CVE-2025-53859

Description

NGINX Open Source and NGINX Plus have a vulnerability in the ngx_mail_smtp_module that might allow an unauthenticated attacker to over-read NGINX SMTP authentication process memory; as a result, the server side may leak arbitrary bytes sent in a request to the authentication server. This issue happens during the NGINX SMTP authentication process and requires the attacker to make preparations against the target system to extract the leaked data. The issue affects NGINX only if (1) it is built with the ngx_mail_smtp_module, (2) the smtp_auth directive is configured with method "none," and (3) the authentication server returns the "Auth-Wait" response header. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

• How can I get the fixes?
• What do statuses mean?

Get expanded security coverage with Ubuntu Pro

Reduce your average CVE exposure time from 98 days to 1 day with expanded CVE patching, ten-years security maintenance and optional support for the full stack of open-source applications. Free for personal use.

References

• MITRE
• NVD
• Launchpad
• Debian

Related Ubuntu Security Notices (USN)

• USN-7715-1
• nginx vulnerability
• 25 August 2025
• USN-8375-1
• nginx vulnerabilities
• 3 June 2026

Other references

• https://www.cve.org/CVERecord?id=CVE-2025-53859
• https://www.openwall.com/lists/oss-security/2025/08/13/5
• https://nginx.org/download/patch.2025.smtp.txt
• https://my.f5.com/manage/s/article/K000152786