Description

A flaw was found in the SAML brokering component of Keycloak, an identity and access management solution. The issue occurs when Keycloak acts as an intermediary between an upstream identity provider and downstream services. Keycloak fails to respect specific security instructions called ProxyRestrictions in the login messages it receives. This allows a user to log into downstream services even when the original identity provider explicitly forbade further sharing of the login session. As a result, users can bypass security boundaries intended to limit where their identity can be used.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.9",
  "pubDate": "2026-11-06T06:16:44.747Z",
  "pubdate": "2026-11-06T06:16:44.747Z",
  "executiveSummary": "This vulnerability involves a failure in the SAML brokering component of Keycloak, an identity and access management solution, to enforce ProxyRestriction attributes within SAML assertion messages. When Keycloak functions as an intermediary (Service Provider/Identity Provider bridge), it fails to honor the restrictions set by an upstream identity provider regarding the propagation of the authentication session to downstream services.\nThe flaw constitutes an improper validation of security assertions, categorized as an access control bypass. By ignoring these constraints, Keycloak allows users to authenticate to downstream services that the upstream identity provider explicitly intended to restrict. This undermines the security boundaries mandated by the identity provider, potentially leading to unauthorized access to applications or resources within the federated environment. An attacker with a valid login session from the upstream provider can leverage this flaw to access services that should have been prohibited, circumventing the intended scope of the user's authentication context. The risk is high for federated environments relying on complex trust chains where specific session constraints are critical for organizational compliance and data security.",
  "technicalDetails": "The vulnerability resides within the SAML brokering implementation of Keycloak, specifically in the logic responsible for processing and relaying SAML response assertions. In a typical SAML federation workflow, an upstream Identity Provider (IdP) may include a 'ProxyRestriction' element within the SAML Assertion, specifically the 'Count' attribute and 'Audience' elements, to limit whether the assertion can be proxied and to which specific relying parties the session can be extended.\nThe root cause of this vulnerability is the failure of the Keycloak SAML broker to parse, validate, or propagate these ProxyRestriction instructions when generating the downstream SAML assertion. Instead of enforcing the restriction policy dictated by the upstream IdP, Keycloak transparently passes or ignores these requirements, essentially acting as an unrestricted pass-through for the authentication session.\nThe attack flow follows a structured path: 1. An attacker initiates an authentication request through Keycloak acting as the brokering agent. 2. The upstream IdP authenticates the user and generates a SAML Assertion containing ProxyRestriction parameters that explicitly forbid the session from being used by downstream target services (Audience restrictions). 3. Keycloak receives this assertion, processes the brokering workflow, but fails to check the ProxyRestriction constraints against the configured downstream service.\nConsequently, Keycloak generates a new SAML assertion or extends the existing context to the downstream service without honoring the original security constraints. The downstream service, receiving a seemingly valid assertion from Keycloak (which it trusts as its configured IdP), grants the attacker access. The attacker successfully gains entry into a restricted service that was intended to be protected by the upstream IdP's security policies.\nThis exploitation does not require advanced administrative privileges on the Keycloak instance itself; rather, it exploits the inherent trust model of the SAML federation. Any authenticated user or potentially malicious actor holding a valid session from the upstream provider can trigger this bypass. Because the vulnerability is situated in the brokering logic, it affects the trust chain between the upstream IdP and any downstream Service Provider (SP) registered within Keycloak. The lack of strict attribute enforcement allows for the lateral movement of authentication sessions across security boundaries, resulting in unauthorized access to downstream systems that rely on the integrity of the SAML assertion chain for policy enforcement."
}