GLM 5.3 autonomously exploits 6 Firefox versions for $1,000 each

The Epoch AI findings put a concrete price tag on a structural security problem. The "patch gap" is the window in which a fix is visible in public code but has not yet reached users. The researchers deliberately set up their test to approximate what a low-skill attacker with limited resources could achieve, giving the model the target Firefox version, basic tools and public information in an offline environment. From there GLM 5.3 worked without human input, identifying upcoming security fixes, selecting vulnerabilities and building multi-stage exploit chains. The fastest successful attack took 12 hours against a 48-hour maximum.

The researchers note that Firefox security fixes are disguised as routine improvements to slow attackers, and that GLM 5.3 saw through that tactic anyway. Across successful attacks on current Firefox versions, the model relied on a previously unknown vulnerability it independently rediscovered in every attempt, bypassing sandbox protections to read files directly from the computer. Anthropic had previously reported that GLM 5.3 can write exploits at roughly the same level as Mythos, and Anthropic's own Mythos model has already been shown exploiting the patch gap in Windows without source code access.

The Firefox result is not isolated. The researchers say patch-gap exploits targeted Chrome in September 2026, and that a vendor released patched PHP builds more than six hours before a coordinated patch release for two critical vulnerabilities, which citing external reporting could have contributed to thousands of exploit attempts before the official release. They are particularly concerned about software users update infrequently, such as router firmware and mobile operating systems. Their recommendations include keeping fixes private until ready to ship, moving them from beta to general release faster, and avoiding patching multiple chainable vulnerabilities at once; Firefox and Google Chrome have already begun shipping weekly updates.