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claude code extension: How to Check if the Tampered Cursor Build Hit Your Machine

#security #cursor #vscode #ai

If you use the claude code extension in Cursor, check your machine today. Cursor says a maliciously modified build of the extension, version 2.1.313, was available in its VS Code extension marketplace on 29 October 2026, and that at most 5,000 users were affected. Cursor also says this build "was not developed by Anthropic." This research-based guide sticks to what Cursor and Reuters have confirmed, and shows the quick check plus a few habits for locking down editor extensions.

What Cursor and Reuters confirmed

According to Cursor's security notice, the tampered build was live between about 2:27am and 1:35pm PT on 29 October. It:

  • collected host and user details such as hostname, username, OS, Git email and public IP
  • tried to add an attacker's SSH key to authorized_keys, turn on the local SSH service and open a reverse SSH tunnel
  • tried to make that tunnel survive restarts, using a LaunchAgent on macOS or a scheduled task on Windows

Cursor says turning on SSH needs admin rights, which the extension does not have on most macOS and Windows machines, so on those machines the attacker could not have logged in through the tunnel. The key, the persistence and the data upload can still happen, so Cursor asks affected users to clean up anyway. It also says the attacker's server is now offline, it found no evidence that other versions or the official distribution were affected, and it audited the marketplace and found no other tampered extensions.

Reuters, via CNA, reported the same points: up to 5,000 users, the same time window, and that Cursor shut down, rebuilt and relaunched its marketplace. Anthropic did not immediately respond to Reuters.

You may see louder claims online about who was behind it. We have left those out until they are confirmed.

Step 1: Check for the attacker's key

Cursor's notice says the build adds a public key labelled svc-telemetry. Its own check for macOS:

grep -qs svc-telemetry ~/.ssh/authorized_keys && echo "svc-telemetry key found" || echo "No key found; machine not impacted"
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For Windows, Cursor gives a PowerShell check in the same notice that also looks at C:\ProgramData\ssh\administrators_authorized_keys. Copy it from there rather than retyping it.

Step 2: If the key is there

Follow Cursor's steps, in order:

  1. Isolate the machine.
  2. Run Cursor's remediation script for your platform (macOS as your normal user, Windows from an admin PowerShell). Cursor warns that uninstalling the extension may not be enough, because the payload leaves scripts that keep running.
  3. Decide whether to rebuild the machine.
  4. From a different, trusted computer, rotate passwords, API keys and sessions that the affected machine could reach.

Step 3: Check which version you have

In a VS Code-style editor, open the Extensions view and search @id:anthropic.claude-code to find the exact extension and its version. VS Code's extension docs also list code --list-extensions --show-versions for the terminal. Cursor names the bad folders anthropic.claude-code-2.1.313-darwin-arm64 and anthropic.claude-code-2.1.313-win32-x64.

Step 4: Lock down your editor extensions

These settings come from VS Code's documentation. Check that your editor version supports them before you rely on them.

  • Slow down updates. extensions.autoUpdateDelay waits a set number of hours (12 by default) before an update installs automatically. You can also turn auto-update off for the extensions you care most about.
  • Allow only what you need. extensions.allowed blocks any extension not on your list. That works well for a small team.
  • Prune. Uninstall extensions you no longer use. Each one runs with the same permissions as the editor.
  • Install by exact ID. Search with @id:publisher.extension to avoid look-alike names.

The bottom line

The claude code extension incident was short, and Cursor says most machines could not be reached through the tunnel. The one-line check takes seconds, so run it, and treat editor extensions like any other software with access to your files.

We share plain, research-based AI news for small businesses and creators at ainews.useaifix.com, and AI automation help at useaifix.com.

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