The problem lies in the jar: protocol implementation used by Firefox and it enables an attacker to conduct XSS and gives them almost limitless possibilitys for malware hosting.

This is an example URI which exploits the issue:

jar:http://www.icmpecho.com/myjarshrine/yarihooo.jpg!/malwareloadingscript.html

Now, instead of copying others work which they have probably spent hours or more on to explain the issue in full, I’ll give you a short recap of the happenings and more and more exposing blog posts:

The problems with the recommendations given by these persons and/or organisations is mainly that the recommend blocking URI’s containing JAR: in webfilters and deep packet inspecting firewalls or avoid following “jar:” links.You should understand why this would be a total waste of time if you have read the above articles and in particular Giorgio’s comments on the issue.

Also you should know why if you have seen one page load another like in most web based exploits (Including the one on the Swedish Parliament’s websites this week (swedish link, sorry)). My feeling is that the first advisories were rushed out “to be first in the corporate sector” and sloppy research took its toll.

If you do want to protect yourselves for real, you might wanna download and install the NoScript extension to Firefox which also handles JAR.

Happy times!