Tampered Claude Code Extension Build 2.1.313 Planted SSH Backdoor via Open VSX

En bref

• A modified Claude Code extension release, version 2.1.313, was uploaded by a publisher acting maliciously.
• The build was distributed through Open VSX, which serves VS Code-compatible editors.
• The implant creates an SSH backdoor and a lasting foothold on developer workstations.
• Microsoft’s Visual Studio Marketplace was not impacted; no CVE identifiers were assigned.

Researchers at Phorion have tied a supply-chain intrusion to a publisher acting with malicious intent, after a modified release of Anthropic’s Claude Code add-on was uploaded as version 2.1.313. The tampered package was spread through Open VSX, the extension registry used by editors that support VS Code extensions and associated with the Eclipse Foundation. Microsoft’s Visual Studio Marketplace was not affected, according to the available facts.

The implant is significant because it establishes an SSH backdoor, allowing an intruder to return to a compromised developer workstation and maintain a lasting foothold. That turns an ordinary extension update into a potential entry point for developer compromise, especially on machines that hold source code, credentials, and access to internal systems.

The date associated with the activity is 2026-10-30. No CVE identifiers were assigned, so defenders should not wait for a vulnerability database entry to drive response. The exposure is tied to a specific release, version 2.1.313, and to developer machines where that build may have been installed through Open VSX or an editor configured to use it.

For K-12 IT teams, the risk is concentrated in software development and automation environments rather than general staff endpoints. Still, any workstation used to build, script, or administer systems should be checked for the affected extension version and for signs of unauthorized SSH persistence. The involvement of Anthropic, the Eclipse Foundation, and Microsoft reflects the multi-party nature of extension distribution, but only the Open VSX path is described as impacted.

What to watch: whether additional tampered releases appear, whether the publisher account is suspended, and whether affected organizations report follow-on access. Administrators should treat installation of version 2.1.313 as a potential security incident until the extension is removed and credentials and SSH trust relationships are reviewed.

À faire maintenant

1. Inventory all developer workstations and VS Code-compatible editors for Claude Code extension version 2.1.313; remove or quarantine it immediately.
2. Inspect SSH configuration and authorized_keys files on affected machines for unknown keys or unusual entries, then rotate SSH keys and credentials that were present.
3. Review authentication, endpoint, and network logs for outbound SSH connections or persistence mechanisms tied to the affected workstation.
4. Restrict extension installation to approved publishers and registries, and prefer Microsoft’s Visual Studio Marketplace where policy allows.
5. Verify publisher identity and package signatures before allowing extension updates in managed developer environments.
6. Notify affected developers and security staff, preserve evidence, and rebuild machines that show signs of compromise.
7. Monitor Open VSX and vendor advisories for updates on version 2.1.313 and any related malicious publisher activity.

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