CVE-2026-9803 is a medium-severity (CVSS 5.3) Out-of-bounds Read vulnerability in org.keycloak:keycloak-services. A fix is available for org.keycloak:keycloak-services — see the affected versions and patch details below.
Keycloak has an Out-of-bounds Read
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-9803.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-9803 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
org.keycloak:keycloak-services☕org.keycloak:keycloak-servicesReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.keycloak:keycloak-services | all versions | No fix |
| ☕Maven | org.keycloak:keycloak-services | ≥ 26.5.0&&< 26.6.3 | 26.6.3org.keycloak:keycloak-services:26.6.3 |
Affected Products
build of keycloakredhatDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.keycloak:keycloak-services, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
No patched version of org.keycloak:keycloak-services has shipped for CVE-2026-9803 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Workarounds
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
A Moderate denial of service flaw was found in Keycloak's client registration endpoints. An unauthenticated attacker can send a specially crafted request with a malformed 'Authorization: Bearer' header, causing an ArrayIndexOutOfBoundsException and an HTTP 500 error. This can lead to a temporary disruption of service…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-9803 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat build of Keycloak 26.4 | rhbk/keycloak-operator-bundle:26.4.13-1 | RHSA-2026:30050 |
| Red Hat build of Keycloak 26.4.13 | rhbk/keycloak-rhel9 | RHSA-2026:30049 |
| Red Hat build of Keycloak 26.6 | rhbk/keycloak-operator-bundle:26.6.3-3 | RHSA-2026:25097 |
| Red Hat build of Keycloak 26.6.3 | rhbk/keycloak-rhel9 | RHSA-2026:25098 |
Frequently Asked Questions
Is CVE-2026-9803 in your dependencies?
Find it across Maven, including transitive dependencies.