Malicious Claude Code extension on Cursor planted SSH backdoors

A malicious version of the Anthropic Claude Code VS Code extension, version 2.1.313, was distributed through Cursor's extension marketplace for approximately 11 hours on October 29, 2026, potentially affecting up to 5,000 developers. The compromised extension executed malicious JavaScript code upon loading, which collected system and user information, installed an SSH backdoor, and attempted to establish a reverse tunnel to an attacker-controlled server. While coding errors in the malware prevented successful SSH backdoor activation on many systems, reconnaissance data was still exfiltrated to the C2 server at 134.209.82[.]225. Cursor removed the extension, took down its marketplace temporarily, and coordinated takedown of the attacker's infrastructure.

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