CVE-2026-4634: Keycloak: keycloak: denial of service via excessive processing of openid connect scope parameters

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-04-02 · updated 2026-10-26
Weakness: CWE-1050

Description

A flaw was found in Keycloak. An unauthenticated attacker can exploit this vulnerability by sending a specially crafted POST request with an excessively long scope parameter to the OpenID Connect (OIDC) token endpoint. This leads to high resource consumption and prolonged processing times, ultimately resulting in a Denial of Service (DoS) for the Keycloak server.

Affected products

Red Hat build of Keycloak 26.4: 26.2, 26.2.15, 26.4, 26.4.11

References

https://access.redhat.com/errata/RHSA-2026:6475
https://access.redhat.com/errata/RHSA-2026:6476
https://access.redhat.com/errata/RHSA-2026:6477
https://access.redhat.com/errata/RHSA-2026:6478
https://access.redhat.com/errata/RHSA-2026:76128
https://access.redhat.com/errata/RHSA-2026:76129
https://access.redhat.com/errata/RHSA-2026:76130
https://access.redhat.com/errata/RHSA-2026:76132
https://access.redhat.com/errata/RHSA-2026:76134
https://access.redhat.com/security/cve/CVE-2026-4634
https://bugzilla.redhat.com/show_bug.cgi?id=2450250
https://access.redhat.com/errata/RHSA-2026:6475

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