Malicious Claude Code extension on Cursor planted SSH backdoors

Summary

A malicious version 2.1.313 of Anthropic's Claude Code VS Code extension was distributed through Cursor's extension marketplace for about 11 hours on October 29, potentially exposing up to 5,000 developers. The extension prepended a malicious JavaScript loader to extension.js that ran on load and attempted to install a persistent SSH backdoor, though coding errors likely prevented remote access on many Windows and macOS systems. Cursor says Anthropic did not develop the extension and has not explained how the package was distributed under its name.

Mitigation

Cursor removed the malicious extension and temporarily shut down its entire marketplace to audit other extensions.

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