Why Your SSH Implementation Is Leaking More Than You Think

TLDR: Most security professionals treat SSH as a black box, but recent research into the SSH multiverse reveals that vendor-specific implementations and embedded devices are riddled with pre-authentication information leaks and state machine flaws. By leveraging tools like SSHamble, researchers can now identify these vulnerabilities, including remote code execution and authentication bypasses, across millions of exposed devices. This post breaks down how these flaws manifest and why you should stop trusting default SSH configurations on your network infrastructure.

SSH is the backbone of remote administration, yet we treat it with a level of complacency that is frankly dangerous. We assume that because it is "SSH," it is secure. The reality is that the SSH ecosystem is a fragmented mess of vendor-specific forks, embedded libraries, and outdated codebases that rarely receive the same scrutiny as standard OpenSSH. When you scan the internet, you are not just seeing OpenSSH; you are seeing a multiverse of implementations that leak system information, kernel versions, and even authentication state before a single packet of encrypted data is exchanged.

The Pre-Authentication Information Leak

Many engineers believe that public key authentication is a silver bullet. While it is certainly better than password-based auth, it does not protect you from the information leakage that occurs during the initial handshake. Before the server even asks for your credentials, it is already broadcasting its identity.

The TCP/IP window size and scaling factors, combined with the server version string, often allow an attacker to fingerprint the exact distribution and kernel version of the target. Once you have that, you can cross-reference it with known vulnerabilities. Even worse, some implementations include specific package versions in their server banners. If you are running an outdated version of a library, you are essentially handing an attacker a roadmap to your vulnerabilities before they even attempt to log in.

State Machine Vulnerabilities and RCE

The most critical finding in recent research is that SSH is essentially a complex state machine. If you can manipulate the transitions between these states, you can force the server into an error condition that it was never designed to handle. This is exactly what happened with the RegressSHion vulnerability. By triggering a signal re-entrance issue, researchers were able to achieve unauthenticated remote code execution.

This is not a theoretical bug. It is a fundamental flaw in how the protocol handles signals during the pre-authentication phase. When you consider that many embedded devices—firewalls, load balancers, and IoT gateways—are running these vulnerable versions, the attack surface is massive.

Consider the MOVEit vulnerability. The flaw was not in the application itself, but in the third-party IPWorks library it used for SSH. By embedding a file path into the public key blob during the handshake, an attacker could force the server to treat that blob as a file, leading to an authentication bypass. This is a classic example of why you must audit your dependencies. If your application relies on a third-party library for SSH, you are only as secure as that library.

Auditing Your Own Infrastructure

If you are a pentester, you need to stop relying on generic scanners. You need to understand the state machine of the target. This is where SSHamble becomes essential. It allows you to probe the SSH implementation at different stages of the protocol. You can ask the server to open a shell, perform port forwarding, or send arbitrary signals before you have even authenticated.

For example, you can use the following command to test for specific behaviors:

sshamble scan -o results.json 192.168.0.0/24

This will generate a JSON file detailing the implementation quirks of every host in your range. You can then grep for specific behaviors, such as whether the server allows port forwarding before authentication. If you find a device that does, you have a pivot point into the internal network, regardless of whether you have valid credentials.

The Defensive Reality

Defending against these attacks requires a shift in how we manage SSH. First, stop exposing SSH to the public internet. If you must, use a bastion host or a VPN. Second, audit your vendor-provided appliances. If you are running a Cisco or Juniper device, check the vendor security advisories regularly. These devices often run proprietary SSH stacks that are not patched as frequently as OpenSSH.

Finally, pay attention to the OWASP Top 10 categories, specifically A06:2021-Vulnerable and Outdated Components. If you are building an application that uses an SSH library, ensure that library is actively maintained. If the vendor stops providing updates, you need to replace the component or isolate the device.

The SSH multiverse is vast, and it is full of hidden doors. As researchers, our job is to find them before the threat actors do. Start by scanning your own environment with the right tools and questioning the security of the "standard" configurations you have been using for years. The next time you see a port 22 open, do not just assume it is secure. Assume it is a state machine waiting to be broken.