Malicious Claude Code Extension Infected Up to 5,000 Developers

It secretly installed a backdoor on the computer, through which attackers could remotely access the system. Users of macOS, Linux, and Windows were at risk.

The issue was first detected by cybersecurity specialists from the company Phorion. They advise companies to urgently check whether this version is installed on employees' devices and, if so, consider the device compromised.

The counterfeit version did not make it into the official Microsoft Visual Studio Marketplace. It has now been replaced with the clean version 2.1.314. However, the update does not remove the backdoor from already infected computers; it must be deleted separately.

Cursor confirmed the incident. According to the company, up to 5,000 users were affected, and each will be contacted personally. The malicious program collected data about the computer and its owner: username, operating system, email linked to Git, IP address, and other details. It then attempted to grant attackers persistent remote access, restoring it approximately every minute. However, due to errors in the code, this usually failed. The exception was when Cursor was run with administrator privileges.

In response, Cursor removed the infected version, temporarily disabled the extension store, checked other extensions (none were affected), changed access keys, and ensured the attackers' server was taken offline.

Cursor states that Anthropic did not create the malicious version. However, it was uploaded through the company's verified account. This was likely a supply chain attack: hackers gained access to the channel through which the official developer releases updates. According to a source, Anthropic did not notify users about the incident and quietly removed the infected version.