A serious flaw in how Firefox handles log-ons could be used by identity thieves to dupe users into disclosing passwords, a noted security researcher said Wednesday.

Aviv Raff, an Israeli researcher best known for ferreting out browser flaws, 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 2.0.0.11 -- Mozilla Corp.'s most current version -- fails to sanitize single quotation marks and spaces in what's called the "Realm" value of an authentication header. "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 e-mail 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 e-mail 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 Inc.'s Checkout payment system; it can be downloaded from here.

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

The company last patched Firefox in late November when it updated the browser to 2.0.0.11. Thursday, Mozilla's chief of security, Window Snyder, would only say that her team is investigating Raff's claims.