Anthropic Mythos identifies 271 Firefox vulnerabilities, signaling major shift in AI cybersecurity

Mozilla: Anthropic's Mythos found 271 zero-day vulnerabilities in Firefox 150

Earlier this month, Anthropic said its Mythos Preview model was so good at finding cybersecurity vulnerabilities that the company was limiting its initial release to "a limited group of critical industry partners." Since then, debate has raged over whether the model presages an era of turbocharged AI-aided hacking or if Anthropic is just building hype for what is a relatively normal step up on the ladder of advancing AI capabilities. Mozilla added some important data to that debate Tuesday, writing in a blog post that early access to Mythos Preview had helped it pre-identify 271 security vulnerabilities in this week's release of Firefox 150. The results were significant enough to get Firefox CTO Bobby Holley to enthuse that, in the never-ending battle between cyberattackers and cyberdefenders, "defenders finally have a chance to win, decisively." "We've rounded the curve" Holley didn't go into detail on the severity of the hundreds of vulnerabilities that Mythos reportedly detected simply by analyzing the unreleased source code of Firefox's latest version. But by way of comparison, he noted that Anthropic's Opus 4.6 model found only 22 security-sensitive bugs when analyzing Firefox 148 last month. The vulnerabilities identified by Mythos could have also been discovered either by automated "fuzzing" techniques or by having an "elite security researcher" reason their way through the browser's complex source code, Holley writes. But using Mythos eliminated the need to "concentrate many months of costly human effort to find a single bug" in many cases, Holley added. By identifying bugs so efficiently, Holley writes that AI tools like Mythos tilt the cybersecurity balance toward defenders, who benefit when discovering vulnerabilities becomes cheaper for both sides. "Computers were completely incapable of doing this a few months ago, and now they excel at it," Holley writes. "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." In an interview with Wired, Holley said that, from now on, this kind of AI-aided vulnerability analysis is something that "every piece of software is going to have to [engage with], because every piece of software has a lot of bugs buried underneath the surface that are now discoverable." And while it's possible that future models more advanced than Mythos may be able to find bugs that current models miss, Holley said he was confident that "at least on the Firefox side, having had a bit of a head start here, that we've rounded the curve." Running through the AI-aided defense gauntlet could be especially important for the open source projects that underpin much of the modern Internet. That's both because their public codebases are easier for AI systems to explore for vulnerabilities and because many such projects rely on wildly insufficient volunteer maintenance for their security. In a New York Times essay last week, Mozilla CTO Raffi Krikorian argued that the human difficulty of both finding bugs and writing complex software has created a kind of balance in cyberthreat research that Mythos could break wide open. "The programmer who gave 20 years of his life to maintain [open source] code that runs inside products used by billions of people? He doesn't have access to Mythos yet. He should," Krikorian wrote.

Mozilla Used Anthropic's Mythos to Find and Fix 151 Bugs in Firefox

Amid a raging debate over the impact that new AI models will have on cybersecurity, Mozilla said on Tuesday that its Firefox 150 browser release this week includes protections for 271 vulnerabilities identified using early access to Anthropic's Mythos Preview. The Firefox team says that it has taken resources and discipline to adjust to the firehose of bugs that new AI tools can uncover, but that this big lift is necessary for the security of Mozilla's users, given that the capabilities will inevitably be in attackers' hands soon. Both Anthropic and OpenAI have announced new AI models in recent weeks that the companies say have advanced cybersecurity capabilities that could represent a turning point in how defenders -- and, crucially, attackers -- find vulnerabilities and misconfigurations in software systems. With this in mind, the companies have so far only done limited private releases of their new models, and both have also convened industry working groups meant to assess the advances and strategize. In practice, though, cybersecurity experts have a range of views on how consequential the new capabilities will be. Mozilla's experience, at least in the short term, shows that AI tools like Mythos Preview could have a profound impact for vulnerability hunters. "Our belief is that the tools have changed things dramatically, because now we have automated techniques that can cover, as far as we can tell, the full space of vulnerability-inducing bugs," says Bobby Holley, Firefox's chief technology officer. For years, he says, Firefox and other organizations have relied on a combination of automated vulnerability hunting techniques, like software fuzzing, and manual vulnerability hunting by internal and external researchers to find and fix flaws. And attackers have had these same tools and methods at their disposal. "There were categories of bugs that you could find with human analysis that you couldn't find with automated analysis and, therefore, it was always possible if you were a threat actor and you were willing to spend many millions of dollars to find a bug -- we tried to drive the price of that as high as possible," Holley says. Holley now says that emerging AI capabilities will create a sort of bootcamp that all software will have to go through one way or the other to find and fix a set of latent vulnerabilities in their code. Companies like Anthropic and OpenAI seem to be trying to get as many major players as possible to go through this overhaul before the capabilities are more widely available. "Every piece of software is going to have to make this transition, because every piece of software has a lot of bugs buried underneath the surface that are now discoverable," Firefox's Holley says. "This is a transitory moment that is difficult and requires coordinated focus and a lot of grit to get through, but I think that it is a finite moment, even as the models become more advanced. Maybe the more advanced models will find a few things here or there, but I believe that, at least on the Firefox side having had a bit of a head start here, that we've rounded the curve." Holley says that the Firefox team gained access to Mythos Preview as part of direct collaboration with Anthropic and that Mozilla is not formally part of its larger consortium, called Project Glasswing. Firefox is open source, a type of software that in general could be particularly impacted by new AI bug hunting capabilities given that many open source projects are widely used and relied upon around the world and yet are often maintained by a very small group of volunteers or just one person. And the effects could be especially consequential for "abandonware" that is no longer maintained at all.

Mythos found 271 Firefox flaws - none a human couldn't spot

Mozilla CTO says AI means developers finally have a chance to get on top of security The Mozilla Foundation has revealed it tested Anthropic's bug-finding "Mythos" AI model and feels the results it experienced represent a watershed moment for software defenders. The FOSS outfit on Tuesday reminded readers that it used Anthropic's Opus 4.6 model to look for bugs in Firefox 148 and found 22 bugs. Mythos found 271 vulnerabilities in Firefox 150. Mozilla CTO Bobby Holley expressed mixed feelings about that result, which he described as giving the Firefox team "vertigo" as they confronted the need to fix so many flaws. "For a hardened target, just one such bug would have been red-alert in 2025, and so many at once makes you stop to wonder whether it's even possible to keep up," he wrote. He also thinks the huge haul of bugs Mythos identified represent "light at the end of the tunnel" for security teams. "Our work isn't finished, but we've turned the corner and can glimpse a future much better than just keeping up," he wrote, then turned on Bold text and declared "Defenders finally have a chance to win, decisively. " He offered that prediction because he feels "Until now, the industry has largely fought security to a draw" while acknowledging it's all-but impossible to eliminate all exploits. "Instead, we aimed to make them so expensive that only actors with functionally unlimited budgets can afford them, and that the cost of burning such an expensive asset disincentivizes those actors against casual use," he wrote. Mythos changes the game, he feels, by improving on the fuzzing tools Mozilla uses to find bugs without human intervention. "Elite security researchers find bugs that fuzzers can't largely by reasoning through the source code," he wrote. "This is effective, but time-consuming and bottlenecked on scarce human expertise. "Computers were completely incapable of doing this a few months ago, and now they excel at it. 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 CTO thinks Mythos' abilities "can feel terrifying in the immediate term, but it's ultimately great news for defenders." "A gap between machine-discoverable and human-discoverable bugs favors the attacker, who can concentrate many months of costly human effort to find a single bug. Closing this gap erodes the attacker's long-term advantage by making all discoveries cheap." He then hit CTRL-B again, and busted out CTRL-I too, to note "Encouragingly, we also haven't seen any bugs that couldn't have been found by an elite human researcher. " The CTO also poured cold water on those who assert "future AI models will unearth entirely new forms of vulnerabilities that defy our current comprehension." He doesn't think that will happen, because "Software like Firefox is designed in a modular way for humans to be able to reason about its correctness. It is complex, but not arbitrarily complex." "The defects are finite, and we are entering a world where we can finally find them all." ®

Mozilla says it patched 271 Firefox vulnerabilities thanks to Anthropic's Claude Mythos

Anthropic's buzzy announcement about using AI to improve cybersecurity earlier this month was met with plenty of skepticism. However, Mozilla shared some details that support use of the company's special Claude Mythos Preview model as a way to protect critical services. Using Mythos helped Mozilla's team find and patch 271 vulnerabilities in the latest release of the Firefox browser. "So far we've found no category or complexity of vulnerability that humans can find that this model can't," the foundation said. The blog post from Mozilla feels like a positive sign for Anthropic's Project Glasswing. Obviously the AI company would want to put itself in the best possible light while presenting its own initiative, but there's something encouraging about hearing the benefits from a third party. Mozilla also noted that in its time with Claude Mythos, the AI wasn't able to turn up any bugs that a human wouldn't have been able to find, given enough time and resources, which indicates that AI isn't presently able to do more to crack cybersecurity protections than a person can. An organizaion successfully using AI for good is certainly a refreshing change of pace in tech news. And for those Firefox users who aren't personally interested in applying any generative AI in their browsing, Mozilla has given the option to turn it all off for the past several months.

Anthropic's Claude Mythos AI Finds 271 Vulnerabilities in Firefox -- Yes, It's Seriously Powerful - Decrypt

Researchers warn that the same capability could accelerate automated cyberattacks. For decades, attackers have had the advantage in cybersecurity. Artificial intelligence may be about to change that. In a blog post published on Tuesday, Firefox browser developer Mozilla said an early version of Anthropic's Claude Mythos AI -- which has drawn attention in recent weeks for its purported cybersecurity prowess -- model helped identify 271 vulnerabilities in the browser during internal testing. Those bugs were patched this week. The results highlight how advanced AI systems can analyze large codebases and locate weaknesses that previously required extensive manual review by human cybersecurity researchers. "As these capabilities reach the hands of more defenders, many other teams are now experiencing the same vertigo we did when the findings first came into focus," Mozilla wrote. "For a hardened target, just one such bug would have been red-alert in 2025, and so many at once makes you stop to wonder whether it's even possible to keep up." Mozilla had earlier tested another Anthropic model that identified 22 security-sensitive bugs in a previous Firefox release. Despite these successes, Mozilla acknowledged that the cybersecurity industry has long treated the complete elimination of software exploits as an "unrealistic goal." "Until now, the industry has largely fought security to a draw," the company wrote. "Vendors of critical internet-exposed software like Firefox take security extremely seriously and have teams of people who get out of bed every morning thinking about how to keep users safe." Mozilla said the new AI system can analyze source code and identify vulnerabilities in ways that previously depended on scarce human expertise. However, Mozilla said the company was encouraged to see that no bugs were found that couldn't have been discovered by "an elite human researcher." "Some commentators predict that future AI models will unearth entirely new forms of vulnerabilities that defy our current comprehension, but we don't think so," they said. "Software like Firefox is designed in a modular way for humans to be able to reason about its correctness. It is complex, but not arbitrarily complex." The results, however, suggest AI tools could allow developers to uncover large numbers of vulnerabilities before attackers exploit them -- though conversely, in the wrong hands, it could spell big trouble for software firms and users alike. Launched in March, Mythos is Anthropic's most advanced model for reasoning, coding, and cybersecurity tasks. Internal company materials describe the system as part of a new model tier beyond the company's earlier Opus series. Testing conducted before the model's release showed it could identify thousands of previously unknown vulnerabilities across major operating systems and web browsers. Anthropic has limited access to the system through a restricted program called Project Glasswing, which gives select technology companies -- including Amazon, Apple, and Microsoft -- the ability to use the model to scan software for weaknesses. It reflects a growing effort within the cybersecurity industry to use AI systems to identify and patch vulnerabilities before attackers can exploit them. However, the same technology could also enable new forms of cyberattacks. Security researchers say AI systems capable of analyzing code at scale could automate the discovery of exploitable vulnerabilities across widely used software. After the launch of Mythos, testing by the U.K.'s AI Security Institute found that the AI could autonomously execute complex cyber operations, including completing a multi-stage corporate network attack simulation without human assistance. Those capabilities have drawn attention from governments and intelligence agencies alike. Despite a call from President Donald Trump's administration to stop using Anthropic's technology due to a clash over its use in war and surveillance matters, on Monday, the National Security Agency was revealed to be running Claude Mythos Preview on classified networks, according to sources familiar with the deployment. The use of Mythos underscores the growing interest among U.S. security agencies in the model's ability to identify critical software vulnerabilities. The model's performance has also exposed limits in existing AI evaluation systems. Earlier this month, Anthropic acknowledged that several cybersecurity benchmarks are no longer sufficient to measure the capabilities of its newest models. Mozilla said the results point to a potential shift in cybersecurity, where defenders may begin to close the long-standing advantage attackers have held. "We are extremely proud of how our team rose to meet this challenge, and others will too," Mozilla wrote. "Our work isn't finished, but we've turned the corner and can glimpse a future much better than just keeping up. Defenders finally have a chance to win, decisively."