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Mozilla Hardened Firefox With Claude Mythos Preview — 271 Security Bugs Found and Fixed Including 15-Year-Old and 20-Year-Old Flaws, 423 Total Fixes in April
Mozilla ran an agentic security pipeline powered by Claude Mythos Preview across Firefox's codebase, finding 271 bugs fixed in Firefox 150. Discoveries include a 15-year-old HTML legend flaw, a 20-year-old XSLT reentrant key() bug, an IPC race condition enabling use-after-free, and a buffer over-read in HTTPS RR/ECH parsing. Three CVEs directly credited to Anthropic. This follows earlier Opus 4.6 work that fixed 22 security-sensitive bugs in Firefox 148. The methodology combines fuzzing, manual inspection, and the new agentic pipeline.
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