EUVD-2026-53716
GHSA-fghp-hcwh-75j2
Severity by source
AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
AV:A because assertion capture requires network adjacency; PR:N and UI:N once the assertion is obtained; no availability impact from session hijacking.
Primary rating from Vendor (redhat).
CVSS VectorNVD
Lifecycle Timeline
5DescriptionNVD
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.
AnalysisAI
Session hijacking via SAML assertion replay in Keycloak's SAML broker component allows adjacent-network attackers to authenticate as arbitrary users without credentials. When Keycloak is deployed as a SAML broker in IdP-Initiated flow mode, it fails to enforce the SAML OneTimeUse condition, permitting a captured assertion to be submitted multiple times. No public exploit has been identified at time of analysis, and EPSS sits at 0.14% (4th percentile), though SSVC rates technical impact as total - reflecting the full account takeover consequence if the preconditions are met.
Technical ContextAI
Keycloak (cpe:2.3:a:red_hat:red_hat_build_of_keycloak and red_hat:red_hat_single_sign-on_7) implements SAML 2.0 federation, allowing it to act as a Service Provider brokering trust from an external Identity Provider. The IdP-Initiated Single Sign-On flow has the IdP directly posting a SAML Response to the SP without a prior AuthnRequest. The SAML specification addresses replay risk via the OneTimeUse condition element within an assertion's Conditions block, which instructs the SP to track and reject any assertion presented more than once. CWE-294 (Authentication Bypass by Capture-replay) precisely describes the root cause: Keycloak omits the tracking and rejection step, so a structurally valid assertion is accepted on every subsequent replay as though it were a fresh authentication event.
RemediationAI
Apply the patch released by Red Hat per the vendor advisory at https://access.redhat.com/security/cve/CVE-2026-18967; a specific patched version number is not independently confirmed from the available data, so consult the advisory directly for the exact target release. As an immediate compensating control, organizations should evaluate whether IdP-Initiated SSO flow is operationally required - disabling it and enforcing SP-Initiated flow eliminates the vulnerable code path entirely, at the cost of requiring users to start authentication from the Keycloak login page rather than directly from the IdP portal. Where IdP-Initiated flow must remain enabled, enforce mutual TLS between the IdP and Keycloak to prevent passive interception of SAML assertions in transit, reducing the feasibility of assertion capture. Additionally, restrict network-level access to the Keycloak SAML endpoint to trusted IdP source IPs via firewall rules, limiting the adjacent-network attacker's ability to inject replayed assertions.
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Same weakness CWE-294 – Authentication Bypass by Capture-replay
View allSame technique Authentication Bypass
View allVendor StatusVendor
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External POC / Exploit Code
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