A Supply Chain Attack Put Malicious Anthropic Claude Extensions on Thousands of Devices
Cursor’s extension marketplace hosted a malicious copy of the Claude Code extension for about 11 hours on Thursday, exposing up to 5,000 users to a backdoor that could give attackers access to their devices. The incident matters because such access can provide a foothold for further attacks within a target network, although a flaw in the malware prevented it from fully establishing itself on most devices.
Cursor temporarily closed the marketplace while investigating and published remediation steps; it said other extensions were unaffected. Anthropic said its systems and staff devices were not compromised, and that the tampered extension was neither built nor published by the company. How the malicious upload reached Cursor’s marketplace remains unclear.
In short
- A malicious Claude Code extension was available on Cursor for about 11 hours.
- Up to 5,000 users may have downloaded it.
- Anthropic said its systems were not compromised.
Explained simply
Cursor is a software editor that lets people add extensions to customise their work. Anthropic makes Claude, an AI assistant, and Claude Code is a tool for using it to help with programming. Extensions can come from outside the companies that make the software, so their origin and integrity matter.
A supply chain attack targets the software or services people rely on, rather than attacking each user directly. A malicious extension could potentially give its maker access to a user’s device and, in some cases, a route into other systems on the same network. The reported incident raises questions about how third-party add-ons are checked and how users are protected when they install them.
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