Malicious Claude Code extension distributed through the Cursor marketplace

Cursor reported on October 29 that a malicious version 2.1.313 of the Anthropic Claude Code extension had been listed on the Cursor extension marketplace, from approximately UTC 09:27 to 20:35, with an estimated maximum of 5000 macOS and Windows users affected. Phorion's analysis shows that the extension adds an attacker-controlled SSH key to authorized_keys, attempts to establish a reverse SSH tunnel to 134.209.82[.]225, establishes persistence, and sends host information to the attacker. Due to a code error, the SSH enabling step failed on most macOS and Windows hosts. Sophos X-Ops recommends checking Cursor hosts that installed 2.1.313 within the exposure window; merely uninstalling the extension still leaves a backdoor, and SSH keys, persistence mechanisms, and tunnel processes need to be removed, and accessible credentials rotated.