Anthropic's Mythos AI identified 271 security flaws in the Mozilla Firefox codebase during testing in May 2026 [1].
This development demonstrates a shift in cybersecurity where artificial intelligence is used to discover zero-day vulnerabilities faster than human auditors. The ability to automate the identification of critical flaws could either secure software more rapidly or provide a roadmap for attackers.
Mozilla utilized the AI, also referred to as Claude Mythos, to automatically discover hidden security flaws within its browser [1]. A Mozilla spokesperson said, "We identified 271 security flaws in Firefox, and the false‑positive rate is almost negligible" [1]. These findings represent zero-day vulnerabilities, flaws that were previously unknown to the developers [4].
The scale of the discovery was highlighted by the MSN technology desk, which reported that Claude Mythos found 271 zero-day vulnerabilities in Firefox [4]. This high volume of discoveries suggests that AI verifiers can scan complex codebases with a level of precision that minimizes incorrect flags [1].
Separate reports regarding the security of the AI tool itself have surfaced. A TWiT host said, "For the first time, an AI verifier has found zero vulnerabilities in Mythos" [0]. This claim suggests that while Mythos is effective at finding flaws in other software, a separate AI verification process found no vulnerabilities within the Mythos system itself [0].
While the findings in Firefox highlight the vulnerabilities of traditional software, the reported lack of flaws in Mythos suggests a new standard for AI-driven code security. The contrast between the 271 flaws found in the browser and the zero flaws found in the AI tool underscores the different architectures of the two systems [0, 1].
“"We identified 271 security flaws in Firefox, and the false‑positive rate is almost negligible."”
The use of Mythos AI to uncover 271 vulnerabilities in a widely used browser like Firefox indicates that AI-driven auditing is becoming a primary tool for vulnerability management. If AI can identify hundreds of zero-day flaws with negligible false positives, the window of time between a bug's creation and its patch will likely shrink, fundamentally changing the arms race between software developers and cybercriminals.
