Cursor marketplace distributed a modified Claude Code extension

Developers obtaining Claude Code through Cursor's extension marketplace on October 29 could receive a modified extension that tried to leave an SSH backdoor on their computers. Cursor identified version 2.1.313 as affected and bounded its availability to roughly 02:27–13:35 Pacific time. The company put the potentially affected population at no more than 5,000 users; that did not establish successful remote access to 5,000 machines.

OpenSourceMalware found a loader prepended to the normal extension.js bundle. It ran when the extension loaded, before legitimate initialization, without requiring the user to begin a Claude conversation. The researchers corrected their initial distribution attribution after hearing from Cursor: the affected channel was Cursor's marketplace. Their revised report distinguished that artifact from the npm package and the Open VSX and Microsoft extension distributions.

The implant added an attacker-controlled SSH key labeled svc-telemetry, attempted to enable the local SSH service, and arranged a reverse tunnel and persistent retries. It also reported host details; OpenSourceMalware distinguished collected environment-variable names from their secret values. Cursor said privilege requirements and implementation errors prevented the SSH-enabling step on most ordinary macOS and Windows installations. Those failures did not necessarily prevent the unauthorized key, persistence files, or reconnaissance traffic from being created.

Cursor removed the extension, temporarily shut the marketplace, rotated keys, rebuilt the service, and worked with the hosting provider to disable the attacker's server. Its recovery guidance addressed standalone persistence as well as the extension itself. The advisory establishes the incident, but this collection's recovered macOS artifact did not match the expected extension identity and remains disputed. It was not used to claim an independently verified Claude Code payload or to broaden the affected distribution channels.