GLM 5.3 exploits Firefox patch gap to steal local files

Epoch AI reports in a research publication that Z.ai's open weight GLM 5.3 model used published security fixes to exploit recent Firefox versions before patches reached users. In tests, the model created a webpage that could read a local file and send it to an attacker when a Firefox user visited it, without further interaction.

The researchers compromised every Firefox version released from September 17 to October 26 using information available when each version shipped. Once the testing setup was complete, each successful attack cost about $1,000. GLM 5.3 identified security related code changes despite efforts to present them as ordinary improvements.

The attacks also used an unpublished vulnerability alongside flaws identified through the patch gap. Epoch AI disclosed the issue to Mozilla, which fixed the unpublished vulnerability in Firefox 159.0 on October 27. The researchers said this change should make similar attacks more difficult.