---
id: CVE-2026-37977
title: A flaw was found in Keycloak
summary: >-
  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 cli…
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N'
cwe:
  - CWE-346
published: '2026-04-06'
updated: '2026-06-26'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-37977'
references:
  - url: 'https://access.redhat.com/errata/RHSA-2026:25097'
    label: secalert@redhat.com
  - url: 'https://access.redhat.com/errata/RHSA-2026:25098'
    label: secalert@redhat.com
  - url: 'https://access.redhat.com/errata/RHSA-2026:30049'
    label: secalert@redhat.com
  - url: 'https://access.redhat.com/errata/RHSA-2026:30050'
    label: secalert@redhat.com
  - url: 'https://access.redhat.com/security/cve/CVE-2026-37977'
    label: secalert@redhat.com
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2455324'
    label: secalert@redhat.com
tags:
  - nvd
epss: 0.00275
epssPercentile: 0.17636
ingestedAt: '2026-06-26T16:43:13.622Z'
---

## Overview

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: ["*"]`.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.