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CVE-2026-85751 – Authentication Bypass – Mailu 2.0 through 2024.06.54 (and helm-charts < 2.7.3)

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Summary

CVE-2026-85751 is a critical authentication bypass vulnerability affecting Mailu, an open-source mail server suite distributed as a set of Docker containers. When Mailu is configured to use header-based proxy authentication via PROXY_AUTH_WHITELIST without also setting REAL_IP_HEADER, an unauthenticated remote attacker can spoof the X-Forwarded-By header to impersonate a trusted proxy and bypass authentication entirely. The flaw carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: Mailu’s nginx configuration uses the proxy_hide_header directive to strip the X-Forwarded-By header from responses sent to the client, but this directive does not overwrite or block that same header on incoming requests. As a result, a client-supplied X-Forwarded-By value is passed through to the application as if it originated from a trusted reverse proxy.
  • Trigger conditions: The vulnerability is only exploitable when an administrator has enabled PROXY_AUTH_WHITELIST (header-based proxy authentication) without also configuring REAL_IP_HEADER. Mailu’s documentation did not clearly mandate REAL_IP_HEADER as a prerequisite for safely using PROXY_AUTH_WHITELIST, making misconfiguration likely.
  • Attack vector: Network — no authentication or user interaction is required. An attacker simply sends a crafted HTTP request containing a forged X-Forwarded-By header to the exposed Mailu instance.
  • Impact: Successful exploitation allows an unauthenticated attacker to bypass authentication, resulting in full compromise of confidentiality, integrity, and availability of the mail platform.
  • The flaw was originally introduced via a code change in Mailu’s nginx proxy configuration merged as part of the 2.0 release line.

Affected software

  • Mailu: versions 2.0 through 2024.06.54
  • Mailu helm-charts: versions prior to 2.7.3

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade Mailu to version 2024.06.55 or later, and upgrade the Mailu helm-charts to version 2.7.3 or later.
  • If immediate patching is not possible: Ensure REAL_IP_HEADER is properly configured on any deployment that uses PROXY_AUTH_WHITELIST, so that client-supplied proxy-identity headers such as X-Forwarded-By cannot be spoofed. Alternatively, disable PROXY_AUTH_WHITELIST until the upgrade can be applied.

References

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