Malicious Claude Code Extension Briefly Infiltrates Cursor Marketplace, Threatening Thousands of Developers

A counterfeit version of Anthropic's Claude Code extension for Visual Studio Code was briefly listed on Cursor's extension marketplace on Oct. 29, exposing an estimated 5,000 developers to malware that silently created persistent SSH backdoors on their machines.

The rogue package, which mimicked the official Claude Code branding, was available for roughly 11 hours before Cursor removed it after the intrusion was detected. During that window, users who downloaded the extension unwittingly installed a hidden payload that modified SSH configuration files and added unauthorized keys, granting attackers long‑term remote access.

Security researchers at BleepingComputer first reported the incident after receiving multiple alerts from users who noticed unexpected SSH keys appearing in their authorized_keys files. Analysis of the malicious code revealed that it leveraged Node.js scripts to locate the user's SSH directory, append a new key pair, and then communicate the private key to a command‑and‑control server operated by the threat actors.

While the exact motive behind the campaign remains unclear, the technique aligns with a growing trend of supply‑chain attacks targeting developer tools. Extensions for popular IDEs are trusted by developers to streamline workflows, making them attractive vectors for inserting covert functionality that can persist across system reboots and even survive OS updates.

Cursor, a platform that aggregates third‑party extensions for VS Code, issued a statement acknowledging the breach and confirming that the malicious package has been removed. The company also pledged to strengthen its vetting process, including additional automated scanning and manual review steps, to prevent similar incidents in the future.

Experts recommend that developers who installed the compromised Claude Code extension immediately revoke any newly added SSH keys, rotate existing credentials, and run malware scans on affected systems. Organizations are also urged to enforce strict extension sourcing policies and to monitor network traffic for unusual outbound connections that could indicate exfiltration.

The episode underscores the importance of supply‑chain hygiene in the software development ecosystem. As more developers rely on third‑party plugins to accelerate coding, security teams must balance convenience with rigorous verification to safeguard both codebases and the infrastructure they touch.