CVE-2025-53859: NGINX ngx_mail_smtp_module vulnerability

Severity: low (CVSS 3.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-08-13 · updated 2026-06-17
Weakness: CWE-125

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.

Affected products

F5 NGINX Open Source: from 0.7.22 before 1.29.1 — fixed in 1.29.1
F5 NGINX Plus: r30, r31, r32, r32-p1, r32-p2, r33 (+4)

How to fix

Upgrade to a fixed version: F5 NGINX Open Source 1.29.1. Check the vendor advisory for mitigations.

References

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