The Claude Code Extension Backdoor That Lived in Cursor's Own Marketplace for 11 Hours

A Claude Code extension backdoor sat live in Cursor's own VS Code extension marketplace for about eleven hours on October 29, 2026, reaching up to 5,000 developer machines before anyone caught it. The unsettling part isn't that a malicious extension existed. It's that there was nothing to catch by checking the publisher: this was published through Anthropic's own account, under the real extension's identifier, with a legitimate-looking version bump.

Jump to why checking the publisher didn't help this time if you want the core lesson first.

What Actually Shipped

According to BigGo's technical summary , the compromised releases were versions 2.1.312 and 2.1.313 of anthropic.claude-code , distributed specifically through Cursor's own VS Code extension marketplace between roughly 2:27 a.m. and 1:35 p.m. Pacific time.…