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

> An unauthenticated attacker can bypass authentication in Mailu by spoofing the X-Forwarded-By header when specific proxy settings are enabled.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 Sep 2026
- **Updated:** 21 Sep 2026
- **Also known as:** GHSA-rfhj-4wcq-74xg

## What is CVE-2026-85751?

CVE-2026-85751 is a critical authentication bypass vulnerability in Mailu, published on 21 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-807 (Reliance on Untrusted Inputs in a Security Decision), CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2026-85751?


## How do I fix CVE-2026-85751?

No fix has been published for CVE-2026-85751 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-85751 being exploited?

There is no known exploitation of CVE-2026-85751 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-85751 and fix it if it is affected.

What it is: CVE-2026-85751 is a critical authentication bypass vulnerability in Mailu, published on 21 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-85751 in the wild and no public exploit code.

Affected product: Mailu. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-85751/ (Markdown: https://insights.sbomify.com/CVE-2026-85751/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-85751
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-85751
- OSV: https://osv.dev/vulnerability/CVE-2026-85751

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-85751/
Summaries are generated and may contain mistakes; the official description above is the record.