X-windows, with or without the font server (XFS) running can be crashed remotely via Mozilla when fonts are set to an unnaturally large size with CSS (Cascading Style Sheets), Tom Vogt of Lemuria.org has reported.
An X bug allows all available memory to be consumed, which causes the system to freeze. The behavior can be duplicated with applications like the Gimp, we're told, but these aren't remotely exploitable. But with Mozilla, a pest can easily set up a malicious Web site which will crash unsuspecting Tuxers' boxen and cause any unsaved data in open apps to go away.
I've confirmed it on Mozilla 0.9.4 with XFree86 4.2.0. It takes all of three seconds to immobilize a machine, and I was unable to kill X or reboot from the keyboard. It affects all known versions of Mozilla, numerous platforms, and is not limited to XF86.
If you wish to give it a go, simply paste this URL http://www.adeliesolutions.com/Projects/ into your Mozilla address bar and enjoy the show. Do make certain that you have no unsaved work, because in most cases a cold-boot is all that will bring the system back under control. Some experimenters have reported different effects, depending on the system configuration and combination of kernel and X versions, but most have met with complete immobilization.
Mozilla has opened a bug report here and work is being done, presumably to limit font sizes. However, Mozilla is merely a vector; the core problem lies somewhere in the way X handles outrageously large fonts. But we're optimistic; it shouldn't require too much rocket science to patch it so that it generates an error instead of attempting the impossible. ®