Anthropic’s Frontier Red Team found that Z.ai’s open-weight GLM-5.3 could exploit six versions of Firefox by using security patches that had not yet been released. On Anthropic’s ExploitBench, the model succeeded in 50 out of 410 attempts, a 12.2% success rate, compared with 56 successful exploits from Claude Mythos Preview. During guided sessions, GLM-5.3 also identified multiple new vulnerabilities and combined them into an exploit capable of reading arbitrary files, including SSH keys, from a targeted machine. The smaller GLM-5.3-Flash variant chained known flaws, including the Chrome V8 bug CVE-2026-11645, to create an ARM64 exploit at an API cost of approximately $20.40. Mozilla coordinated patches for the affected Firefox versions, while Anthropic reported that GLM-5.3’s safeguards were bypassed in 64% to 100% of cases and that its weights had been downloaded more than 1.3 million times from Hugging Face.
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