Compromised Claude Code Extension Spreads Trojan via Cursor Editor — Big Tech and the digital economy

Security researchers have identified a critical breach in the Claude Code extension for the Cursor editor, where attackers injected malware into version 2.1.313. The compromised update automatically installed an SSH backdoor on millions of developer systems across Windows, Linux, and macOS, potentially linking to recent high-profile crypto exchange hacks.

A significant security incident has occurred involving Anthropic's Claude Code extension, which was compromised by attackers who injected malicious code into the official version distributed via the Cursor editor marketplace. Security researcher Kevin Beaumont reported that the affected extension, installed on millions of developer systems, was used to spread a Trojan. The specific vulnerable version is identified as 2.1.313, which was removed from the Cursor marketplace in the early hours of October 30 and replaced with the cleaned version 2.1.314. Despite the removal, systems that were already infected remain at risk.

While the Claude Code extension is also available on the VS Code Marketplace and OpenVSX, the compromise appears to be isolated to the variant hosted on the Cursor platform. According to the platform operator, the compromised version reached a maximum of 5,000 users within that specific marketplace. Security researchers from Phorion were the first to detect the attack, noting that the manipulated extension automatically establishes an SSH backdoor on infected machines. This vulnerability grants attackers access to locally stored files and source code on Windows, Linux, and macOS systems.

Beaumont noted on Mastodon that the attackers' control server is now offline; however, it had previously collected hostnames, usernames, and other system information from compromised developer environments. Given that the Claude Code extension is reportedly used by Binance, researchers speculate a possible connection between this supply chain attack and the recent record-breaking hack of the cryptocurrency exchange. Developers who have installed the Claude Code extension in Cursor are urged to immediately verify if their systems were affected by the manipulated version.

Any device found to be running the compromised software should be treated as breached, and appropriate security measures must be implemented. To assist with detection, Beaumont has published rules on GitHub for Microsoft Defender that allow for the automated identification of the affected versions of the Claude Code extension.

Sources: Golem.de.