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Vulnerability record · CVE-2026-85751 · published 21 September 2026

CVE-2026-85751: Authentication bypass by spoofing vulnerability

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.

9.8 CVSS 3.1 Critical EPSS 0.56% · top 55.8% CWE-290 · Authentication bypass by spoofingCWE-807 · CWE-807 Deferred
9.8CVSS 3.1 base score
0.56%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
6References
24 Sep 2026Last modified by NVD

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.

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

References

LinkTags
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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Source: NIST National Vulnerability Database (record CVE-2026-85751), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.