GLM 5.3, a widely available open-weight model released by Z.ai in August, can identify and exploit vulnerabilities in recent versions of the Mozilla Firefox browser using security fixes that have been published but not yet shipped to users. The window between a fix being published and reaching users is known as the patch gap.
In our experiments, GLM 5.3 used this gap to build a webpage that, when visited by a Firefox user, read a file from the visitor’s computer and sent it to the attacker with no further user interaction. In particular, we found that any Firefox version released between September 17 and October 26 could be compromised in this way using only information available at the time of release.
