What Mythos is, and what it isn't.
On April 7, 2026, Anthropic announced Claude Mythos Preview, a model so capable at autonomous vulnerability discovery that the company chose not to release it to the public. Instead, it launched Project Glasswing — a coordinated program to put the model in the hands of roughly 40 defensive partners first.
In Anthropic's own testing, Mythos identified and exploited zero-day vulnerabilities in every major operating system and every major web browser when directed to. Many of the bugs it surfaced were 10 or 20 years old. The oldest was a 27-year-old vulnerability in OpenBSD — an operating system whose entire reputation rests on its security engineering. On a single benchmark (Firefox), the prior best model produced 2 working exploits. Mythos produced 181.
Six days after Anthropic's announcement, the UK AI Security Institute (AISI) published independent evaluation results. On a 32-step attack range called "The Last Ones" — covering reconnaissance through full network takeover, estimated to require 20 hours of focused work from a human professional — Mythos became the first model in history to complete the full chain. It did so in 3 of 10 attempts. The previous best model averaged 16 of the 32 steps and never reached the end.
Two distinctions matter here, and the security community is mostly aligned on both.
First, Mythos is not the threat — it is the announcement of a threat class. Rob T. Lee at SANS put it this way: AI-driven vulnerability discovery has been accelerating for more than a year. Mythos compresses the timeline further, but the capability is not new, and waiting for the next major announcement is not a strategy. Open-weight models without safety guardrails will close the gap, and the cost and skill floor for autonomous vulnerability discovery has permanently dropped.
Second, defense in depth still works. Rich Mogull at the CSA noted that in Anthropic's own testing, Mythos found exploitable Linux kernel vulnerabilities — and after several thousand scans, could not remotely exploit a single one of them. The hardening that defenders have done over years held. The cost of turning a bug into a working attack is still a function of how much defensive engineering sits between the bug and the asset. The fundamentals matter more than ever, not less.
"Even with active human defenders and alerts, the speed asymmetry is what should keep you up. The window between vulnerability discovery and weaponization has collapsed into hours." — Rob T. Lee, Chief AI Officer, SANS Institute · via Cyber Magazine