---
id: "CVE-2026-117553"
kind: "vulnerability"
aliases: ["CVE-2026-117553"]
title: "Umbraco CMS: Host header poisoning lets attackers hijack back-office sessions"
severity: "high"
cvss: 7.7
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["asp.net", "cms", "runtime", "self-hosted", "umbraco"]
not_affected_if: ["patched-version"]
published_at: "2026-10-27T14:16:35+00:00"
first_seen_at: "2026-10-27T14:32:18+00:00"
updated_at: "2026-10-27T14:32:54+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-117553", "first_seen_at": "2026-10-27T14:32:18+00:00"}]
---

# Umbraco CMS: Host header poisoning lets attackers hijack back-office sessions

Umbraco CMS before 17.7.2 and 18.2.2 trusts the Host header when registering OAuth redirect URIs, so an unauthenticated attacker can register their own redirect URI. Attackers send signed-in editors authorization links using response_mode=form_post to capture codes, then redeem them to take over back-office sessions and run code through Razor templates.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed Umbraco CMS version in the project's package references; versions below `17.7.2` on the 17 line or below `18.2.2` on the 18 line are affected.
- unknown

## Fix

- Upgrade Umbraco CMS to `17.7.2` or later on the 17 line, or `18.2.2` or later on the 18 line.

## Timeline

- 2026-10-27T14:32:18+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-117553

## From the sources

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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_