NGINX
CVE-2025-53859: NGINX ngx_mail_smtp_module vulnerability
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.
Published
Automatic summary from the vendor's CVE record. We haven't written a full analysis of this vulnerability yet.
Affected and fixed versions
| Product / branch | Fixed in |
|---|---|
| NGINX Plus R35 | Not affected |
| NGINX Plus R34 | R34 P2 or later |
| NGINX Plus R33 | R33 P3 or later |
| NGINX Plus R32 | R32 P3 or later |
| NGINX Plus R31 | * or later |
| NGINX Plus R30 | * or later |
| NGINX Open Source 0.7 | 1.29.1 or later |
Always confirm against the vendor advisory, which lists every fixed hotfix.
What it is
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.
Sources
KEV status, EPSS score and vendor data refreshed automatically, last on 12 Oct 2026.