CVE-2026-37977 Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim
Attackers can inject CORS headers via unvalidated JWT claims to expose low-sensitivity error data, requiring misconfigured webOrigins.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-37977?
CVE-2026-37977 is a low-severity vulnerability in Redhat Build of Keycloak, published on 6 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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: ["*"]`.
Weakness: CWE-346
Which versions are affected by CVE-2026-37977?
4 open source packages ship an affected version, including org.keycloak:keycloak-services. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
org.keycloak:keycloak-services Maven | All versions | 26.4.13 |
org.keycloak:keycloak-services Maven | All versions | 26.4.13 |
org.keycloak:keycloak-services Maven | From 26.5.0 | 26.6.3 |
org.keycloak:keycloak-services Maven | From 26.5.0 | 26.6.3 |
Products include Redhat Build of Keycloak.
How do I fix CVE-2026-37977?
Upgrade org.keycloak:keycloak-services to a fixed version for your release line: 26.4.13 or 26.6.3.
Is CVE-2026-37977 being exploited?
There is no known exploitation of CVE-2026-37977 in the wild and no public exploit code.
Other vulnerabilities in Redhat Build of Keycloak
- CVE-2026-15572 Keycloak-services: keycloak-services: dcr protocol mapper type-swap policy bypass allows privilege escalation
- CVE-2026-3047 Keycloak SAML Broken has Authentication Bypass by Primary Weakness
- CVE-2026-15573 Keycloak-services: keycloak-services: authorization bypass via unnormalized uri matching in pathmatcher
- CVE-2026-16102 Keycloak-services: keycloak-services: default dcr policy allows role forgery via user property mappers
- CVE-2026-1609 Org.keycloak/keycloak-quarkus-server: keycloak: unauthorized access via jwt authorization grant with disabled users
Sources
Also known as ECHO-84f9-e9cf-32fc, GHSA-5v8v-xvjv-57x7, MINI-4jx5-hmg6-3qmv, MINI-7wm3-grjx-7m6q.