Open Redirect in Umbraco Umbraco-Cms
CVE-2026-117553
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.
Vulnerability class: Open Redirect
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
Weakness classification (CWE)
References
- github.com/umbraco/Umbraco-CMS/commit/490b9b27dcbb8fb3fe868a36203590968b0569d6 (patch)
- github.com/umbraco/Umbraco-CMS (product)
- github.com/umbraco/Umbraco-CMS/security/advisories/GHSA-rpv9-qp4c-g469 (vendor-advisory)
- github.com/umbraco/Umbraco-CMS/blob/release-18.2.1/src/Umbraco.Cms.Api.Manageme… (technical-description)
- www.vulncheck.com/advisories/umbraco-cms-before-17.7.2-and-18.2.2-account-takeo… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-117553?
- CVE-2026-117553 is a high-severity vulnerability in Umbraco Umbraco-Cms, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 7.5/10. Published 2026-10-27.
- How severe is CVE-2026-117553?
- High severity. CVSS v3 base score is 7.5 out of 10.