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Build Of KeycloakApplication · Redhat

CVE-2026-9803

MEDIUM · 5.3 CVSS v3.1 Published 2026-05-28
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

A vulnerability in Keycloak's ClientRegistrationAuth component allows remote unauthenticated attackers to cause a denial of service by sending POST requests with malformed 'Authorization: Bearer' headers to client registration endpoints. The malformed input triggers an unhandled ArrayIndexOutOfBoundsException, resulting in HTTP 500 errors and service unavailability.

MitigationApply the official Keycloak security patch when available; in the interim, consider implementing rate limiting or a WAF rule to detect and block malformed Authorization headers before they reach the vulnerable component.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Build Of KeycloakApplication
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm Keycloak is running
    Check for running Keycloak processes or verify the service is listening on its configured port (default 8443 for HTTPS or 8080 for HTTP). Use commands like 'ps aux | grep keycloak' or 'netstat -tlnp | grep -E "(8080|8443)"'
    Affected if Keycloak process is running and accessible on the network
  2. Identify the Client Registration endpoint availability
    Attempt to access the client registration endpoint for your realm. Common paths are /realms/{realm}/clients/registrations or /auth/realms/{realm}/clients/registrations. Use curl or a browser to send a GET request to these endpoints.
    Affected if The endpoint returns a 200 or 401 response, indicating the client registration service is accessible
  3. Verify Client Registration Service is enabled
    Check Keycloak configuration files (standalone.xml, standalone-ha.xml, or the Keycloak admin console) for the 'Client Registration' capability. In the admin console, navigate to Realm Settings > Client Registration and confirm it is enabled.
    Affected if Client Registration Service is enabled and publicly accessible without authentication
  4. Test for vulnerability with malformed Authorization header
    Send a POST request to the client registration endpoint with a malformed 'Authorization: Bearer' header that is missing the token value, for example: 'Authorization: Bearer'. Use curl: curl -X POST -H 'Authorization: Bearer' https://keycloak-host/realms/{realm}/clients/registrations
    Affected if The server returns an HTTP 500 error with an ArrayIndexOutOfBoundsException in the response or logs

If Keycloak is running with the Client Registration Service enabled and accessible, and malformed Authorization headers cause HTTP 500 errors, the environment is vulnerable to CVE-2026-9803.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the official Keycloak security patch when available; in the interim, consider implementing rate limiting or a WAF rule to detect and block malformed Authorization headers before they reach the vulnerable component.

Fix this in Build Of Keycloak Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-9803 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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