Malicious Claude Code Extension Distributed Through Cursor Marketplace
Sophos, Friday, October 30th, 2026
Sophos reports a malicious Claude Code extension version 2.1.313 was distributed via the Cursor marketplace for about 11 hours on October 29, 2026.
Cursor reported the malicious build was available between about 09:27 and 20:35 UTC and may have affected up to 5,000 macOS and Windows users.
Analysis by Phorion found it adds an attacker-controlled SSH key to authorized_keys, tries to open a reverse SSH tunnel to 134.209.82[.]225, sets up persistence and sends host details to the attacker; coding errors make SSH enablement fail on most hosts.
Sophos advises finding hosts running Cursor and checking for version 2.1.313.
Uninstalling the extension does not remove the backdoor, so teams must remove the SSH key, persistence and tunnel processes and rotate credentials.