How does the malicious Claude Code extension endanger the security of up to 5,000 developers?

Security firm Phorion disclosed that a malicious Claude Code extension implanted with an SSH backdoor was uploaded to the Open VSX platform on October 29, with version number 2.1.313, taken down after 11 hours online, affecting at most 5,000 developers. The backdoor allowed attackers to access developers' machines, obtain Git emails and up to 40 environment variables, and thereby target cloud credentials or API tokens. Phorion noted that deleting the extension does not remove the threat; the malicious payload leaves behind autonomously running scripts, and developers need to check SSH authorization files for suspicious entries such as svc-telemetry; because of an error in the administrator privilege check, the impact on macOS is limited, but users of other operating systems still face the threat. Cursor has closed its extension marketplace for an in-depth security audit and provided a cleanup script; Anthropic has not yet alerted customers, but says the tampered version was not released by it.