CVE-2025-53859 is an out-of-bounds read vulnerability classified under CWE-125 found in the ngx_mail_smtp_module of F5 NGINX Plus and NGINX Open Source. This vulnerability arises during the SMTP authentication process when NGINX is configured with the smtp_auth directive set to "none" and the authentication server responds with an "Auth-Wait" header. Under these conditions, an unauthenticated attacker can craft SMTP requests that cause NGINX to over-read memory buffers, potentially leaking arbitrary bytes from the server's memory related to the authentication process. The vulnerability requires no privileges or user interaction but has a high attack complexity because the attacker must prepare the target system to extract leaked data effectively. The affected versions include NGINX Plus releases R30 through R34. The flaw does not allow modification of data or denial of service but poses a confidentiality risk by exposing memory contents. No patches or exploits are currently publicly available, and versions past their End of Technical Support are not evaluated. The vulnerability is network exploitable remotely without authentication, but the impact is limited to partial information disclosure with no integrity or availability impact.

For European organizations, the impact of CVE-2025-53859 is primarily related to confidentiality leakage in SMTP authentication processes handled by NGINX Plus. Organizations using NGINX Plus as a mail proxy or gateway with the vulnerable configuration could inadvertently expose sensitive authentication data or other memory contents to remote attackers. While the vulnerability does not allow code execution or denial of service, leaked information could aid attackers in further reconnaissance or targeted attacks. This risk is heightened in sectors with stringent data protection requirements such as finance, healthcare, and government institutions across Europe. However, the high complexity of exploitation and specific configuration requirements limit widespread impact. Organizations running affected versions in critical mail infrastructure should assess their exposure, especially if they rely on the smtp_auth "none" method and external authentication servers that return "Auth-Wait" headers. The vulnerability does not affect availability or integrity, reducing the risk of operational disruption but still posing a privacy concern under GDPR and other European data protection regulations.

To mitigate CVE-2025-53859, European organizations should first verify whether their NGINX Plus deployments use the ngx_mail_smtp_module with smtp_auth configured to "none" and if their authentication servers return the "Auth-Wait" header. If so, consider changing the smtp_auth method to a more secure option that does not trigger this vulnerability. Applying the latest NGINX Plus updates or patches from F5 as they become available is critical, even though no patches are currently listed, monitoring vendor advisories is essential. Network-level protections such as restricting access to SMTP proxy services to trusted networks and implementing strict firewall rules can reduce exposure. Additionally, enabling detailed logging and monitoring for unusual SMTP authentication requests may help detect exploitation attempts. Organizations should also conduct security reviews of their mail proxy configurations and consider alternative authentication mechanisms that do not rely on vulnerable code paths. Finally, ensure that all systems are running supported software versions to benefit from ongoing security updates.