CVE-2026-117553: Umbraco-CMS (High, CVSS 7.7)
- CVSS
- 7.7 High (v4.0)
- EPSS
- 0.5% chance of exploitation in the next 30 days (higher than 42.3% of CVEs)
- Exploited in the wild
- Not on CISA’s KEV list
- Public exploit
- None reported in public exploit databases
- Weakness
- CWE-601
- 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- Published
- Oct 27, 2026
What it is
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.
Description from the National Vulnerability Database.
What to do
Check whether you run Umbraco-CMS, then apply the vendor’s fix or mitigation from the advisories below.
Sources: NVD, FIRST EPSS, CISA KEV, and public exploit databases (availability only). ThreatRoundup reports whether public exploit code exists but never links to it: fake proof-of-concepts that infect security researchers are common. This page updates automatically.