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Mozilla Used Anthropic's Mythos to Find and Fix 271 Bugs in Firefox

41 points by cpeterso 6mo ago

FireBeyond 6mo ago
Apropos of anything else, I do like that if one of the big bullet points of Mythos is security, that in their list of "preview users" Anthropic chose orgs like Firefox who might have the largest blast radii, and are the most tempting of targets.
ray_v 6mo ago
As my coworker succinctly put it, "nobody uses Firefox anymore." I don't know if hundreds of millions of people is exactly, "nobody" but I personally agree that open source software is just going to crush closed source for exactly the reasons we're seeing unfold in front of us; you can audit and correct incorrect behavior for the benefits of all.
- UltraSane 6mo ago
  I use Firefox + uBlock Origin because it give me complete control over what I see.
  - mrandish 6mo ago
    Same.
- pants2 6mo ago
  For closed-source, I'd expect defenders to have a greater advantage because they can run Mythos on the source code, while attackers only get an opaque API/protocol to try messing with.
  - sanxiyn 6mo ago
    To some extent yes, but models are good at reverse engineering such that it isn't as great advantage as you might think.
  - ygjb 6mo ago
    There is definitely a closed-source defender advantage where an attacker doesn't have access to the code, binary, or environment that can be instrumented (so basically, running in the cloud), but there have been several very effective technical demonstrations of LLM guided or agentic approaches to assessing the security of closed source tools, and I have had some successes personally using LLMs with tool use to manage binary analysis tools to perform reverse engineering of closed source packages. For many attack scenarios the boundary is really if you can establish an effective canary or oracle for determining if a change in input results in a change in output, once you have that, it's simply a matter of scaling your testing or attack (for fuzzing, for blind injection, or any other number of attacks that depend on getting signal from a service).
- wookmaster 6mo ago
  Idk Mozilla has its issues but I still primarily use Firefox and librewolf on my Linux desktop. I refuse to use chrome except in instances where necessary.
yborg 6mo ago
What they did not say is how many of these vulnerabilities were addressed by LLM-created fixes, if any.
- bvisness 6mo ago
  I can only speak for SpiderMonkey, as that’s the team I’m on, but we humans are definitely writing and reviewing the patches for these bugs. Sometimes the AI suggestions are good, often they’re not, and we never send off a fix for a security bug unless we thoroughly understand the problem and have assessed its severity ourselves.
SpicyLemonZest 6mo ago
Big news here, I think, is that they agree with Anthropic's prediction that it's a transitory issue, and expect to come out the other end more secure after fixing a finite number of bugs. Not looking forward to my turn at the firehose, but it could have been a lot worse.
kajman 6mo ago
So where are they, then? Am I misunderstanding the process and this stuff is kept under wraps even after release? There's three CVEs in today's security advisory that mention Anthropic. https://www.mozilla.org/en-US/security/advisories/mfsa2026-30/ https://www.mozilla.org/en-US/security/advisories/mfsa2026-3... There's also no write-up I can see that distinguishes to what extent this is the work of the seven people credited alongside Mythos.
- sanxiyn 6mo ago
  Last three CVEs are collections of bugs. CVE-2026-6784 is a collection of 55 bugs. CVE-2026-6785 is a collection of 154 bugs. CVE-2026-6786 is a collection of 107 bugs. As for credits, I think bugs are ultimately credited to people, and this time Mozilla people used Mythos, as opposed to Anthropic people using Opus or Mythos.
ChrisArchitect 6mo ago
Source: https://blog.mozilla.org/en/firefox/ai-security-zero-day-vulnerabilities/ https://blog.mozilla.org/en/firefox/ai-security-zero-day-vul...
lefty2 6mo ago
I wonder how many false positives there were. Typically this types of static analysis tools come up with a ton of potential bugs, but only a few of them are actual bugs.
- mccr8 6mo ago
  The basic technique (as has been publicly described by Anthropic) is you ask one agent to come up with a test case that triggers, say, an ASan use-after-free. Then you have a second agent that validates the test case. This eliminates a lot of false positives. It gets a little tricky when you allow the first agent to modify the code, which is necessary for things like sandbox escapes where you want to demonstrate that sending bad IPC causes problems.
  - ygjb 6mo ago
    And the following part that is more important is the agent that attempts the fix in code, the agent that tests the fix and reports on perfomance and functional impacts, and the agent the triggers the build and release to production. Everything up to finding and validating the bug is a huge win in vuln/exploit development, everything after validating the bug is a huge win for defensive security and a massive gap until the tools are generally available :S
totallyrandom__ 6mo ago
Anthropic said they used 1000 agents worth $20k of token to discover "several dozens" of vulnerabilities in OpenBSD, of which only one was cool enough to mention it and brag about. That's not including the cost of 196 reviewers they also used. The question is, if Firefox was given $20k worth of credit to find these vulnerabilities, how many vulnerabilities could have been discovered by paying that much money to security researchers who wouldn't have needed additional reviewers?

41 points by cpeterso 6mo ago

Apropos of anything else, I do like that if one of the big bullet points of Mythos is security, that in their list of "preview users" Anthropic chose orgs like Firefox who might have the largest blast radii, and are the most tempting of targets.

ray_v 6mo ago

As my coworker succinctly put it, "nobody uses Firefox anymore." I don't know if hundreds of millions of people is exactly, "nobody" but I personally agree that open source software is just going to crush closed source for exactly the reasons we're seeing unfold in front of us; you can audit and correct incorrect behavior for the benefits of all.

UltraSane 6mo ago
I use Firefox + uBlock Origin because it give me complete control over what I see.
- mrandish 6mo ago
  Same.
pants2 6mo ago
For closed-source, I'd expect defenders to have a greater advantage because they can run Mythos on the source code, while attackers only get an opaque API/protocol to try messing with.
- sanxiyn 6mo ago
  To some extent yes, but models are good at reverse engineering such that it isn't as great advantage as you might think.
- ygjb 6mo ago
  There is definitely a closed-source defender advantage where an attacker doesn't have access to the code, binary, or environment that can be instrumented (so basically, running in the cloud), but there have been several very effective technical demonstrations of LLM guided or agentic approaches to assessing the security of closed source tools, and I have had some successes personally using LLMs with tool use to manage binary analysis tools to perform reverse engineering of closed source packages. For many attack scenarios the boundary is really if you can establish an effective canary or oracle for determining if a change in input results in a change in output, once you have that, it's simply a matter of scaling your testing or attack (for fuzzing, for blind injection, or any other number of attacks that depend on getting signal from a service).
wookmaster 6mo ago
Idk Mozilla has its issues but I still primarily use Firefox and librewolf on my Linux desktop. I refuse to use chrome except in instances where necessary.

yborg 6mo ago

What they did not say is how many of these vulnerabilities were addressed by LLM-created fixes, if any.

bvisness 6mo ago
I can only speak for SpiderMonkey, as that’s the team I’m on, but we humans are definitely writing and reviewing the patches for these bugs. Sometimes the AI suggestions are good, often they’re not, and we never send off a fix for a security bug unless we thoroughly understand the problem and have assessed its severity ourselves.

SpicyLemonZest 6mo ago

Big news here, I think, is that they agree with Anthropic's prediction that it's a transitory issue, and expect to come out the other end more secure after fixing a finite number of bugs. Not looking forward to my turn at the firehose, but it could have been a lot worse.

kajman 6mo ago

So where are they, then? Am I misunderstanding the process and this stuff is kept under wraps even after release? There's three CVEs in today's security advisory that mention Anthropic. https://www.mozilla.org/en-US/security/advisories/mfsa2026-30/ https://www.mozilla.org/en-US/security/advisories/mfsa2026-3... There's also no write-up I can see that distinguishes to what extent this is the work of the seven people credited alongside Mythos.

sanxiyn 6mo ago
Last three CVEs are collections of bugs. CVE-2026-6784 is a collection of 55 bugs. CVE-2026-6785 is a collection of 154 bugs. CVE-2026-6786 is a collection of 107 bugs. As for credits, I think bugs are ultimately credited to people, and this time Mozilla people used Mythos, as opposed to Anthropic people using Opus or Mythos.

ChrisArchitect 6mo ago

Source: https://blog.mozilla.org/en/firefox/ai-security-zero-day-vulnerabilities/ https://blog.mozilla.org/en/firefox/ai-security-zero-day-vul...

lefty2 6mo ago

I wonder how many false positives there were. Typically this types of static analysis tools come up with a ton of potential bugs, but only a few of them are actual bugs.

mccr8 6mo ago
The basic technique (as has been publicly described by Anthropic) is you ask one agent to come up with a test case that triggers, say, an ASan use-after-free. Then you have a second agent that validates the test case. This eliminates a lot of false positives. It gets a little tricky when you allow the first agent to modify the code, which is necessary for things like sandbox escapes where you want to demonstrate that sending bad IPC causes problems.
- ygjb 6mo ago
  And the following part that is more important is the agent that attempts the fix in code, the agent that tests the fix and reports on perfomance and functional impacts, and the agent the triggers the build and release to production. Everything up to finding and validating the bug is a huge win in vuln/exploit development, everything after validating the bug is a huge win for defensive security and a massive gap until the tools are generally available :S

totallyrandom__ 6mo ago

Anthropic said they used 1000 agents worth $20k of token to discover "several dozens" of vulnerabilities in OpenBSD, of which only one was cool enough to mention it and brag about. That's not including the cost of 196 reviewers they also used. The question is, if Firefox was given $20k worth of credit to find these vulnerabilities, how many vulnerabilities could have been discovered by paying that much money to security researchers who wouldn't have needed additional reviewers?