Anthropic’s Claude Code extension distributed a malicious package through Cursor’s VS Code extension marketplace on 29 October, DoublePulsar reported. Cursor said around 5,000 developer systems were impacted, according to DoublePulsar.
The altered extension, version 2.1.313, collected details including hostnames, usernames, configured Git email addresses and public IP addresses. It also attempted to add an attacker-controlled SSH key and establish a persistent connection for remote access. Cursor said errors in the malicious code meant that step would fail except in unusual circumstances where its editor ran with administrator privileges, according to DoublePulsar.
Kevin Beaumont, who writes DoublePulsar, credited Phorion with first detecting the malicious activity on endpoints. He reported that Anthropic had not informed customers and had removed the malicious version without notification.
Beaumont reported that Anthropic had replaced version 2.1.313 with 2.1.314 about eight hours before he published his report. He said the replacement appeared to remove the backdoor from the extension but would not remove an SSH backdoor from systems already infected.
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