Controversy over IE-to-Firefox exploit

MS and Mozilla in row over blame for cross-browser attack.

An exploit which involves browsing to a malicious website using Internet Explorer, but then launches an attack via a loophole in Mozilla Firefox, is causing controversy over which piece of software is 'vulnerable'.

The attack, apparently discovered simultaneously by two separate groups of researchers, uses the 'FirefoxURL' function, which can allow executable code to run from a remote source. When such a call is made from within Firefox security measures detect the action as suspicious and prevent it, but Explorer accepts the command and passes it straight to the system, allowing malicious code to be run without restraint. The attack is similar to a problem found with the Windows version of the Safari browser some time ago.

While one of the researchers (whose release on the discovery is here) describes the problem as an 'Internet Explorer 0day', vulnerability specialist Secunia has issued an alert, labelled 'Highly Critical', which puts the blame for the flaw on Firefox (see here).

Mozilla has released a statement (here) saying that, while it believes the problem is down to Internet Explorer passing on dangerous data, it nevertheless plans to issue a fix for the problem. According to a report in The Register here, Microsoft has denied that its browser is at fault and has no plans to make changes to the behaviour.

Windows security expert Jesper Johansson has said that IE is dealing appropriately with the data passed to it. Johansson was unable to get the exploit to work - his blog entry on the matter, here, includes instructions for disabling the dangerous functionality in Firefox. Another sample exploit is online here. The problem is thought only to affect Firefox version 2.0.2 or later.

11 July 2007

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