CVE-2026-85751

CRITICALPre-NVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.8 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.6%, top 55% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
Trending — 3 sources updated this week
9.8EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1%CVSS: 9.8Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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 v3
9.8
EG Score
9.8(medium)
EG Risk
49(Track)
EG Risk 49/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity98% × 45%
Exploitation1% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
1%
EPSS %ILE
45%
KEV
Not listed

Published

September 21, 2026

Last Modified

September 21, 2026

Advisory Details (6)

Auto-updated Sep 21, 2026
Patch available. Sources: github_commit, github_pr, github_release, github.
github Patch Available

Authentication bypass in header-based proxy authentication via spoofable `X-Forwarded-By` trust · Advisory · Mailu/Mailu · GitHub

https://github.com/Mailu/Mailu/security/advisories/GHSA-rfhj-4wcq-74xg
github_release Patch Available

2024.06.55

Patch available: Mailu/Mailu 2024.06.55

https://github.com/Mailu/Mailu/releases/tag/2024.06.55
github_pr Patch Available

Ensure we set X-Forwarded-By (backport #4070)

Patch available: Mailu/Mailu 2024.06.55 (PR #4071 merged 2026-07-18)

https://github.com/Mailu/Mailu/pull/4071
github_pr

Ensure we set X-Forwarded-By

Fix merged in Mailu/Mailu PR #4070 on 2026-07-18 — awaiting tagged release

https://github.com/Mailu/Mailu/pull/4070
github_commit Patch Available

commit ff4003d04575 (Mailu/Mailu)

Patch available: Mailu/Mailu 2024.06.55 (contains commit ff4003d04575)

https://github.com/Mailu/Mailu/commit/ff4003d045753013470945336448dbd790cac778
github_commit

commit dffa97cbd889 (Mailu/Mailu)

Fix landed in Mailu/Mailu commit dffa97cbd889 — awaiting tagged release

https://github.com/Mailu/Mailu/commit/dffa97cbd889ab208246372ce9c86e99abb27ae9

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 7× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-09-23 19:08 UTCEG score recompute
  2. 2026-09-23 17:54 UTCEPSS rescore
  3. 2026-09-22 16:42 UTCEG score recompute
  4. 2026-09-22 16:01 UTCEPSS rescore
  5. 2026-09-21 16:24 UTCEG score recompute
  6. 2026-09-21 15:45 UTCEG score recompute
  7. 2026-09-21 15:45 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-85751?
CVE-2026-85751 is a critical vulnerability published on September 21, 2026. 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 PROXYAUTHWHITELIST configured but REALIPHEADER unset trusted a client-controlled X-Forwarded-By header for header-based proxy authentication. The…
When was CVE-2026-85751 disclosed?
CVE-2026-85751 was first published in the National Vulnerability Database on September 21, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-85751 actively exploited?
CVE-2026-85751 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1% probability of exploitation in the next 30 days, which ranks it in the top 54.7% of all scored CVEs.
What is the CVSS score of CVE-2026-85751?
CVE-2026-85751 has a CVSS v3 base score of 9.8 (NVD).
How do I remediate CVE-2026-85751?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-85751, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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