A tampered copy of Claude Code extension appeared in Cursor’s extension marketplace for about 11 hours on Thursday, October 29, reaching up to 5,000 users. The malicious version attempted to plant an SSH backdoor on developers’ machines and could also open a reverse tunnel to the attacker’s server. Cursor noted the extension did not come from Anthropic, and that the attacker gained access through Cursor’s marketplace, not Anthropic’s codebase. A security firm, Phorion, flagged the compromise, reporting that the malicious version was posted under Anthropic’s official account and that the extension’s affected versions were 2.1.312 and 2.1.313 for Windows and macOS. The breach could extract machine and user details and attempt to install SSH keys, though administrator rights were often required for some actions. Cursor shut down its marketplace during investigation, rotated keys, and later reinstated it after confirming no further risk. They advised users to update to Claude Code for VS Code version 2.1.314 and provided guidance to remove the backdoor. Anthropic stated that it did not build or publish the malicious extension and that only Cursor users were affected. The security incident underscores the risk of supply-chain attacks on developer tools. Cursor and Anthropic are cooperating on the incident, with Anthropic insisting there was no breach of its systems or employee devices.