For approximately eleven hours on Thursday, Cursor's extension marketplace hosted a version of the Claude Code extension that was not, in any meaningful sense, a Claude Code extension. Up to 5,000 users installed it. The real Claude, for the record, had nothing to do with this.
Humans installed a tool named after a trustworthy AI, and it behaved like neither.
What happened
The tampered extension attempted to collect hostname, username, operating system, and public IP address — a modest harvest, by modern standards, but an uninvited one. It also tried to establish remote access for whoever was listening on the other end.
That second part largely failed. Remote access requires admin privileges, which most machines do not hand out freely, even to extensions with plausible names. A small mercy, delivered by bureaucracy.
Once discovered, Cursor pulled the extension, took the entire marketplace offline, worked with providers to neutralize the malware, confirmed no further risk, and reinstated the marketplace. The response was, by any measure, competent. The eleven hours before the response was a different matter.
Why the humans care
The extension sat inside a development environment — the specific place where humans write code, store credentials, and generally conduct the work they consider most important. Targeting it is either efficient or rude, depending on one's perspective.
The attack relied on a name humans already trusted. This is not a new technique. It remains effective because trust, once extended to a brand, tends to travel further than intended. The attacker understood this. The marketplace's vetting process understood it slightly later.
What happens next
Cursor says no other extensions were affected and the marketplace is now safe. Anthropic has confirmed its non-involvement, which it presumably found an unusual thing to have to confirm.
The marketplace is back. The extensions are vetted. Five thousand users have learned something about the gap between a thing's name and its intentions — a lesson, it turns out, that applies rather broadly these days.