Supply chain attack installs malicious Anthropic Claude extension on thousands of devices

A malicious Anthropic Claude Code extension containing a backdoor was listed on Cursor's IDE extension marketplace for about 11 hours, and Cursor said at most 5,000 users downloaded it. The extension covered Windows, macOS and Linux and was designed to give attackers access to victims' devices; Cursor briefly shut down the extension marketplace and published remediation steps, saying other extensions were unaffected. Detection firm Phorion first found the extension on a customer's Mac, and its co-founder and CEO Calum Hall said design flaws in the malware itself prevented it from fully persisting on the vast majority of targets. Anthropic technical staff member Felix Rieseberg said this was not a compromise of Anthropic systems or employee computers, but rather someone using access to the Cursor marketplace to publish a tampered copy of the extension, and that the version on official channels was unaffected; how the malicious extension entered the marketplace remains unclear.