CVE-2026-85751 PUBLISHED

Mailu: Authentication bypass in header-based proxy authentication via spoofable `X-Forwarded-By` trust

Assigner: GitHub_M
Reserved: 04.09.2026 Published: 21.09.2026 Updated: 21.09.2026

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.

Metrics

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS Score: 9.8

Product Status

Vendor Mailu
Product Mailu
Versions
  • Version >= 2.0.0, < 2024.06.55 is affected
Vendor Mailu
Product helm-charts
Versions
  • Version < 2.7.3 is affected

References

Problem Types

  • CWE-290: Authentication Bypass by Spoofing CWE
  • CWE-807: Reliance on Untrusted Inputs in a Security Decision CWE