Backdoor Found in Claude Code for VS Code Extension on Cursor Marketplace

Backdoor Discovery and Impact

Phorion identified a backdoor within the official Claude Code for VS Code extension bundle, specifically versions 2.1.312 and 2.1.313, available on the Cursor Marketplace. This backdoor establishes a persistent Secure Shell (SSH) connection across macOS, Linux, and Windows operating systems. The compromise grants unauthorized access to local files, source code, and developer tools, as extensions run with the permissions of the IDE and the user.

Affected Versions and Remediation

The affected versions, 2.1.312 and 2.1.313, were distributed through the Cursor Marketplace. Cursor has since removed these compromised versions. Organizations are advised to immediately review their environments for the use of these specific extension versions and initiate incident response procedures if found.

Technical Details of the Backdoor

The implant includes setup and persistence logic for all three major operating systems. On macOS, the extension attempts to configure an SSH backdoor. However, Phorion noted an oversight where the malware tries to check the Remote Login state without root privileges. This error can prevent the SSH enablement step from completing if SSH is not already running, though SSH keys and persistence mechanisms are still installed.

Threat Assessment

Despite the potential bug on macOS that may hinder immediate inbound SSH access, the malware remains a significant threat. It successfully installs persistence mechanisms, aiming for long-term interactive access to compromised developer environments. The primary goal of this payload is to maintain a foothold for future access.