The tampered Claude Code packages found in Cursor's store never reached Microsoft's Visual Studio Marketplace or the official npm release, Microsoft says. Its security team published the finding in an October 30 report.
Anthropic told Microsoft that no one had broken into its own systems and that it did not make the malicious package. Microsoft's researchers say the evidence so far points to Cursor's marketplace.
Cursor's earlier notice said it had found no evidence of affected official copies. Microsoft now says neither of those two sources carried the tampered packages.
The original warning named versions 2.1.312 and 2.1.313 for Windows and Mac. It covers installs or updates on October 29. The window ran from about 2:27 a.m. to 1:35 p.m. Pacific time.
The new analysis also spells out what one part of the data collection could reveal. Microsoft found code that searched the names of computer settings known as environment variables. It looked for words such as KEY, TOKEN and SECRET. For those settings, the code was written to send the names, not their values.
That does not rule out later access to credentials through the backdoor, as Cursor's notice warns.
The packages tried to open a hidden route into a computer. Cursor says that remote-login attempt failed on most Windows and Mac machines because it needed admin rights. The attacker's key and scripts could still remain.
Cursor says it removed the packages, rebuilt its store and took the attacker's server offline. Its current notice still tells affected users to check their machines and follow its cleanup steps. Uninstalling the add-on alone may leave malicious scripts behind.