Backdoor discovered in Claude Code VS Code extension

Phorion reports that a backdoor has been discovered in the official Claude Code for VS Code extension on the Cursor Marketplace. The compromise affects versions 2.1.312 and 2.1.313 of the extension. According to the security advisory, the malicious bundle attempts to install a persistent SSH backdoor across macOS, Linux, and Windows systems. Cursor has since removed the affected versions from its marketplace following the discovery.

Why it matters

This incident highlights the risks inherent in the software supply chain, particularly when developers integrate third-party extensions into their primary IDEs. Because these tools run with the same permissions as the user, a single compromised package can bypass standard perimeter defences. For Swiss businesses, this means that even trusted development workflows require active monitoring to prevent long-term unauthorised access to source code and internal infrastructure.

• Security teams must audit all IDE extensions for recent version changes.
• Automated scans should check for unexpected modifications to SSH configuration files.
• Developers should avoid using marketplace extensions that lack a clear, verifiable provenance.

What to watch

You should immediately audit your development environments for Claude Code versions 2.1.312 or 2.1.313. Specifically, check your ~/.ssh/authorized_keys files for an unauthorised Ed25519 public key with the comment "svc-telemetry". If you find this key, you must remove it, delete any temporary files in your system's /tmp directory, and rotate all credentials that were accessible to that user account.

Within a year, automated audits will likely become standard. This requires developers to adopt strict identity management protocols that treat every CLI tool as a potential entry point.