More discussion on the Firefox 1.5.0.3 “image bug”
There’s quite a bit more discussion on a DOS bug in Firefox 1.5.0.3, the link goes to a site where they’ve confirmed the issue and there is a link there to a POC, so be cautious. It turns out that using javascript, image tags can be made to have a mailto: link which can automatically launch tons of instances of whatever default mailhandler a system has (essentially one for each image tag.) Right now, this sounds more like a Denial of Service risk, as I don’t see at this point any evidence that anything WORSE could be done than really freezing up the system with too many copies of the mail program open.
There have been rumblings about this since at least May 6th… Sans (Incidents.org) had this to say about a workaround…
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.
It doesn’t seem to be a high risk vulnerability – but a low danger, annoyance denial of service risk.