CVE-2026-9803 Keycloak: keycloak: denial of service via malformed authorization header
Remote unauthenticated attackers can cause a denial of service in Keycloak by sending malformed authorization headers to client registration endpoints.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-9803?
CVE-2026-9803 is a medium-severity denial of service vulnerability in Redhat Build of Keycloak, published on 28 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
A flaw was found in Keycloak's ClientRegistrationAuth component. A remote unauthenticated attacker can exploit this vulnerability by sending a specially crafted POST request with a malformed 'Authorization: Bearer' header to any client registration endpoint. This can lead to an ArrayIndexOutOfBoundsException, causing the server to return an HTTP 500 error and resulting in a Denial of Service (DoS) for the affected service.
Weakness: CWE-125
Which versions are affected by CVE-2026-9803?
4 open source packages ship an affected version, including org.keycloak:keycloak-services. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
org.keycloak:keycloak-services Maven | All versions to 26.4.7 | None |
org.keycloak:keycloak-services Maven | All versions to 26.4.7 | None |
org.keycloak:keycloak-services Maven | From 26.5.0 | 26.6.3 |
org.keycloak:keycloak-services Maven | From 26.5.0 | 26.6.3 |
Products include Redhat Build of Keycloak.
How do I fix CVE-2026-9803?
Upgrade org.keycloak:keycloak-services to 26.6.3 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-9803 being exploited?
There is no known exploitation of CVE-2026-9803 in the wild and no public exploit code.
Other vulnerabilities in Redhat Build of Keycloak
- CVE-2026-15572 Keycloak-services: keycloak-services: dcr protocol mapper type-swap policy bypass allows privilege escalation
- CVE-2026-3047 Keycloak SAML Broken has Authentication Bypass by Primary Weakness
- CVE-2026-15573 Keycloak-services: keycloak-services: authorization bypass via unnormalized uri matching in pathmatcher
- CVE-2026-16102 Keycloak-services: keycloak-services: default dcr policy allows role forgery via user property mappers
- CVE-2026-1609 Org.keycloak/keycloak-quarkus-server: keycloak: unauthorized access via jwt authorization grant with disabled users
Sources
Also known as ECHO-1814-da4f-7983, GHSA-cpf7-j4cf-vqx4, MINI-22vm-xrp4-58qr, MINI-r3mc-q43m-mq53.