A tampered copy of Anthropic's Claude Code extension spent about 11 hours in Cursor's integrated development environment marketplace on Thursday before the company pulled it. Cursor, which Wired reports is owned by SpaceX, says the rogue extension was downloaded by at most 5,000 users across Windows, macOS and Linux. The marketplace was briefly shut down while Cursor investigated.
The company says no other extensions were affected. The malicious version carried a backdoor designed to give attackers a foothold on victim machines — a classic first step toward deeper network access. Calum Hall, cofounder and CEO of detection firm Phorion, was the first to spot it after the implant appeared on a customer's Mac.
Hall calls the outcome lucky. A flaw in the malware's own code stopped it from fully establishing itself on the vast majority of targets. Early speculation pointed to a compromised Anthropic supply chain. Cursor's Thursday security alert pushed back: the malicious versions were not developed by Anthropic.
Felix Rieseberg, a member of Anthropic's technical staff, posted on X Friday confirming the company's systems and employee laptops were not breached. A malicious actor used access to Cursor's marketplace to publish the tampered copy. David Dworken, who works on security at Anthropic, echoed that the official channels were never affected and the incident only impacts Cursor users.
Cursor has not explained how the malicious upload happened. The company did not respond to Wired's request for comment on whether the marketplace itself was breached. Remediation steps for the roughly 5,000 affected targets were published Thursday. The episode points to how IDE marketplaces have become high-value targets.
A single compromised extension can reach thousands of developer machines in hours. Cursor's investigation continues. Anthropic's official Claude Code extension remains unchanged on its own distribution channels.
TBFiled by The Tech Business Desk
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