{"cve":"CVE-2026-37977","nvd":{"published":"2026-04-06T09:16:17.850","modified":"2026-10-26T21:10:00.170","status":"Modified","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: [\"*\"]`.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N","score":5.3},"cwes":["CWE-346"],"references":["https://access.redhat.com/errata/RHSA-2026:25097","https://access.redhat.com/errata/RHSA-2026:25098","https://access.redhat.com/errata/RHSA-2026:30049","https://access.redhat.com/errata/RHSA-2026:30050","https://access.redhat.com/security/cve/CVE-2026-37977"],"products":["redhat:build_of_keycloak"],"source":"nvd","synced_at":"2026-10-26T21:38:48.864438205+00:00"},"epss":{"cve":"CVE-2026-37977","score":0.00253,"percentile":0.16658,"date":"2026-10-26"},"kev":null,"osv":[{"id":"GHSA-5v8v-xvjv-57x7","url":"https://osv.dev/vulnerability/GHSA-5v8v-xvjv-57x7","summary":"Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim","modified":"2026-07-15T19:01:07.571326237Z","packages":[{"ecosystem":"Maven","name":"org.keycloak:keycloak-services","purl":"pkg:maven/org.keycloak/keycloak-services","ranges":["<26.4.13"],"fixed":["26.4.13"]},{"ecosystem":"Maven","name":"org.keycloak:keycloak-services","purl":"pkg:maven/org.keycloak/keycloak-services","ranges":[">=26.5.0 <26.6.3"],"fixed":["26.6.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}