CVE-2025-61984 is a vulnerability identified in OpenSSH versions before 10.1, related to improper handling of control characters in usernames. Specifically, the ssh client allows control characters in usernames that originate from certain untrusted sources, namely the command line and %-sequence expansions in configuration files. This improper handling can lead to code execution when the ProxyCommand directive is used, as ProxyCommand executes arbitrary commands and may be manipulated via crafted usernames containing control characters. The vulnerability is categorized under CWE-159 (Improper Handling of Invalid Use of Special Elements), indicating that the software does not correctly sanitize or validate special control characters in input data. Notably, usernames provided as complete literals in configuration files are not considered untrusted and thus not vulnerable. The CVSS 3.1 base score is 3.6, reflecting low severity due to the requirement for local access vector (AV:L), high attack complexity (AC:H), low privileges (PR:L), no user interaction (UI:N), and limited confidentiality and integrity impacts (C:L/I:L/A:N). There are no known exploits in the wild at the time of publication. The vulnerability arises from the way OpenSSH processes usernames in conjunction with ProxyCommand, which can be exploited to execute arbitrary code if an attacker can supply a username containing control characters from untrusted sources. This flaw highlights the importance of strict input validation and sanitization in security-critical software components.

For European organizations, the impact of CVE-2025-61984 is generally low but non-negligible. The vulnerability could allow an attacker with local access and low privileges to execute arbitrary code via crafted usernames when ProxyCommand is enabled. This could lead to partial compromise of client systems, potentially exposing sensitive data or allowing lateral movement within networks. Organizations relying heavily on OpenSSH for secure remote access, especially those using ProxyCommand for advanced SSH tunneling or jump hosts, may face increased risk. Critical infrastructure sectors, government agencies, and enterprises with stringent security requirements could be targeted if attackers find ways to supply malicious usernames through untrusted sources such as automated scripts or configuration management tools. However, the requirement for local access and the high complexity of exploitation reduce the likelihood of widespread impact. Confidentiality and integrity could be affected to a limited extent, but availability is not impacted. Overall, the threat is moderate for organizations with vulnerable OpenSSH deployments but can be mitigated effectively.

1. Upgrade OpenSSH to version 10.1 or later, where this vulnerability is fixed. 2. Audit and restrict the use of ProxyCommand in SSH configurations, especially in environments where usernames may originate from untrusted sources. 3. Implement strict input validation and sanitization for usernames, particularly when usernames are derived from command line inputs or configuration file expansions. 4. Avoid using %-sequence expansions in SSH configuration files for usernames unless absolutely necessary and ensure these inputs are sanitized. 5. Monitor SSH logs for unusual username patterns containing control characters or unexpected sequences. 6. Employ host-based intrusion detection systems (HIDS) to detect anomalous SSH client behavior. 7. Educate system administrators about the risks of using ProxyCommand with untrusted inputs and enforce least privilege principles for SSH users. 8. Review and harden configuration management and automation scripts that may supply usernames to SSH commands to prevent injection of control characters. 9. Consider network segmentation and access controls to limit local access to systems running vulnerable OpenSSH versions. 10. Maintain up-to-date vulnerability management processes to promptly apply patches and monitor for new exploit developments.