CVE-2026-37977 is a low-severity (CVSS 3.7) CWE-346 vulnerability in org.keycloak:keycloak-services. A fix is available for org.keycloak:keycloak-services — see the affected versions and patch details below.
Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim
Exploitation Status
No confirmed exploitation observed yet
- 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-2026-37977.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
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. A remote attacker can exploit a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint. This flaw occurs because the azp claim from a client-supplied JSON Web Token (JWT) is used to set the Access-Control-Allow-Origin header before the JWT signature is validated. When a specially crafted JWT with an attacker-controlled azp value is processed, this value is reflected as the CORS origin, even if the grant is later rejected. This can lead to the exposure of low-sensitivity information from authorization server error responses, weakening origin isolation, but only when a target client is misconfigured with webOrigins: ["*"].
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.keycloak:keycloak-services | all versions | 26.4.13org.keycloak:keycloak-services:26.4.13 |
| ☕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
Update org.keycloak:keycloak-services to 26.4.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-37977 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
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.
Low impact: This vulnerability in Keycloak's UMA token endpoint allows for CORS header injection when a client is misconfigured with `webOrigins: ["*"]`. This can lead to the exposure of low-sensitivity information from authorization server error responses. Exploitation requires a specific client misconfiguration and…
| 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-2026-37977 in your dependencies?
Find it across Maven, including transitive dependencies.