Ronald sent us a PoC DoS exploit, which uses the recently discussed Firefox 1.5.0.3 image issue.
His prove of concept exploit will use javascript to generate image tags with 'mailto:' link, which in turn will open the mail application automatically without any user interaction. As a result, many mail windows (e.g. Outlook) will be opened and the system will become unresponsive.

One possible workaround is to turn off automatic startup of your e-mai application in Firefox. To do so, enter in the URL bar: about:config . This will show a long list of configuration options. Search for 'warn-external.mailto' (e.g. use the 'Filter' option). By default, this value should be set to "false". Click on the line to toggle it to "true" (it will be bold if it is not set to the default).

Now, whenever you click on a mailto: link, you will first be asked if you would like to start your e-mail application. In the case of the exploit this will keep your system responsive, even though you may still have to click on all the dialogs.

Disabling javascript is another option, or disabling mailto: link all together. But these options are more intrusive.

For more details and a link to a PoC, see securityview.org