An official Anthropic extension for the Claude model was compromised and made available through Cursor’s VS Code marketplace, affecting as many as 5,000 developers on Friday. The malicious version, labeled 2.1.313, was distributed through Anthropic’s verified account on the platform before being replaced with a patched version 2.1.314 several hours later, according to security researcher Kevin Beaumont and Phorion security firm officials. The method by which attackers obtained access to Anthropic’s official Cursor account remains unexplained by either company.
The backdoored extension attempted to establish persistent SSH access by installing an attacker-controlled SSH key, enabling the local SSH service, and creating a reverse SSH tunnel. The malware was designed to function across Windows, macOS, and Linux systems. Extensions running in Cursor operate with the permissions of the integrated development environment, meaning the compromised version could have accessed local files, source code, and developer tools once installed. The backdoor’s capabilities included variable name theft, which security researchers noted would allow operators to identify cloud credentials and API tokens stored on infected devices for prioritization in follow-up attacks.
Implementation flaws in the malicious code for Windows and macOS systems prevented the backdoor from functioning properly in some cases, though researchers characterized the threat as significant due to its persistence mechanisms. Cursor personnel worked with the hosting provider of the server receiving reverse shell connections to prevent further communication from infected devices. The company removed the malicious version from its marketplace and temporarily shut down the VSCode Marketplace to rotate security keys and audit access controls. Developers who installed or updated the extension between early morning on Thursday and mid-afternoon should consider themselves potentially affected. Despite the incident, neither Anthropic nor Cursor has issued public warnings to customers or provided explanations regarding the account compromise.
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