Claude Mythos Spots a Recoverable Math.random() Signing Key in Rejetto HFS and Turns It Into Auth Bypass and RCE (CVE-2026-61500)

Rejetto HTTP File Server (HFS) 3.x, the TypeScript rewrite of a widely deployed file server

Horizon3.ai disclosed on September 30, 2026 a session-forgery flaw that Anthropic's Claude Mythos identified during Project Glasswing work. HFS generated its session signing key with `Math.random()`, which uses V8's non-cryptographic xorshift128+ generator, and a separate code path leaked raw random outputs. The chain runs from user enumeration to sampling the leaked outputs, recovering the generator state with the Z3 constraint solver, reconstructing the signing key, forging an admin cookie, and executing code through the built-in administrative APIs. Horizon3 said Mythos "identified off the bat that recovering the cryptographic seed was viable" without follow-up prompting, and that the bug class would previously have been abandoned for the time and specialist math it required

A non-cryptographic PRNG used for a security secret, with its output observable elsewhere in the application. The model collapsed the expertise and effort that had kept the flaw unexploited