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Mozilla Uses Anthropic's Mythos To Fix 271 Bugs In Firefox (nerds.xyz) 166

BrianFagioli writes: Mozilla says it used an early version of Anthropic's Claude Mythos Preview to comb through Firefox's code, and the results were hard to ignore. In Firefox 150, the team fixed 271 vulnerabilities identified during this effort, a number that would have been unthinkable not long ago. Instead of relying only on fuzzing tools or human review, the AI was able to reason through code and surface issues that typically require highly specialized expertise.

The bigger implication is less about one release and more about where this is heading. Security has long favored attackers, since they only need to find a single flaw while defenders have to protect everything. If AI can scale vulnerability discovery for defenders, that dynamic could start to shift. It does not mean zero days disappear overnight, but it suggests a future where bugs are found and fixed faster than attackers can weaponize them.
"Computers were completely incapable of doing this a few months ago, and now they excel at it," says Mozilla in a blog post. "We have many years of experience picking apart the work of the world's best security researchers, and Mythos Preview is every bit as capable. So far we've found no category or complexity of vulnerability that humans can find that this model can't."

The company concluded: "The defects are finite, and we are entering a world where we can finally find them all."

Mozilla Uses Anthropic's Mythos To Fix 271 Bugs In Firefox

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  • How many? (Score:3, Insightful)

    by eneville ( 745111 ) on Tuesday April 21, 2026 @06:05PM (#66105734) Homepage

    How many of these bugs are around VPN promotions?

    I still prefer Firefox, but it'd be nice if it was more like Phoenix and less like a Windows upgrade notice.

    • Are these changes pushed to Firefox nightlies? Does that mean that, as of this very second, Firefox is 1000x more secure than Chrome??? :-O

    • by AmiMoJo ( 196126 )

      Maybe they could turn it on optimizing performance and fixing compatibility issues next.

      They seem to want AI in the browser, so here's an idea for free. Have an AI agent that launches when you click a "site is broken" button, and it figures out why it is broken and fixes it in the browser. Bonus points if it can handle privacy enhancing add-ons breaking sites too.

      • by gtall ( 79522 )

        The site of the article seems slashdotted, but the precis above does not seem to imply that AI is being used to fix bugs, only find them. And who would trust AI to fix their browser on the fly?

      • by nmb3000 ( 741169 )

        I've been hoping to see an in-browser AI that does realtime ad-blocking, privacy protection, and readability adjustments.

        Extensions like uBlock are amazing and I have all the respect for the people who build it and maintain the definition lists, but it's always an arms race. Something capable of doing it dynamically and on-the-fly could be pretty powerful.

        Is an LLM capable of this? Probably not reliably or quickly (especially a self-hosted one like what's shipped with Firefox today), but maybe it can iter

  • Identify != Fix (Score:3, Interesting)

    by SeaFox ( 739806 ) on Tuesday April 21, 2026 @06:08PM (#66105740)

    The headline and the summary don't seem to quite agree here. The AI analyzing code to identify vulnerabilities is not the same as fixing i.e. writing new code to patch those vulnerabilities.

    • Re:Identify != Fix (Score:5, Insightful)

      by higuita ( 129722 ) on Tuesday April 21, 2026 @06:25PM (#66105764) Homepage

      >The headline and the summary don't seem to quite agree here
      why not!?

      The main pain of security issues is finding them!!
      After claude found the issues, humans could check and fix them, that for many issues isn't that hard. Again the hard part is pinpoint that some check fails to catch a corner case or a buffer may have the wrong size

      • Re: (Score:2, Insightful)

        by SeaFox ( 739806 )

        I found my front door lock was not working.
        I called a locksmith to come repair it.
        Now it is working.

        I didn't fix the front door. The locksmith did.

        • It was the locksmiths door in this case.
        • by higuita ( 129722 )

          critical failure in this comparation:

          >I found my front door lock was not working.

          you found it, great, it was not working!! Easy detection!

          now lets put this in the same apples to apples:

          the door is working fine, you lock and unlock the door without problems. you do not call the locksmith, everything seems fine!
          Your next neighbor kid tried to use a screw drive ( or a finger nail if you want, it is not brute force) in to the lock and it unlocked without any issue. you learn about it and call the locksmith a

    • by Gavino ( 560149 )
      Mod parent up, not down!
    • Re:Identify != Fix (Score:5, Insightful)

      by DamnOregonian ( 963763 ) on Tuesday April 21, 2026 @06:41PM (#66105786)
      I wouldn't say they disagree, so much as the headline is poorly written to be ambiguous.

      Saying, "I used a floodlight to fix my car" isn't inherently inaccurate. It's just ambiguous.
      Identify would have been a much better word than fix, which includes identification in the process.
    • by gweihir ( 88907 )

      Indeed. And here is the thing: There is growing evidence that LLMs trying to fix vulnerabilities does tend to break functionality and to introduce new vulnerabilities. Maybe the reason the are always only boasting about "finding" them.

      • If slashdot humans tried to fix the many bugs here (notifications button does not work, non-ascii characters are mangled, etc.), do you think they would break more than if AI did it for them?

        • If slashdot humans tried to fix the many bugs here (notifications button does not work, non-ascii characters are mangled, etc.), do you think they would break more than if AI did it for them?

          Depends which humans. If it was you or I that did it then we'd definitely introduce more bugs. The ones that did the final fixes will be the internal firefox engineers that spend their entire working time trying to improve Firefox. What's impressive here is that the generic AI model is playing at the same level as the dedicated human engineers and really giving them a huge step forward in finding the bugs. Likely a future dedicated version of the model which has been taught (through training prompting or wh

      • by Tailhook ( 98486 )

        There is growing evidence

        Weasel words.

        Cite your "evidence."

        • by gweihir ( 88907 )

          Like the other side did cite all its evidence? I have done that. (Well, no. But the did not either.)

    • I mean, given how good clod clod is at writing code, it wouldnâ(TM)t surprise me if it also helped fix them.

  • by schwit1 ( 797399 ) on Tuesday April 21, 2026 @06:40PM (#66105782)

    What does this mean for older software that's no longer being patched?

    The next few patch Tuesdays could be interesting.

    • by Himmy32 ( 650060 )

      Same as it ever did, if a bug there's someone could find it and exploit it. There's already AI scanning tools besides all the traditional ones. Just they are getting better and reducing the effort needed to find it.

      However browsers are in an entirely different class of problem because they connect willy-nilly to possibly bad servers, do all sorts of complex things, are used to connect to very sensitive data, can be scanned by any new tools as are they are mostly open source, and are expected to run untrust

    • by evanh ( 627108 )

      I suspect AI has made the last few patch Tuesdays very, say, "fragile."

      • For the past half year every patch Tuesday is like cutting the red wire; you never know if it will cause the bomb to explode. The March patch Tuesday is the one that finally did me in. It wrecked the whole network stack. DISM couldn't repair its image, even with physical media. Every attempt I made at fixing things just made it worse. I finally had to completely reformat my whole PC and start from scratch (luckily, I had just completed a NAS so I was able to back stuff up).

    • by Hentes ( 2461350 )

      This is why I migrated to Wayland. I'm not a fan of it, but running unmaintained software like X is going to be a very bad idea very soon.

  • by davidwr ( 791652 ) on Tuesday April 21, 2026 @06:41PM (#66105788) Homepage Journal

    "The defects are finite, and we are entering a world where we can finally find them all."

    We may be entering a world where we can find 99.44% of bugs and we may find the "easy to find ones" a lot faster than we would find them today, but it's very arrogant to declare "we are entering a world where we can finally find them all" given how many unknowns are still out there.

    Yes, the progress is good, but we need some humility and we need to be realistic with our expectations.

  • Before release, you add a phase of vulnerability discovery, to find as much as you can with the latest and greatest models and fix those before release.
    It makes sense to defuse the threat before it becomes an issue, otherwise your attackers will do it for you and won't tell you what they find.

  • If there's a finite number of bugs and AI can find all of them, sure this can let the defenders win, just find and fix every bug. But the usual model is there's too many bugs to find and fix them all. The defender has to find and fix all the bugs the attacker is able to afford to find. That's the attacker's cost time the number of bugs at the price the attacker's willing to pay or less. Even if there's some economies of scale of finding and fixing bugs, the defender has to pay at least as much as the attack

  • by btroy ( 4122663 )
    I like Firefox. I'll be curious what breaks because of the bug fixes.
    • by Ksevio ( 865461 )

      Probably a lot of hacking tools will break.

      Other than that I think the Mozilla team is pretty competent and can implement fixes with proper testing

  • AI being able to find bugs/exploits is fine, as long as its the maintainer doing it and fixing them.

    Only a matter of time before someone uses AI to find a vulnerability in an open source product and uses it as an attack vector.

  • Mythos and co. are going to reignite the debate between proprietary code and open-source code.
    Mythos can find these bugs ‘easily’ because it has access to the source code.
    How many bugs will it find in the Linux kernel? And how many others won’t be found by Mythos but by other, foreign AIs?
    • I have not followed it very closely, but there seems to be some strong progress in getting LLMs involved in decompilation. Nothing may be safe!

  • > The company concluded: "The defects are finite, and we are entering a world where we can finally find them all."

    Wrong.

    "There's always one more bug"
            -- Elen Ullman

  • It's all well and good using a Mythos Preview to do work. If it is useful then great. However, it will not be a free tool forever. How many thousands of AI bucks would have been burnt to achieve this?
  • Okay, so Mozilla finds and fixes obscure security bugs with the aid of AI tools. Then they just need to keep doing that for the new code.

    This quickly makes code more robust and instead we think about slaying the messenger. If Mozilla would not uncover and fix the bugs an attacker could.

    • > Then they just need to keep doing that for the new code.

      "You Insensitive Claude, why haven't you made Thunderbird multi-threaded yet?"

      (there appears to be evidence of significant limitations in its understanding of complex code)

  • I suspect most of this is because Anthropic paid top bug bounty hunters 7 figure salaries for creating better datasets and RLHF.

  • This is great! Mythos = lots of eyeballs. Now tell us how many of each severity level were found, how many of those could be fixed automatically, and how many fixes both auto and manual then were found to introduce a vulnerability upon reanalysis. Though if there was even one critical severity bug found out of 271 that makes it worth it.

  • So Mozilla gets access to this Mythical LLM, but will LadyBird? Do only certain blessed browser devs get access?
  • The number of bugs is proportional to the efficiency of the code. Once all the bugs are gone, all the fixes will make everything run ten times slower and take double the memory.

  • Was the joke I was looking for...

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