What’s Vulnerable › Vulnerabilities

CVE-2026-85751: Mailu: Authentication bypass in header-based proxy authentication via spoofable `X-Forwarded-By` trust

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-21 · updated 2026-09-24
• Weakness: CWE-290

Description

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.

Affected products

• Mailu helm-charts: before 2.7.3
• Mailu: from 2.0.0, < 2024.06.55

References

• https://github.com/Mailu/Mailu/commit/dffa97cbd889ab208246372ce9c86e99abb27ae9
• https://github.com/Mailu/Mailu/commit/ff4003d045753013470945336448dbd790cac778
• https://github.com/Mailu/Mailu/pull/4070
• https://github.com/Mailu/Mailu/pull/4071
• https://github.com/Mailu/Mailu/releases/tag/2024.06.55
• https://github.com/Mailu/Mailu/security/advisories/GHSA-rfhj-4wcq-74xg

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