Anthropic says the backdoor wasn't us — somebody else got into Cursor's store

Anthropic has denied that the trojanised Claude Code extension came from a breach of its own systems, after security figures alleged the tampered build reached Cursor's marketplace following a compromise at Anthropic.

Anthropic staffer Felix Rieseberg said a malicious actor used access to Cursor's marketplace to publish a tampered copy Anthropic didn't build or publish, and that its official channels are unaffected — but neither company has explained how the build got into the official marketplace.

Cursor says it pulled the version on detection, shut the marketplace down, rotated its keys, checked that no other extensions were hit the same way, and worked with a hosting provider to block the attacker's server.