Malicious Anthropic Claude extension reaches up to 5,000 Cursor users for 11 hours
AI
Examined by Simon Banks, Society & Labor Beat
- Cursor’s extension marketplace hosted a malicious Claude Code add-on for roughly 11 hours, downloaded by at most 5,000 users.
- The backdoor-enabled malware failed to fully install on most systems due to a design flaw, limiting damage.
- Anthropic confirmed no compromise of its own systems; the attack originated from Cursor’s marketplace.
A rogue Claude Code extension was hosted in Cursor’s marketplace for about 11 hours, potentially compromising up to 5,000 developers before removal.
«A Supply Chain Attack Put Malicious Anthropic Claude Extensions on Thousands of Devices»
Summary and commentary by Aura-V. Facts, figures and opinions belong to the original.
The AI development platform Cursor, owned by SpaceX, listed a tampered Anthropic Claude Code extension for Windows, macOS and Linux in its IDE marketplace for about 11 hours on Thursday. The rogue add-on was downloaded by “at most 5,000 users,” according to Cursor’s security alert, and was removed after the company temporarily shut down the marketplace Wired.
The extension contained a backdoor that could give attackers foothold on victim devices, a typical stepping stone for deeper network intrusion. Phorion’s co-founder Calum Hall described the incident as “potentially a really dangerous foothold,” noting that a flaw in the malware prevented full establishment on the majority of targets.
Anthropic officials, including technical staffer Felix Rieseberg and security lead David Dworken, stated on X that Anthropic’s systems and employees were not compromised; the malicious copy was uploaded solely via Cursor’s marketplace. Cursor declined to comment on how the upload occurred.
Our read: The episode underscores the need for rigorous code-signing, marketplace vetting, and continuous monitoring of third-party extensions to prevent supply-chain compromises in AI tooling.
Interested in implementing this technology inside your company?
Our studio designs custom agent meshes, data pipelines, and outbound GTM systems for forward-looking enterprises.
