CVE-2026-18209: Keycloak-services: keycloak-services: oidc redirect_uri fragment bypass in http parameter pollution check

Severity: medium (CVSS 4.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-31 · updated 2026-09-16
Weakness: CWE-1288

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.

Affected products

Red Hat build of Keycloak 26.4: all versions

References

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