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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-37977 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 |
Detection & 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-37977 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-37977. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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?
O3 Security finds CVE-2026-37977 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.