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HIGH severity

CVE-2026-117553 — Umbraco

HIGHFix: umbraco/Umbraco-CMS@490b9b2

CVE-2026-117553 is a high-severity (CVSS 7.5) Open Redirect vulnerability. No vendor fix is recorded yet; mitigation options are listed below.

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…

Published
Updated
Affected
—
Patched
See advisory
Exploits
None indexed
Exploitation data as of Oct 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs42th percentile — riskier than 42% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-117553 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 396,257 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

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.

Detection & mitigation playbook

Vulnerability

  1. Detect

    Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

  2. Remediation status

    No fixed release is recorded for CVE-2026-117553 yet. Track the upstream advisory, and apply the exposure-reduction steps below in the meantime.

  3. Mitigate without a patch

    Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

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.
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CVE-2026-117553: Umbraco Open Redirect (High 7.5)