mozdeco · HackerTrans

vor 2 Monatendiscuss

As described in our blog posts, our harness/pipeline only looks for crashes so all of the bugs resulting from that do have PoCs. There is a smaller number of bugs found by manual auditing that didn't have PoCs but I'd say easily more than 90% of all of the bugs we are talking about had a PoC.

mozdeco

vor 2 Monatendiscuss

> But report [1] says that "Some of these bugs showed evidence of memory corruption...", which implies that majority of these (which includes 271 bugs from Mythos) don't have evidence at all. Do I not understand something?

This is just the standard sentence we've been using for years. It has nothing to do with Mythos and for Mythos, almost all bugs show evidence of memory corruption (we do have a handful of bugs in JS IPC / JS Actors, one is in the blog post).

> Mythos is supposed to be pretty good at writing actual exploits, so (as I understand) there shouldn't be any serious problems with checking if bug is vulnerability or not.

Yes but if we have a choice between writing exploits and scanning more source, potentially finding more bugs, then of course we prioritize the latter.

mozdeco

vor 2 Monatendiscuss

No, it's a new post, see also

https://hacks.mozilla.org/2026/05/behind-the-scenes-hardenin...

mozdeco

vor 2 Monatendiscuss

No, we actually just posted a follow-up story with more details and opened several bugs, see also:

https://hacks.mozilla.org/2026/05/behind-the-scenes-hardenin...

mozdeco

vor 2 Monatendiscuss

Mythos did in fact write PoCs for all bugs that crash with demonstration of memory-unsafe behavior (e.g. use-after-free, out-of-bounds reads/writes, etc).

For us this is substantial enough evidence to consider it a security vulnerability at that point, unless shown otherwise and it has always been this way (also for fuzzing bugs).

mozdeco

vor 4 Monatendiscuss

The bugs are at least of the same quality as our internal fuzzing bugs. They are either crashes or assertion failures, both of these are considered bugs by us. But they have of course a varying value. Not every single assertion failure is ultimately a high impact bug, some of these don't have an impact on the user at all - the same applies to fuzzing bugs though, there is really no difference here. And ultimately we want to fix all of these because assertions have the potential to find very complex bugs, but only if you keep your software "clean" wrt to assertion failures.

The curl situation was completely different because as far as I know, these bugs were not filed with actual testcases. They were purely static bugs and those kinds of reports eat up a lot of valuable resources in order to validate.

mozdeco

vor 4 Monatendiscuss

[working for Mozilla]

That's because there were none. All bugs came with verifiable testcases (crash tests) that crashed the browser or the JS shell.

For the JS shell, similar to fuzzing, a small fraction of these bugs were bugs in the shell itself (i.e. testing only) - but according to our fuzzing guidelines, these are not false positives and they will also be fixed.

mozdeco

vor 4 Monatendiscuss

[work at Mozilla]

I agree that LLMs are sometimes wrong, which is why this new method here is so valuable - it provides us with easily verifiable testcases rather than just some kind of analysis that could be right or wrong. Purely triaging through vulnerability reports that are static (i.e. no actual PoC) is very time consuming and false-positive prone (same issue with pure static analysis).

I can't really confirm the part about "local" bugs anymore though, but that might also be a model thing. When I did experiments longer ago, this was certainly true, esp. for the "one shot" approaches where you basically prompt it once with source code and want some analysis back. But this actually changed with agentic SDKs where more context can be pulled together automatically.

mozdeco

vor 4 Monatendiscuss

And the Firefox side of the story: https://blog.mozilla.org/en/firefox/hardening-firefox-anthro...

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