A Supply Chain Attack Put Malicious Anthropic Claude Extensions on Thousands of Devices

Summary

A tampered Anthropic Claude Code extension was live in Cursor's IDE extension marketplace for roughly 11 hours on Thursday, and Cursor says at most 5,000 users downloaded it. The extension carried a backdoor, but a flaw in its design kept it from fully establishing itself on most targets. Anthropic says it did not build, publish or host the malicious copy and that its own systems were not compromised, while how the upload reached the marketplace remains unclear.

Mitigation

Cursor published remediation steps for the affected users on Thursday. The source does not state their content.

Topics

• Model and package supply chain

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