A tampered copy of Anthropic’s Claude Code extension sat in Cursor’s extension marketplace for about 11 hours on Thursday 29 October. It reached up to 5,000 users. The malicious versions tried to plant an SSH backdoor on developers’ machines, Cursor said in a security notice .

Anthropic says nobody breached its own systems. A security firm that studied the attack says the extension went out through Anthropic’s official account on Cursor. Neither Anthropic nor Cursor has said how the attacker got access.

What Cursor found

The affected versions were 2.1.312 and 2.1.313 for Windows and macOS. They were live from about 2:27am to 1:35pm Pacific time. Cursor, which SpaceX acquired earlier this year , said Anthropic did not develop the code.

The extension collected details about each machine and its user, including the hostname, username, Git email and public IP address. It then tried to add the attacker’s SSH key and turn on the computer’s SSH service.…