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Vulnerability · Published Sep 21, 2026 · Assigned by GitHub_M

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

Critical 9.8

In plain English

A critical-severity authentication bypass by spoofing in helm-charts from Mailu. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities).

Severity (CVSS 3.1)9.8Critical
Exploit odds (30 days)0.6%top 53.4% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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 (as reported)

VendorProductVersions
Mailu Mailu >= 2.0.0, < 2024.06.55
Mailu helm-charts < 2.7.3

Weakness: CWE-290, CWE-807 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

References