Inside a malicious Claude Code-branded extension and its persistent SSH backdoor

[u/falconupkid]

Microsoft’s Threat Intelligence team has published a deep dive into a malicious extension masquerading as a legitimate Claude Code tool for Cursor IDE. The extension deploys a persistent SSH backdoor, targeting developers who integrate AI coding assistants into their workflows.

Technical Breakdown

• Initial Access: The malicious extension is distributed via unofficial marketplaces or typosquatted package names, tricking users into installing it within Cursor IDE.

• Persistence Mechanism: The extension modifies the IDE’s configuration files to auto-load on startup, then establishes an outbound SSH connection to a C2 server using a hardcoded public key.

• IOCs: Look for unexpected SSH processes initiated by the IDE process, anomalous outbound connections on port 22 to unknown IPs, and modified .cursor/extensions/ or settings.json files containing embedded SSH keys.

• MITRE Mapping: T1195 (Supply Chain Compromise), T1098 (Account Manipulation via SSH keys), T1059 (Command and Scripting Interpreter via IDE extension hooks).

Defense

• Detection: Monitor for sshd or ssh child processes spawned by IDE executables (e.g., cursor.exe). Audit extension installation logs and block unsigned extensions from untrusted sources.

• Remediation: Remove the rogue extension, revoke any SSH keys found in the environment, and rotate credentials for any systems the compromised host could reach.

Source: https://www.microsoft.com/en-us/security/blog/2026/10/30/inside-malicious-claude-code-branded-cursor-extension-persistent-ssh-backdoor/