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Stop MitM on the first SSH connection, on any VPS or cloud provider

This deep dive introduces a novel, provider-independent technique to secure the critical first SSH connection to cloud VMs, preventing Man-in-the-Middle attacks where traditional Trust On First Use (TOFU) falls short. It leverages cloud-init to inject a temporary host key, then rotates to a persistent one, safeguarding against compromised cloud-init data and network attackers. The author challenges existing security practices, offering a robust solution for a common cloud deployment vulnerability.

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#7
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18h
on Front Page
First Seen
May 10, 6:00 PM
Last Seen
May 11, 11:00 AM
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The Lowdown

The article details a method to prevent Man-in-the-Middle (MitM) attacks during the initial SSH connection to newly provisioned virtual machines in the cloud. It highlights a script that uses cloud-init to securely establish trust, overcoming the inherent vulnerabilities of the "Trust On First Use" (TOFU) model and the risks associated with exposing long-term keys in cloud-init user data.

  • The Problem: The first SSH connection to a new VM is vulnerable to MitM attacks because the host's authenticity is unknown, and simply accepting an unknown host key (TOFU) is insufficient. This is exacerbated by some providers lacking built-in secure initial connection mechanisms.
  • The Solution: The author proposes injecting a temporary SSH host private key via cloud-init. This temporary key is used only long enough to securely generate and retrieve the "real" (long-term) SSH host keys.
  • Security Advantages: This technique ensures that the temporary key is never permanently saved in ~/.ssh/known_hosts and, crucially, that the valuable long-term private key is never exposed through cloud-init user data or metadata services, which can often be accessed by an attacker.
  • Hardened Implementation: The accompanying script provides a robust implementation, leveraging OpenSSH's key rotation features to write only valid, controlled keys to the administrator's known_hosts file.
  • Threat Model: The article thoroughly analyzes how this method protects against various attack vectors, including network attackers, compromised administrator workstations, and even compromised VM/provider infrastructure, assuming the OpenSSH protocol itself remains secure.
  • MitM Nuances: An important aside clarifies the conditions under which a network MitM attack might succeed (e.g., if the administrator can be tricked into entering a password or forwarding an agent/X11 connection) versus when it likely fails.

This innovative approach offers a significant improvement in initial VM connection security by strategically using temporary keys and cloud-init, addressing a critical gap in many cloud deployment scenarios.