Google has fixed a vulnerability in its popular GMail web mail service that creates a means for hackers to steal users' contact lists.

The cross-site scripting flaw stemmed from the decision by GMail to store contact lists in a JavaScript file. GMail always saves contact lists as JavaScript code using the same URL, so a script featuring this URL can read out the fields of a users' contact list. GMail failed to check what sites were attempting to run this "callback" function.

As a result users logged into GMail, or other Google services sharing the same login, are liable to hand over their contact list to spammers or other miscreants providing they are tricked into visiting a maliciously constructed website. Exploitation would have been as simple as fooling users into visiting a hostile website through spam messages sent to users' email accounts.

The flaw, similar to previous problems with GMail, was first reported by Googlified. The security bug was introduced through an unannounced feature on Google Video that was designed to allow users' to email clips to their contacts more easily.

Coders failed to take into account that it was a bad idea to save sensitive data as JavaScript, under predictable URLs, a problem Google watchers spotted shortly after Google made the coding changes last week. ®

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