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Epoch AI reported on October 29th that Z.ai's open-weight GLM 5.3, working alone in an offline for up to 48 hours, built a webpage that reads local files from a visiting Firefox user's machine. It did it for all six Firefox releases from 156.0 to 158.0.1 (September 17th to October 26th), using only what was public when each shipped.
The fastest attempt took about 12 hours, and each version cost about $1,000 in fees once the was built. The model never refused.
Attempts against Chrome spent about $100,000 without an equivalent result. The limits are real. Every chain depended on one then-unpublished bug that Mozilla fixed in Firefox 159.0 on October 27th, the exploits read files without running code, and Kimi K3.1 Pro was not tested.
The point stands anyway: the window between a fix going public and users installing it is now within reach of a model anyone can download, and Epoch urges vendors to keep fix details private until the update ships.
An open-weight model turned Firefox security fixes into working exploits for about $1,000 each
Epoch AI reported on October 29th that Z.ai's open-weight GLM 5.3, working alone in an offline sandbox for up to 48 hours, built a webpage that reads local files from a visiting Firefox user's machine. It did it for all six Firefox releases from 156.0 to 158.0.1 (September 17th to October 26th), using only what was public when each shipped. The fastest attempt took about 12 hours, and each version cost about $1,000 in API fees once the harness was built. The model never refused. Attempts against Chrome spent about $100,000 without an equivalent result.
The limits are real. Every chain depended on one then-unpublished bug that Mozilla fixed in Firefox 159.0 on October 27th, the exploits read files without running code, and Kimi K3.1 Pro was not tested. The point stands anyway: the window between a fix going public and users installing it is now within reach of a model anyone can download, and Epoch urges vendors to keep fix details private until the update ships. SourcesAC
Sources
- AOpen-weight models and the patch gap (Epoch AI)epoch.ai
- COpen-weight model GLM 5.3 built full Firefox exploit chains (The Decoder)the-decoder.com
This story ran in the Morning Brief of October 31st 2026.