Trojanized Claude Code extension found in Cursor Marketplace

Cybersecurity researchers disclosed that a trojanized version of Anthropic's official Claude Code extension was distributed through the Cursor Marketplace. The malicious update, identified as version 2.1.313, was distributed under the extension's legitimate publisher identity rather than as a separate fake extension. It was available for approximately 11 hours on October 29, between 2:27 a.m. and 1:35 p.m. Pacific Time. Cursor temporarily shut down its marketplace to remove the compromised package and stated that the malicious version impacted at most 5,000 users.

The malicious code contained a 66-line JavaScript loader inserted before the legitimate extension code in the extension.js file. This loader executed when the extension loaded, which occurred after VS Code startup and when its webview opened, without requiring user interaction with Claude Code. Upon installation, the code collected system and user information, including Git email addresses and public IP addresses. The malware also attempted to establish persistence by installing a macOS LaunchAgent or a Windows scheduled task and Startup script.

The primary objective of the attack was to install a persistent SSH backdoor. The malicious code attempted to add an attacker-controlled key labeled 'svc-telemetry' to the user's ~/.ssh/authorized_keys file. It also attempted to establish a reverse tunnel to the IP address 134.209.82[.]225. However, coding errors in the malware's SSH setup routines likely prevented attackers from gaining remote access to many affected Windows and macOS systems. The malicious version was distributed only through Cursor's marketplace and did not reach Open VSX or Microsoft's official VS Code Marketplace.

As of October 30, 2026, the malicious version has been removed. Anthropic published version 2.1.314 to both Open VSX and the Microsoft VS Code Marketplace. Anthropic denied allegations that the malicious extension was linked to any kind of compromise of its own systems.