CVE-2026-27651: NGINX ngx_mail_auth_http_module vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.0% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-24 · updated 2026-07-15
Weakness: CWE-476

Description

When the ngx_mail_auth_http_module module is enabled on NGINX Plus or NGINX Open Source, undisclosed requests can cause worker processes to terminate. This issue may occur when (1) CRAM-MD5 or APOP authentication is enabled, and (2) the authentication server permits retry by returning 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 0.5.15 through 0.9.7; from 1.0.0 before 1.28.3; from 1.29.0 before 1.29.7 — fixed in 1.28.3, 1.29.7
F5 NGINX Plus: from r33 before r35; r32, r32-p1, r32-p2, r32-p3, r32-p4, r35 (+4) — fixed in r35

How to fix

Upgrade to a fixed version: F5 NGINX Open Source 1.28.3, 1.29.7; F5 NGINX Plus r35. Check the vendor advisory for mitigations.

References

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