Fake Claude Code Extension Pops Up on Cursor Marketplace, Threatening Thousands of Developers with SSH Backdoors

For about eleven hours on October 29, a fake copy of Anthropic’s Claude Code extension for Visual Studio Code appeared in Cursor’s extension marketplace, putting roughly 5,000 developers at risk of malware that could install long‑lasting SSH backdoors on their computers. Researchers uncovered the malicious package after detecting abnormal network traffic originating from machines that had just installed the extension.

Designed to look identical to the genuine Claude Code tool—a utility that helps programmers produce code snippets via Anthropic’s large language model—the rogue extension exploited the familiar brand to entice users into fetching the tainted package from Cursor’s official distribution channel. This episode highlights the expanding threat of supply‑chain compromises within the software development landscape.

After installation, the extension ran a concealed payload that created SSH keys and appended them to the victim’s authorized_keys file, thereby providing the attacker with enduring, password‑free entry to the compromised system. The backdoor was engineered to endure reboots and persist through updates, offering threat actors a stable foothold for possible further intrusion or data theft. Since the malicious code operated in stealth, most developers did not notice the breach until security solutions reported the unexpected SSH connections.

Cursor pulled the malicious extension soon after the report and temporarily shut down the marketplace to conduct a comprehensive review of its vetting processes. Anthropic released a statement affirming that its official Claude Code extension remained untouched and advised users to confirm the origin of any downloads. The event underscores the necessity of checking extension provenance, particularly for tools that plug directly into development environments.

Specialists note that the incident demonstrates how even brief exposure can leave lasting effects on the open‑source and developer communities. Companies are recommended to watch for rogue SSH keys, implement multi‑factor authentication when feasible, and enforce rigorous extension approval processes. While the probe proceeds, security firms are dissecting the malware’s command‑and‑control architecture to identify the perpetrators and to evaluate whether targets beyond the Cursor marketplace were also compromised.