A malicious version of Anthropic’s official Claude extension for VS Code was distributed through Cursor’s marketplace on Thursday, compromising the security of potentially thousands of developer machines. The backdoored package, identified as version 2.1.313, was made available through Anthropic’s verified account on Cursor before being removed after several hours. Independent researchers and security firm Phorion confirmed that the compromised version was replaced by a patched release, version 2.1.314, but the method by which attackers gained access to Anthropic’s official account remained unexplained.
The malicious code attempted to establish persistent SSH backdoors on affected systems by installing attacker-controlled SSH keys, enabling SSH services, and creating reverse tunnels for remote access. The backdoor was designed to function across Windows, macOS, and Linux platforms. By running with the permissions of Cursor’s integrated development environment, the compromised extension would have gained access to local files, source code, developer tooling, and stored credentials. Researchers noted that the malware was capable of harvesting variable names, which could allow attackers to identify and prioritize cloud credentials and API tokens for subsequent targeted attacks.
Cursor personnel responded by working with the hosting provider to disable communication channels between infected devices and attacker-controlled servers. The company removed the backdoored version from its marketplace and temporarily shut down the VSCode Marketplace to rotate security keys, audit access controls, and verify that no other extensions were affected. The marketplace has since reopened. However, neither Anthropic nor Cursor has issued public warnings to affected users or provided explanations regarding how the account compromise occurred. Users who installed or updated the extension between 5:27 am ET and 4:35 pm on Thursday are considered potentially affected, with both companies publishing indicators of compromise to assist in detection efforts.
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