# CVE-2026-117553: Umbraco CMS authentication bypass via Host header poisoning

Source: https://junglewise.ai/threats/cve-2026-117553-umbraco-cms-before-17-7-2-and-18-2-2-contains-an-authentication  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-27. Updated: 2026-10-27  
License: CC BY 4.0 with a link back

CVE-2026-117553 · Severity: high · CVSS 7.5 · Published 2026-10-27

Umbraco CMS is a content management system that powers website creation and management. This vulnerability allows attackers to bypass authentication by poisoning HTTP headers and manipulating OAuth settings, enabling them to hijack administrative accounts and execute malicious code on the website without needing valid credentials. An attacker could gain full control of the website's back office, modify content, and run arbitrary code.

Technologies: [Umbraco CMS](https://junglewise.ai/threats/technologies/umbraco-cms-1). Vendors: [Umbraco](https://junglewise.ai/threats/vendors/umbraco).

## Quick answers

**Is CVE-2026-117553 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-117553 being exploited in the wild.

**How severe is CVE-2026-117553?** High severity, with a CVSS score of 7.5, published 27 October 2026.

**Which products and versions are affected?** Umbraco CMS before 17.7.2 and before 18.2.2.

**Is there a fix for CVE-2026-117553?** Yes: fixed in 17.7.2, 18.2.2. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-117553 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-117553?** Check whether you run Umbraco CMS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 17.7.2, 18.2.2

## Technical details

The vulnerability is an authentication bypass in Umbraco's back-office authorization that exploits Host header poisoning combined with OAuth redirect URI manipulation. An unauthenticated attacker can register attacker-controlled OAuth redirect URIs, then trick signed-in users into authorization links using response_mode=form_post to capture authorization codes, which are subsequently redeemed via HTTP Basic authentication to hijack back-office sessions and execute code through Razor templates. The attack requires user interaction (clicking a malicious link) but no prior authentication.

## Affected products

- Umbraco CMS before 17.7.2 and before 18.2.2

## Timeline

- 2026-10-27: disclosed

## References

- [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](https://github.com/umbraco/Umbraco-CMS/blob/release-18.2.1/src/Umbraco.Cms.Api.Management/Middleware/BackOfficeAuthorizationInitializationMiddleware.cs)