Anthropic partnered with Mozilla to run a two-week security audit of Firefox using Claude Opus 4.6, uncovering 22 vulnerabilities — 14 rated high-severity. The team started in the JavaScript engine before expanding to other parts of the codebase. Most bugs were patched in Firefox 148. While Claude excelled at finding vulnerabilities, it struggled to write working exploits: $4,000 in API credits yielded only two successful proof-of-concept exploits out of many attempts.
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