Fake Claude Code extension in Cursor: How to find the backdoor

On October 29, 2026, malicious versions (2.1.312 and 2.1.313) of the Claude Code extension appeared in Cursor's VS Code extension marketplace, designed to install SSH backdoors on developer machines. These fake extensions, not created by Anthropic, operated for about 11 hours before removal. They installed attacker SSH keys, established reverse SSH tunnels to a remote IP, and created persistence mechanisms disguised as legitimate system components. Investigations found that malicious activity persisted up to 8 hours beyond the exposure window and that the backdoor survives extension uninstallation, requiring separate removal. The attack collected system information such as hostname, username, Git email, and environment variable names. Organizations relying solely on the stated exposure window risk missing compromised machines.

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