Cursor says up to 5,000 users affected by malicious Claude Code extension

AI coding tool Cursor has released the findings of a security investigation. A tampered copy of the Anthropic Claude Code extension was planted in its VS Code marketplace and stayed online for about 11 hours, affecting up to 5,000 users, while most customers were not impacted.

The probe found the attacker used a leaked Fireworks access key to steal data from a Cursor storage bucket, then used a few unmasked credentials to get into some SaaS tools and view code repos and user credentials. Once spotted, Cursor killed the key, rotated credentials and shut down the attacker's servers.

Users on privacy mode were not affected. Among those not using it, less than 0.05% had their agent trace data leaked. Anyone who installed the malicious 2.1.312 or 2.1.313 version should follow official steps, as uninstalling alone won't fully remove the risk. The incident is now under control, and affected customers have been notified one by one.