GLM-5.3 exploits Firefox vulnerabilities, prompting a Mozilla patch

Anthropic's red team found Z.ai's open-weight model could turn unshipped security fixes into working browser exploits An open-weight AI model can now do something that used to require a seasoned exploit developer and a lot of coffee. Researchers found that Z.ai’s GLM-5.3 could exploit six versions of Firefox by working from security fixes that had not yet shipped to users. Mozilla patched a related vulnerability after the findings. What the red team found The details come from a September 29 report by Anthropic’s Frontier Red Team. The team evaluated GLM-5.3, which Z.ai released on August 14, 2026. The model’s specialty, at least in this testing, is JavaScript engines. Those are the parts of a browser that run the code on every website you visit. On Anthropic’s ExploitBench benchmark, GLM-5.3 produced working exploits in 50 out of 410 attempts. That works out to a success rate of 12.2%. For comparison, Anthropic’s own Claude Mythos Preview managed 56 exploits on the same benchmark. The Firefox piece is the most striking result. According to the findings, GLM-5.3 exploited six Firefox versions using security fixes that were written but not yet released. Guided sessions and a cheaper...