Malicious Claude Extension Hit 5,000 Cursor Users — SkimNews
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- Cursor hosted a tampered Anthropic Claude Code extension in its marketplace for approximately 11 hours on Thursday, which was downloaded by at most 5,000 users across Windows, macOS, and Linux.
- Phorion detected the malicious extension on a customer’s Mac, with CEO Calum Hall calling it a potentially dangerous foothold that could have enabled deep network access.
- Anthropic confirmed it did not build or publish the compromised extension, with technical staff member Felix Rieseberg stating the versions on official channels remain unaffected.
- David Dworken of Anthropic clarified on X that no Anthropic systems or employee laptops were compromised in the incident.
- Cursor temporarily shut down its extension marketplace to investigate the breach but has not explained how the malicious upload occurred or whether its systems were breached.
Why it matters: Up to 5,000 developers were exposed to a backdoored AI tool that could grant attackers persistent access, yet the flaw in the malware and rapid detection limited harm. The incident shifts liability questions to Cursor, which controls the distribution channel, despite Anthropic’s repeated assurances of no internal compromise.
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