SAML Broker One-Time Use (OTU) Bypass in Keycloak
CVE-2026-18967 Published on August 6, 2026
Keycloak-services: keycloak-services: saml onetimeuse assertion replay in idp-initiated broker flow
A flaw was found in the SAML broker component of Keycloak, an identity and access management solution. When configured as a SAML broker using the IdP-Initiated flow, Keycloak fails to enforce the OneTimeUse condition in SAML assertions. This allows an attacker who captures a valid, unused assertion to replay it multiple times. Successful exploitation could allow an attacker to hijack a user's session and gain unauthorized access to the system as that user.
Vulnerability Analysis
Timeline
Reported to Red Hat.
Made public.
Weakness Type
Authentication Bypass by Capture-replay
A capture-replay flaw exists when the design of the software makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Capture-replay attacks are common and can be difficult to defeat without cryptography. They are a subset of network injection attacks that rely on observing previously-sent valid commands, then changing them slightly if necessary and resending the same commands to the server.
Products Associated with CVE-2026-18967
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