Is Claude Mythos dangerous? Mozilla says it discovered 271 critical bugs in Firefox that humans missed

Anthropic's Mythos AI model has quickly become one of the most discussed tools in cybersecurity. The company behind it has itself warned that the system is powerful enough to locate hidden software weaknesses, including zero-day vulnerabilities that attackers could exploit before developers even know they exist. Anthropic described this moment as a "watershed moment for cybersecurity," because a model that can find dangerous flaws at scale could help defenders fix systems faster but could also become risky if misused. Now, Mozilla's latest Firefox report offers one of the clearest real-world examples of why Mythos is being seen as both dangerous and useful. Mozilla says Mythos Preview helped discover and fix a large batch of security flaws (271 bugs) in Firefox, including 12 serious examples it publicly shared, some of which had remained buried in the browser code for years.

Why Anthropic says Mythos could be dangerous

The concern around Mythos is not because it attacks systems on its own, but because of what it can help humans do. Anthropic has said the model is capable of identifying hidden weaknesses in major operating systems, browsers, and software platforms. In theory, a bad actor could use such a tool to search for security gaps much faster than before.

Traditionally, finding deep bugs in software takes skilled researchers, time, and expensive testing setups. AI models like Mythos can shorten that process by reading huge codebases, spotting suspicious logic, and creating proof-of-concept test cases. That is why governments and organisations are paying attention. If used responsibly, it can strengthen digital infrastructure. If used carelessly, it could raise cyber risk.

Mozilla's latest findings suggest the model's capabilities are real, not just theoretical.

Firefox report shows Mythos found hidden flaws humans missed

Mozilla said AI-generated bug reports were once mostly low quality and created extra work for engineers. But newer systems have improved sharply. By combining Mythos with internal testing tools, Mozilla was able to uncover bugs that traditional methods had missed.

Among the 12 major examples shared by Mozilla were several severe memory corruption and sandbox-related flaws. These types of bugs are often valuable to attackers because they may help run malicious code or break out of restricted browser environments.

One case involved an incorrect equality check that could allow Firefox's JIT engine to skip initialising a WebAssembly structure, potentially opening the door to arbitrary memory read and write actions. Another bug involved a race condition over inter-process communication that could let a compromised content process manipulate IndexedDB reference counts and trigger a use-after-free flaw.

Mozilla also disclosed a 15-year-old issue in the HTML

element caused by rare edge-case interactions across different browser systems. Another was a 20-year-old XSLT bug where repeated key() calls could free memory while it was still in use.

The company further revealed a flaw where raw NaN values crossing an IPC boundary could be mistaken for JavaScript object pointers, creating another potential route for sandbox escape. One more bug exploited special rowspan=0 table behaviour by adding over 65,535 rows, causing an overflow in a 16-bit layout field that had gone unnoticed for years.

271 bugs linked to Mythos, 423 fixed in one month

Mozilla said Mythos Preview helped identify 271 bugs tied to the Firefox 150 release cycle. In total, Firefox shipped fixes for 423 security bugs in April 2026 when including findings from fuzzing systems, internal reviews, external researchers, and other AI tools. Of the 271 bugs associated with Mythos, Mozilla said 180 were rated high severity, 80 were moderate, and 11 were low.

So, is Mythos really dangerous?

Mozilla's report suggests Mythos is dangerous in the same way a powerful lockpick can be dangerous. It depends on who is using it. The model appears capable of uncovering serious weaknesses that had remained hidden for years, which validates Anthropic's warnings. It could be misused in the wrong hands. But the Firefox case also shows the upside. Instead of helping attackers, Mythos was used to patch bugs before they could be abused in the wild. So, the danger is real, but so is the opportunity to build safer software faster than ever before if Anthropic manages to keep the AI tool away from hackers.