CVE-2026-9803: Red Hat Keycloak denial of service in ClientRegistrationAuth

CVE-2026-9803 · Severity: medium · CVSS 5.3 · Published 2026-05-28

Keycloak, an open-source identity and access management solution, is vulnerable to a denial-of-service attack. By sending a specifically malformed authorization request, an unauthenticated attacker can cause the server to crash or return errors. This disrupts the ability of users and applications to log in or register, potentially halting business operations that rely on Keycloak for authentication.

Technologies: Keycloak, org.keycloak:keycloak-services (Maven), Red Hat build of Keycloak, Red Hat build of Keycloak. Vendors: Keycloak, Maven, Red Hat.

Quick answers

Is CVE-2026-9803 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-9803 being exploited in the wild.
How severe is CVE-2026-9803?
Medium severity, with a CVSS score of 5.3, published 28 May 2026.
Which products and versions are affected?
Keycloak <= 26.4.7, >= 26.5.0, < 26.6.3; org.keycloak:keycloak-services (Maven) ECOSYSTEM:introduced=0,last_affected=26.4.7; Red Hat build of Keycloak 26.4, 26.6.
Is there a fix for CVE-2026-9803?
A patch was published on 2026-06-10: Red Hat release 26.6.3 published Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-9803 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-9803?
Check whether you run Keycloak, org.keycloak:keycloak-services (Maven), Red Hat build of Keycloak or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

An out-of-bounds read vulnerability (CWE-125) exists in Keycloak's ClientRegistrationAuth component within the keycloak-services package. The flaw is triggered when the server processes a POST request containing a malformed 'Authorization: Bearer' header at any client registration endpoint. This malformed input leads to an ArrayIndexOutOfBoundsException, resulting in an HTTP 500 error. A remote, unauthenticated attacker can exploit this to cause a Denial of Service (DoS) condition. The issue is addressed in Keycloak version 26.6.3 and corresponding Red Hat builds.

Affected products

  • Keycloak Keycloak <= 26.4.7, >= 26.5.0, < 26.6.3
  • Red Hat Red Hat build of Keycloak 26.4, 26.6

Identifiers

  • CVE-2026-9803
  • GHSA-cpf7-j4cf-vqx4

Timeline

  • 2026-05-28: disclosed
  • 2026-05-28: advisory
  • 2026-06-10: patched: Red Hat release 26.6.3 published

References

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