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.

CVSS v3.1 Score

3.7

LOW

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Weakness Type (CWE)

CWE-125 Out-of-bounds Read

Affected Products

Vendor	Product	Versions
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_plus	All
f5	nginx_open_source	0.7.22 — 1.29.1

References

Advisories & Patches

https://my.f5.com/manage/s/article/K000152786

Other References

http://www.openwall.com/lists/oss-security/2025/08/13/5

Frequently Asked Questions

What is CVE-2025-53859? +

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. It has a CVSS v3.1 base score of 3.7 (LOW).

How severe is CVE-2025-53859? +

CVE-2025-53859 has a CVSS v3.1 score of 3.7 out of 10, rated LOW. This is a low-severity vulnerability with limited impact.

What products are affected by CVE-2025-53859? +

CVE-2025-53859 affects products from f5, specifically: nginx_open_source, nginx_plus. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-53859? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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