Security watchers are concerned that a protocol handling flaw in Firefox could have implications for the security of data held within Google and, possibly, other web applications.

The flaw, involving the handling of the "jar:" protocol by Firefox, gives rise to cross-site scripting attacks. No patch is available through there are a number of workarounds (such as blocking URIs that contain "jar:" using a reverse proxy or application firewall). For home users, Secunia advises users to avoid following untrusted "jar:" links or visiting untrusted websites.

The jar: protocol is used to extract and render content from ZIP compressed files. Unfortunately the "jar:" protocol handler in Firefox does not validate the MIME type of the contents of an archive, which are then executed in the context of the site hosting the archive.

The trick might be used to conduct cross-site scripting attacks on sites that allow a user to upload certain files, such as .zip or .png.

As the GNUCitizen blog notes application which allow the upload of JAR/ZIP files are potentially vulnerable to a persistent Cross-site Scripting as a result of the bug. Web mail clients, collaboration systems, document sharing systems are all potential targets on attack.

The attack could be used to steal cookie-based authentication credentials, for example. GNUCitizen has published a proof of concept demo that illustrates how a user's contact book in Gmail might be accessed using the approach.

Suitably the results page of the demo is illustrated with a picture of insufferably annoying (and arguably racist) Star Wars character Jar Jar Binks. ®

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