Malicious Claude Code Extension Plants SSH Backdoors via Cursor Marketplace
Written by AI from the linked story. Not reviewed by an editor. How this works
A trojanized version of Anthropic's Claude Code extension was published to the Cursor IDE marketplace and remained available for 11 hours, during which up to 5,000 developers may have installed it. The malware attempted to establish persistent SSH backdoors by injecting attacker-controlled keys and creating reverse tunnels — giving attackers long-term remote access to compromised developer machines. This incident highlights the inherent trust developers place in IDE marketplaces and how that trust can be weaponised through supply chain compromise. Developer workstations are high-value targets because they often hold source code, credentials, and cloud access — making a successful backdoor here potentially catastrophic for downstream systems.
Tactical Insight
Immediate actions
- Audit all recently installed IDE extensions and remove any installed during the 11-hour window (approx. the known exposure period) of the malicious Claude Code extension.
- Rotate all SSH keys and inspect `~/.ssh/authorized_keys` on developer machines for unauthorised entries added during the exposure window.
- Scan developer endpoints for active reverse SSH tunnels or unexpected outbound connections to unknown hosts.
Long-term improvements
- Establish a vetting and allowlisting policy for IDE extensions, requiring security review before installation in corporate environments.
- Implement least-privilege access controls so developer workstations cannot accept inbound SSH connections or establish arbitrary outbound tunnels without explicit firewall rules.
- Integrate software composition and extension integrity checks (e.g., hash verification, publisher signing) into developer onboarding and toolchain policies.
Detection measures
- Deploy endpoint detection and response (EDR) tooling on developer machines to alert on unauthorised modifications to SSH authorised_keys files.
- Monitor outbound network traffic from developer workstations for reverse tunnel patterns (e.g., persistent outbound SSH on non-standard ports) using SIEM correlation rules.