What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-27 · updated 2026-10-27
• Weakness: CWE-601

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.

Affected products

• Umbraco-CMS: before 17.7.2; from 18.0.0 before 18.2.2 — fixed in 17.7.2, 18.2.2

How to fix

Upgrade to a fixed version: Umbraco-CMS 17.7.2, 18.2.2. Check the vendor advisory for mitigations.

References

• 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/commit/490b9b27dcbb8fb3fe868a36203590968b0569d6
• 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

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