Tampered Claude Code extension briefly planted SSH backdoor via Cursor marketplace
Affected developers should check for an unfamiliar 'svc-telemetry' SSH key, since the malicious build sat in Cursor's marketplace for about eleven hours.
A tampered build of Anthropic's Claude Code extension, labeled version 2.1.313, sat in Cursor's extension marketplace for about eleven hours on October 29, and was reportedly installed by as many as 5,000 users. The rogue code added a hidden SSH key, tried to open a reverse tunnel to an attacker-run server, set up persistence on Windows, Linux and macOS, and sent host details out in plaintext. Phorion found flaws in the SSH setup routines, so the backdoor mostly failed to grant access, though the key and persistence still landed on affected machines. Cursor pulled the extension, shut its marketplace briefly, and took the server offline, while Anthropic says its own systems were not breached and it has released 2.1.314 through its official channels. Developers who ran the extension in that window should look for an unfamiliar 'svc-telemetry' key in their SSH configuration, remove any persistence entries, and rotate credentials, since how the attacker got a malicious build onto Cursor's marketplace under Anthropic's name remains unexplained.
Why it matters
The incident shows that an extension carrying a trusted vendor's name can reach developers through a third-party marketplace without the vendor's systems being breached. Anthropic says its own infrastructure and official versions were unaffected, so the exposure is tied to Cursor's distribution channel and to the window when the rogue build was listed. Developers who installed it during that period may hold a planted SSH key and need to remove it. How the malicious build got into Cursor's marketplace under Anthropic's name has not been explained.
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