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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-85751
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-21 16:17:25 UTC
Updated2026-09-21 18:17:11 UTC
DescriptionMailu 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.

Risk And Classification

Primary CVSS: v3.1 9.8 CRITICAL from [email protected]

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

EPSS: 0.005570000 probability, percentile 0.452820000 (date 2026-09-22)

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

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary9.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
3.1CNADECLARED9.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Mailu Mailu affected >= 2.0.0, < 2024.06.55 Not specified
CNA Mailu Helm-charts affected < 2.7.3 Not specified

References

ReferenceSourceLinkTags
github.com/Mailu/Mailu/commit/ff4003d045753013470945336448dbd790cac778 [email protected] github.com
github.com/Mailu/Mailu/pull/4071 [email protected] github.com
github.com/Mailu/Mailu/releases/tag/2024.06.55 [email protected] github.com
github.com/Mailu/Mailu/security/advisories/GHSA-rfhj-4wcq-74xg [email protected] github.com
github.com/Mailu/Mailu/commit/dffa97cbd889ab208246372ce9c86e99abb27ae9 [email protected] github.com
github.com/Mailu/Mailu/pull/4070 [email protected] github.com
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