Bug 2508304 (CVE-2026-18208)

Summary: CVE-2026-18208 keycloak-services: keycloak-services: Inactive out-of-audience token introspection leaks signed JWT claim
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: anujha, aschwart, asoldano, aszczucz, bbaranow, bmaxwell, boliveir, bstansbe, dlofthou, drichtar, istudens, ivassile, iweiss, mosmerov, mposolda, msvehla, nwallace, pberan, pesilva, pjindal, pmackay, rmartinc, rstancel, ssilvert, sthorger, thjenkin, vdosoudi, vmuzikar
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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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.
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Description OSIDB Bzimport 2026-07-29 08:13:48 UTC 
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.