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On October 29, 2026, Epoch AI released a study stating that GLM 5.3, an open-weight model released by Z.ai in August 2026, was able to exploit the patch gap window in browsers to compromise six recent versions of Mozilla Firefox, at a marginal cost of about $1,000 per version. Experiments showed that the model, without internet access and with only publicly available information, autonomously completed vulnerability identification and a multi-stage exploitation chain, with the fastest run taking 12 hours, and repeatedly rediscovered an undisclosed vulnerability. Epoch AI has notified Mozilla of the vulnerability, which the latter fixed in Firefox 159.0, released on October 27, 2026. Epoch AI also stated that the total API cost for targeting Firefox exceeded $100,000, while using the same set of tools to attack Google Chrome cost about $100,000 and was still unsuccessful.
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Was I wrong about open models and cybersecurity risk?Developments
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Oct 31
Interconnects
Was I wrong about open models and cybersecurity risk?
Nathan Lambert reflects on his earlier judgment about the cybersecurity risks of open-weight models: more than a month after the release of GLM-5.3, public evidence does not show a step-change in cyber risk; if Claude Mythos were accidentally leaked as open weights, the world would most likely see an acceleration of risk rather than catastrophe. Epoch AI used GLM-5.3 to find a vulnerability in Firefox that could silently exfiltrate files, showing that exploiting a vulnerability still requires multiple coordinated steps. He acknowledges that he should have taken more seriously the reasonable warnings from the AI safety community about a new phase of cyber instability.
Oct 30
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Epoch AI report: open-source model GLM 5.3 exploits Firefox patch gap to launch attacks
Epoch AI published research on October 29, 2026, stating that GLM 5.3, an open-weight model released by Z.ai in August 2026, can exploit browser patch gaps to compromise six recent versions of Mozilla Firefox, at a marginal cost of about $1,000 per version. In the experiment, the model, without internet access and with only public information, autonomously completed vulnerability identification and a multi-stage exploit chain, with the fastest run taking 12 hours, and repeatedly rediscovered an undisclosed vulnerability; Epoch has notified Mozilla of the vulnerability, which the latter fixed in Firefox 159.0 released on October 27, 2026. Epoch said total API spending for the Firefox effort exceeded $100,000, while attacking Google Chrome with the same toolset cost about $100,000 and still did not succeed.
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