Announcing cross_fuzz, a potential 0-day in circulation, and more

Type securityvulns
Reporter Securityvulns
Modified 2011-01-03T00:00:00


Hi list,


I am happy to announce the availability of cross_fuzz - an amazingly effective but notoriously annoying cross-document DOM binding fuzzer that helped identify about one hundred bugs in all browsers on the market - many of said bugs exploitable - and is still finding more.

The fuzzer owes some of its efficiency to dynamically generating extremely long-winding sequences of DOM operations across multiple documents, inspecting returned objects, recursing into them, and creating circular node references that stress-test garbage collection algorithms. More info about the exact algorithm used is given here:

The design of the fuzzer makes it unexpectedly difficult to get clean, deterministic repros; to that effect, in the current versions of all the affected browsers, we are still seeing a collection of elusive problems when running the tool - and some not-so-elusive ones. I believe that at this point, a broader community involvement may be instrumental to tracking down and resolving these bugs.

I also believe that at least one of the vulnerabilities discovered by cross_fuzz may be known to third parties - which makes getting this tool out a priority.


  • Internet Explorer: MSRC notified in July 2010. Fuzzer observed to trigger several exploitable crashes - e.g.: well as some security-relevant GDI corruption issues.

Reproducible, exploitable faults still present in current versions of the browser. I have reasons to believe that one of these vulnerabilities is known to third parties:

Comment: Vendor has acknowledged receiving the report in July (case 10205jr), but has not contacted me again until my final ping in December. Following that contact attempt, they were able to quickly reproduce multiple exploitable crashes, and asked for the release of this tool to be postponed indefinitely. Since they have not provided an explanation as to why these issues could not be investigated earlier, I refused; more info here:

  • All WebKit browsers: WebKit project notified in July 2010. About two dozen crashes identified and addressed in bug 42959 and related efforts by several volunteers. Relevant patches generally released with attribution in security bulletins. Some extremely hard-to-debug memory corruption problems still occurring on trunk.

  • Firefox: Mozilla notified in July 2010. Around 10 crashes addressed in bug 581539, with attribution in security bulletins where appropriate. Fuzzing approach subsequently rolled into Jesse Ruderman's fuzzing infrastructure under bug 594645 in September; from that point on, about fifty additional bugs identified (generally with no specific attribution at patch time). Several tricky crashes still occurring on trunk.

Note: Flash-related (npswf32.dll) bad read/write offset crashes are also common if the plugin is installed.

  • Opera: vendor notified in July 2010. Update provided in December stated that Opera 11 fixes all the frequent crashes, and that a proper security advisory will be released at a later date. Release notes state: "Fixed a high severity issue; details will be disclosed at a later date". Several hard-to-debug crashes reportedly still waiting to be resolved.

Note that with Opera, the fuzzer needs to be restarted frequently due to OOM conditions.


Please see:


Cross_fuzz can be easily extended to fuzz any DOM-enabled documents or browser plugins simply by providing new target documents. This may be an interesting area for future research.

I believe that releasing the tool at this point is considerably more prudent than the approach taken with ref_fuzz in 2008-2010:

For updates, you can actually follow me on Twitter (gasp):