libssh ProxyCommand Username Injection Vulnerability

Executive Summary

This vulnerability in libssh involves improper handling of username expansion in ProxyCommand. When a username contains %r and is expanded, malicious shell metacharacters can be injected. This bypasses validation in ssh_check_username_syntax() and allows attackers to manipulate shell behavior, potentially exposing environment variables or causing unintended actions.

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Detection Guidance

Check libssh versions for affected releases and inspect logs for ProxyCommand handling with username expansion containing %r. Look for unusual shell metacharacters in username fields during SSH connections.

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Impact Analysis

If you use libssh with ProxyCommand and untrusted username input, an attacker could craft a username to execute shell commands or leak sensitive environment variables. This requires the attacker to control part of the username and the system to use ProxyCommand with shell evaluation.

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Compliance Impact

This vulnerability could lead to information disclosure, which may violate GDPR (data protection) or HIPAA (privacy) if sensitive data like environment variables are exposed. Compliance depends on whether the affected system processes regulated data and the impact of the disclosure.

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Mitigation Strategies

Update libssh to the latest patched version. Disable ProxyCommand usage if not required. Validate all username inputs to prevent shell metacharacter injection before expansion.

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