CVE-2025-61984 is a vulnerability identified in OpenSSH versions prior to 10.1, related to CWE-159 (Improper Handling of Invalid Use of Special Elements). The issue stems from the SSH daemon allowing control characters within usernames that are sourced from potentially untrusted inputs, specifically the command line and %-sequence expansions in configuration files. These control characters can be maliciously crafted to manipulate the ProxyCommand feature of OpenSSH, potentially leading to arbitrary code execution. However, usernames provided as complete literals in configuration files are not considered untrusted and thus not vulnerable. The vulnerability requires local access (attack vector: local), has high attack complexity, and requires low privileges but no user interaction. The impact on confidentiality and integrity is limited, and availability is unaffected. The vulnerability has a CVSS v3.1 base score of 3.6, indicating low severity. No public exploits or active exploitation have been reported. The root cause is insufficient sanitization and validation of special control characters in usernames when processed in certain contexts, allowing an attacker to inject commands via ProxyCommand expansions. This vulnerability highlights the importance of strict input validation in security-critical components like SSH, especially when handling user identifiers and configuration expansions.

For European organizations, the impact of CVE-2025-61984 is relatively low but non-negligible. Organizations that rely on OpenSSH versions prior to 10.1 and utilize ProxyCommand in their SSH configurations could face risks of unauthorized code execution if an attacker can supply crafted usernames through local access or configuration manipulations. This could lead to limited confidentiality and integrity breaches, such as unauthorized command execution or privilege escalation within the SSH session context. Although the attack complexity is high and requires local access with low privileges, environments with shared access or multi-tenant systems could be vulnerable. Critical infrastructure, government agencies, and enterprises with complex SSH setups might be more exposed. The absence of known exploits reduces immediate risk, but the vulnerability could be leveraged in targeted attacks or insider threat scenarios. The overall availability of systems is unlikely to be affected. European organizations should consider this vulnerability in their risk assessments, especially those with stringent security requirements and compliance obligations.

1. Upgrade OpenSSH to version 10.1 or later, where this vulnerability is addressed. 2. Audit and sanitize all SSH configuration files, especially those using %-sequence expansions, to ensure usernames do not contain control characters or other special elements. 3. Avoid using ProxyCommand with untrusted or dynamically generated usernames. 4. Implement strict access controls to limit local access to trusted users only, reducing the risk of exploitation via local attack vectors. 5. Monitor SSH logs for unusual username patterns or ProxyCommand usage that could indicate exploitation attempts. 6. Employ host-based intrusion detection systems (HIDS) to detect anomalous command executions related to SSH sessions. 7. Educate system administrators on the risks of improper username handling and the importance of secure SSH configuration practices. 8. Consider deploying application whitelisting or sandboxing for SSH-related processes to limit potential damage from code execution.