---
title: "CVE-2026-117553: Umbraco CMS - Authentication Bypass"
description: "Umbraco CMS < 17.7.2 and < 18.2.2 contains an authentication bypass caused by Host header poisoning in OAuth redirect URI registration, letting unauthenticated attackers hijack back-office sessions and execute code via Razor templates, exploit requires sending signed-in users authorization links."
url: https://panop.io/threat-heartbeat/CVE-2026-117553/
---

# CVE-2026-117553 Umbraco CMS - Authentication Bypass

Umbraco CMS < 17.7.2 and < 18.2.2 contains an authentication bypass caused by Host header poisoning in OAuth redirect URI registration, letting unauthenticated attackers hijack back-office sessions and execute code via Razor templates, exploit requires sending signed-in users authorization links.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.50% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-117553)

## Details

Type

Authentication Bypass

Impact

Remote Code Execution, Privilege Escalation

CVSS vector

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

Published

27 October 2026

Updated

29 October 2026

Weakness

[CWE-601](https://cwe.mitre.org/data/definitions/601.html)

## Vendor fix

Upgrade to versions 17.7.2 or 18.2.2 or later.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-117553](https://nvd.nist.gov/vuln/detail/CVE-2026-117553)
- [https://github.com/umbraco/Umbraco-CMS](https://github.com/umbraco/Umbraco-CMS)
- [https://github.com/umbraco/Umbraco-CMS/blob/release-18.2.1/src/Umbraco.Cms.Api.Management/Middleware/BackOfficeAuthorizationInitializationMiddleware.cs#L74-L112](https://github.com/umbraco/Umbraco-CMS/blob/release-18.2.1/src/Umbraco.Cms.Api.Management/Middleware/BackOfficeAuthorizationInitializationMiddleware.cs#L74-L112)
- [https://github.com/umbraco/Umbraco-CMS/commit/490b9b27dcbb8fb3fe868a36203590968b0569d6](https://github.com/umbraco/Umbraco-CMS/commit/490b9b27dcbb8fb3fe868a36203590968b0569d6)
- [https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-rpv9-qp4c-g469](https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-rpv9-qp4c-g469)
- [https://www.vulncheck.com/advisories/umbraco-cms-before-17.7.2-and-18.2.2-account-takeover-via-host-header-poisoning](https://www.vulncheck.com/advisories/umbraco-cms-before-17.7.2-and-18.2.2-account-takeover-via-host-header-poisoning)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

Panop answers these inside your environment, for this CVE and every other finding.

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