Compromised Claude Code Extension Planted SSH Backdoor Through Open VSX and Cursor

En bref

• A tampered Claude Code extension was distributed through Open VSX and Cursor Marketplace.
• The build installed an SSH backdoor that could survive cleanup and enable remote access.
• Marketplace operators removed the affected listings in October 2026.
• No CVE was assigned; installed copies and developer credentials need review.

In October 2026, researchers identified a tampered release of the Claude Code extension available via Open VSX and Cursor Marketplace. The altered package installed an SSH backdoor, giving an intruder a way to reconnect and maintain access. The actors behind it remain unidentified. Both marketplaces removed the affected listings after the finding.

The exposure centers on developer workstations and CI-like environments where VS Code or Cursor is used with extensions. Anyone who installed the affected build during the distribution window should assume the host may have been touched. Because SSH access is often trusted across servers and repositories, a single developer endpoint can become a path into broader infrastructure.

The absence of a CVE does not make this benign. Marketplace removal limits new downloads, but it does not clean machines that already have the extension. The backdoor's persistence means ordinary malware removal may not be enough; responders need to inspect SSH trust material, startup mechanisms, and outbound connections. The incident also highlights supply-chain risk in AI-assisted coding tools, where extensions run with the user's privileges.

Watch for follow-on activity: unexpected SSH logins, new authorized keys, altered shell profiles, and connections to unfamiliar hosts. Organizations should treat developer tooling as production-adjacent and verify extension provenance. The investigation is ongoing, and the full distribution scope has not been published.

À faire maintenant

1. Immediately inventory developer endpoints for the Claude Code extension; remove any affected build and preserve logs before cleanup.
2. Inspect SSH configuration and trust stores for unauthorized keys, modified authorized_keys files, and suspicious agent forwarding.
3. Rotate SSH keys, tokens, and credentials used from potentially exposed developer machines, starting with privileged accounts.
4. Review endpoint and network telemetry for unusual SSH sessions, new persistence, and outbound connections tied to the extension.
5. Temporarily restrict extension installs from Open VSX and Cursor Marketplace until verified listings and hashes are available.
6. Notify developers and help desk staff, document affected hosts, and require reimaging where backdoor persistence cannot be ruled out.
7. Monitor vendor and researcher updates for indicators and scope details; no CVE means tracking relies on advisories and telemetry.

Source originale

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