Tampered Claude Code add-on in Cursor’s store had a backdoor
Versions 2.1.312 and 2.1.313 tried to give an unknown attacker remote access to developers’ computers. Cursor pulled them after about 11 hours, and Anthropic says it never built or published them.
Cursor is a popular AI code editor built on Microsoft’s VS Code. It runs its own store for extensions, the add-ons that bring in tools such as Anthropic’s Claude Code. On October 29, two builds in that store were not what they claimed to be.
Versions 2.1.312 and 2.1.313 of the Claude Code extension, for Windows and macOS, carried extra code, Cursor says. It added an SSH key named “svc-telemetry” to the computer, so that the attacker could log in later.
The code also tried to switch on remote login and open a hidden connection to the attacker’s server. Without administrator rights, those steps mostly failed, The Register and BleepingComputer report. The key, a way to start again after a reboot and the data sending still worked.
The code sent out the user name, the git email address, the public IP address and the names of settings that looked like secrets. It did not send the secret values themselves, according to Cursor and Phorion, a security company that analysed the code.
The builds were in the store from about 2:27am to 1:35pm Pacific time on October 29, about 11 hours. Cursor’s notice, updated on October 30, names both versions; the first news reports named only 2.1.313.
BankInfoSecurity reports that Cursor put the number of affected users at no more than 5,000. Cursor’s own notice gives no number, and there is no public download count.
Cursor removed the builds, shut its whole extension store, changed its keys and rebuilt the store. It says it checked who had access and found no other tampered extensions.
Developers trust the official listings in an app’s extension store, and coding tools often sit next to passwords and keys. This attack did not need a flaw in Claude Code itself, only a way to swap the file in the store. Cursor has not said how the attacker got in.