The SSH ProxyCommand feature is not activated by default in a standard OpenSSH installation. Users need to specifically set up their SSH client settings and define the ProxyCommand directive to make use of this functionality. Therefore, even though the feature is present, it remains inactive unless configured appropriately.
Vulnerability Overview
The identified vulnerability emerges when an invalid user or hostname, containing shell metacharacters, is supplied to SSH. If a ProxyCommand, LocalCommand directive, or “match exec” predicate references the username or hostname using expansion tokens, an attacker may exploit the situation for command injection. This risk is particularly notable in untrusted Git repositories that house submodules with shell metacharacters in usernames or hostnames.
Exploitation Scenario
To exploit this vulnerability, an attacker could manipulate SSH by providing arbitrary user/hostnames. The injection could be carried out within an untrusted Git repository, leveraging a submodule with shell metacharacters in either the username or hostname.
Key Steps for Mitigation
Recognizing the gravity of this security flaw, vendors and maintainers of affected implementation applications, including LibSSH, OpenSSH, Debian, and others, have promptly released fixes. Users are strongly advised to refer to their individual vendor advisories corresponding to their operating systems and promptly install the provided patches.
Stay Informed
Keep abreast of vendor advisories and security updates related to SSH implementations in use. Regularly check for updates from your operating system and application providers.
Apply Patches Promptly
Immediately install the patches released by vendors. Timely application of these patches is crucial to closing the vulnerability gap and fortifying your system against potential exploits.
Review SSH Configurations
Examine your SSH configurations to ensure they adhere to security best practices. This includes scrutinizing ProxyCommand, LocalCommand directives, and “match exec” predicates that may incorporate expansion tokens.
Monitor Git Repositories
Exercise caution with Git repositories, particularly those considered untrusted. Be vigilant about submodule content, especially if they involve usernames or hostnames with shell metacharacters.
The critical vulnerability in SSH ProxyCommand underscores the importance of proactive security measures. By promptly applying patches, staying informed about security updates, and reviewing SSH configurations, users can mitigate the risks associated with this flaw. Stay secure, stay vigilant. We can provide the solutions required, our security experts are able to help you.
