CVE-2026-18208: Keycloak-services: keycloak-services: inactive out-of-audience token introspection leaks signed jwt claim
A flaw was found in the OIDC token introspection endpoint of the keycloak-services component. Keycloak is an open-source identity and access management solution used to secure modern applications and services. The issue occurs when a confidential client, configured to receive signed JWT introspection responses, attempts to introspect a token issued for a different audience. Although the endpoint correctly identifies the token as inactive for that client, it still returns the full set of token claims within a signed JWT field. This allows an unauthorized client to bypass audience-based restrictions and access sensitive information contained in the token.
Other sources
An information disclosure vulnerability was identified in Keycloaks OIDC token introspection endpoint. This flaw is an incomplete fix for CVE-2026-37979. While the original fix correctly enforced audience checks by marking out-of-audience tokens as inactive, it failed to sanitize the jwt field in the response when JWT introspection responses are enabled. The vulnerability exists in the keycloak-services component. When a confidential client with the client.introspection.response.allow.jwt.claim.enabled attribute set to true requests an application/jwt response, the server includes a signed JWT representation of the token even if the audience check fails. An attacker who controls a confidential client in the same realm and obtains a valid access token issued for a different client can exploit this to: Recover sensitive claims and attributes from lightweight or opaque tokens. Bypass audience-based access control mechanisms intended to restrict introspection metadata. Access internal identity information (roles, groups, user attributes) they are not authorized to view.
— Red Hat
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
In the affected realm, set client.introspection.response.allow.jwt.claim.enabled to false for confidential clients that use OIDC token introspection responses, to prevent Keycloak from returning a signed JWT representation (jwt field) even when audience checks fail.
Keycloak OIDC token introspection (keycloak-services) client.introspection.response.allow.jwt.claim.enabled = false
Event History
Frequently Asked Questions
What is the severity of CVE-2026-18208?
CVE-2026-18208 has a medium severity rating of 6.5.
What does CVE-2026-18208 affect?
CVE-2026-18208 affects the Keycloak-services component, specifically the OIDC token introspection endpoint.
How do I mitigate CVE-2026-18208?
To mitigate CVE-2026-18208, ensure that your Keycloak configuration does not expose signed JWT claims unnecessarily.
What type of attack does CVE-2026-18208 enable?
CVE-2026-18208 may allow an attacker to exploit the inactive out-of-audience tokens to leak signed JWT claims.
When was CVE-2026-18208 published?
CVE-2026-18208 was published on July 29, 2026.