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9.8

Mailu mail server can be tricked to skip login

CVE-2026-85751 · published today
Summary

Mailu installations that use proxy authentication but do not set the REAL_IP_HEADER may trust a client‑controlled X‑Forwarded‑By header. An attacker can fake this header to appear as a trusted proxy and gain access without logging in. Update Mailu to the latest version or apply the newer helm chart to resolve the issue.

What to do

The CVE record does not list a fixed version. Check the vendor's site or the advisory links below - a fix may already be released.

Affected software
VendorProductAffected versions
mailu mailu >= 2.0.0, < 2024.06.55
mailu helm-charts < 2.7.3
Original advisory text
Mailu: Authentication bypass in header-based proxy authentication via spoofable `X-Forwarded-By` trust
Mailu is a mail server distributed as a set of Docker images. From Mailu 2.0 until 2024.06.55 and prior to Mailu helm-charts 2.7.3, deployments with PROXY_AUTH_WHITELIST configured but REAL_IP_HEADER unset trusted a client-controlled X-Forwarded-By header for header-based proxy authentication. The proxy_hide_header directive in the nginx template at core/nginx/conf/proxy.conf hid the header from upstream responses but did not overwrite the incoming request value in this configuration. An unauthenticated remote attacker could therefore spoof the trusted proxy identity and bypass authentication. This issue is fixed in Mailu 2024.06.55 and Mailu helm-charts 2.7.3.
References
Severity
9.8 Critical
CVSS 3.1: 9.8 (MITRE)
Type
CWE-290Authentication Bypass by Spoofing
CWE-807Reliance on Untrusted Inputs in a Security Decision
Timeline
Published21 Sep 2026
Updated21 Sep 2026
First seen21 Sep 2026
Sources
CVE-2026-85751 · NVD
CVE-2026-85751 · MITRE
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