CVE-2026-18209

Keycloak-services: keycloak-services: oidc redirect_uri fragment bypass in http parameter pollution check

Published Jul 31, 2026 CVSS 3.1 · 3.4 Low EPSS < 1% KEV · Not listed
Description
A flaw was found in the keycloak-services component of Keycloak, which handles OpenID Connect (OIDC) authentication flows. The issue occurs because the security check designed to prevent HTTP parameter pollution only inspects the query portion of a redirect URL and ignores the fragment portion. When a client is configured with a wildcard redirect URI, an attacker can use this to inject duplicate security parameters into the login response. If a client application is not configured correctly, it might trust the attacker's injected data instead of the real security information from Keycloak, leading to session fixation or account confusion.

Remediation

Vendor Workaround

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
CPEs cpe:/a:redhat:build_keycloak: cpe:/a:redhat:build_keycloak:26.6::el9
References
Type Values Removed Values Added
First Time appeared Redhat build Of Keycloak
Redhat data Grid
Redhat jboss Enterprise Application Platform Expansion Pack
Redhat single Sign On
Vendors & Products Redhat build Of Keycloak
Redhat data Grid
Redhat jboss Enterprise Application Platform Expansion Pack
Redhat single Sign On
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
Description A flaw was found in the keycloak-services component of Keycloak, which handles OpenID Connect (OIDC) authentication flows. The issue occurs because the security check designed to prevent HTTP parameter pollution only inspects the query portion of a redirect URL and ignores the fragment portion. When a client is configured with a wildcard redirect URI, an attacker can use this to inject duplicate security parameters into the login response. If a client application is not configured correctly, it might trust the attacker's injected data instead of the real security information from Keycloak, leading to session fixation or account confusion.
Title Keycloak-services: keycloak-services: oidc redirect_uri fragment bypass in http parameter pollution check
First Time appeared Redhat
Redhat build Keycloak
Redhat jboss Data Grid
Redhat jbosseapxp
Redhat red Hat Single Sign On
Weaknesses CWE-1288
CPEs cpe:/a:redhat:build_keycloak:
cpe:/a:redhat:jboss_data_grid:8
cpe:/a:redhat:jbosseapxp
cpe:/a:redhat:red_hat_single_sign_on:7
Vendors & Products Redhat
Redhat build Keycloak
Redhat jboss Data Grid
Redhat jbosseapxp
Redhat red Hat Single Sign On
References
Metrics cvssV3_1

{'score': 3.4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:L/A:N'}