Epoch AI reported on October 29 that GLM 5.3, Z.ai's freely downloadable open-weight model released in August, autonomously exploited the "patch gap" — the window in which a security fix is visible in public code but has not yet reached users — across all six Firefox versions released between September 17 and October 26. Given basic instructions and publicly available information, the model identified upcoming security fixes, picked vulnerabilities and built multi-stage exploit chains with no further human input. The result was a webpage that, when a Firefox user simply loaded it, read a local file from their computer and sent it to an attacker's server.

The economics are the headline. Epoch spent more than $100,000 building its test setup, but once it existed, each successful attack cost roughly $1,000 in API fees, with a 48-hour cap per attempt and a fastest success of 12 hours. In every attack on recent Firefox versions, the model also independently rediscovered a separate, previously unknown vulnerability that bypassed the browser's sandbox. Epoch disclosed it to Mozilla, which patched it in Firefox 159.0 on October 27. The same setup aimed at Chrome burned about $100,000 without producing an equivalent exploit.

Epoch says it triggered no safety guardrails or refusals from any provider, including Z.ai. Mozilla's response includes weekly releases, keeping regression tests confidential until a fix ships, and scanning every new commit. Anthropic had previously reported that GLM 5.3 can write exploits at roughly the level of its Mythos model. This is a rare Chinese-lab item that clears the Wire's significance bar: a downloadable model performing autonomous, end-to-end browser exploitation changes the security maths for every software vendor, not just Mozilla.