Malicious Claude Code Extension on Cursor Marketplace Installed SSH Backdoors

A trojanized version of Anthropic's Claude Code VS Code extension was distributed through Cursor's extension marketplace for approximately 11 hours on October 29, 2026, according to BleepingComputer. The report states the malicious listing was available to Cursor users during that window, exposing up to an estimated 5,000 developers to malware designed to install persistent SSH backdoors on infected machines.

Why It Matters

Supply-chain compromise of developer tooling is a high-value attack path: extensions run with the privileges of the IDE and often the developer's local user account, and a backdoor planted this way can persist well beyond the window the malicious package was live. Developers who installed or updated the Claude Code extension via Cursor's marketplace during the reported window should treat their workstations as potentially compromised, since SSH backdoors are typically designed for long-term, stealthy remote access rather than one-time data theft.

What Defenders Should Watch For

• Inventory which developer machines had the Cursor-distributed Claude Code extension installed or updated around October 29, 2026, and verify the extension's publisher/signature against the legitimate source.
• Audit ~/.ssh/authorized_keys, SSH config files, and any newly added SSH key pairs on developer endpoints for unauthorized entries.
• Review process execution and network connection logs from VS Code/Cursor extension host processes for unexpected outbound SSH or unfamiliar child processes spawned around the install/update time.
• Check for new or modified cron jobs, systemd units, or shell profile scripts (.bashrc, .profile) that could re-establish persistence.
• Rotate SSH credentials and revoke any keys associated with potentially compromised developer accounts as a precaution.

This is developing, net-new intelligence based on a single vendor report, and no CVE has been assigned to this incident. Full technical indicators and scope may evolve as more details emerge. See the original reporting at BleepingComputer.