Epoch Brief: GLM 5.3 autonomously builds Firefox exploits from patch gaps for about $1,000 each

Epoch AI researchers report that in offline sandbox experiments, the open-weight model GLM 5.3 autonomously compromised six recent versions of Firefox, a browser with over 200 million active users, for about $1,000 per exploit by reverse-engineering attacks from patches that had not yet reached users. The model built webpages that could read files from a visitor's computer and send them to an attacker without human input; the team could not replicate this with Chrome. The same brief notes that Anthropic's Claude Fable 5.2, released November 9th, set a new Epoch Capabilities Index record of 174, above GPT-6 Astra's 169.