Bug 2508306 (CVE-2026-18211)

Summary: CVE-2026-18211 keycloak-services: keycloak-services: secure-client-uris policy bypass via localhost-prefixed domains
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: anujha, aschwart, asoldano, aszczucz, bbaranow, bmaxwell, boliveir, bstansbe, dlofthou, drichtar, istudens, ivassile, iweiss, mosmerov, mposolda, msvehla, nwallace, pberan, pesilva, pjindal, pmackay, rmartinc, rstancel, ssilvert, sthorger, thjenkin, vdosoudi, vmuzikar
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the secure-client-uris client policy executor within Keycloak core services. This component is responsible for enforcing security requirements on client configurations, such as requiring encrypted connections for redirect URIs. Due to an improper check that only looks at the start of a web address rather than properly verifying the host, an attacker can bypass these security restrictions by using a specially crafted domain name. This could allow an attacker to intercept sensitive authentication codes over unencrypted connections.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-07-29 08:24:42 UTC 
A flaw was found in the secure-client-uris client policy executor in Keycloak. The executor permits non-HTTPS redirect URIs when allow-http-on-localhost is enabled by performing a simple string prefix check for http://localhost or http://127.0.0.1 instead of parsing the URI to validate the host component.
An attacker can exploit this by registering a client via Dynamic Client Registration using a redirect URI that starts with the allowed prefix but points to an attacker-controlled domain (e.g., http://localhost.evil.test). If the policy is scoped to authorization_code requests, it fails to block the registration or the subsequent use of the malicious URI. When a victim authenticates through the attacker's client, the authorization code is transmitted over cleartext HTTP to the attacker's server. This allows the attacker to intercept the authorization code and potentially gain unauthorized access to the victim's session.