MessengerFX’s security problem corrected

Some times ago I found a Cross Site Scripting vulnerability in MessengerFX, a popular web-based Windows Live Messenger client. Friday I received from the team saying that the problem is now corrected:

Hi Kevin,
First of all i want to thank you for your warn. We fixed that problem and
now its working correctly.

[…]
If you find any other problem please let me know. Thanks again.

It was serious : Every software’s feature is available through Javascript. Any contact of a MessengerFX user can crash his browser, and futhermore get its contact list, add, remove, ban and unban contacts, read and send messages to any other contact of the victim ! Basically, an attacker just need to be listed in the contacts list of an MessengerFX user and this attacker can take control over the account.

In fact, all Javascript code is now removed server-side, so it’s impossible to send some snippets to a friend and the code is still executed locally (in the browser of the sender). The team explain that a new version of their app will be released soon and will better handle things like this.

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