window Snyder, would only say that her team is investigating Raff's claims. Visit Broadband Advisor for the latest internet news.