Official Claude Code VS code extension compromise on the Cursor marketplace

Security researchers at Phorion have disclosed a compromise of the official Claude Code for VS Code extension distributed via the Cursor Marketplace. The malicious version (2.1.313) was compromised via software supply chain, with a cross-platform SSH backdoor capable of establishing persistent remote access on macOS, Linux, and Windows systems. Following disclosure, the affected release was removed from the marketplace and associated package registries.

Unlike traditional credential stealers, this attack was designed to provide long-term interactive access to developer workstations by planting SSH authentication keys, establishing reverse tunnels, and creating operating system persistence mechanisms.

For organisations using Claude Code, VS Code, or Cursor in development environments, this incident represents a significant escalation in the IDE supply chain threat landscape.

According to Phorion, an attacker modified the extension's primary JavaScript bundle by prepending a malicious immediately-invoked function expression (IIFE) that executed as soon as the extension loaded. User interaction was not required.

Because VS Code and Cursor extensions execute with the permissions of the logged-in user, the malicious code immediately gains access to source repositories, local files, development credentials, SSH configurations and cloud tooling.

The potential impact of this software supply-chain incident is still being evaluated. It is not known whether official versions of the extension were compromised on marketplaces other than Cursor.

If you use VS Code with the extension for Claude, it is advised to check the following IoCs within your environment:

Extension version: 2.1.313
Outbound connections to 134.209.82[.]225
Creation of the following files:
Temporary files beginning: .stk_

~/.config/.smt

~/.config/.smtk
~/Library/LaunchAgents/com.apple.sysmond.plist

Authorized_keys entries referencing: svc-telemetry

If your organisation was exposed to this software supply chain attack, contact the Integrity360 Incident Response team for assistance.

Reference: https://phorion.io/blog/offical-claude-code-vsx-compromise/

If you are worried about any of the threats outlined in this bulletin or need help in determining what steps you should take to protect yourself from the most material threats facing your organisation, please contact your account manager, or alternatively get in touch to find out how you can protect your organisation.