CVE-2026-97846 in Keycloak Single Sign-On
Summary
by MITRE • 09/25/2026
Keycloak provides a feature called mTLS holder-of-key binding which ensures that a token can only be used by the client that originally requested it by binding it to their digital certificate. A flaw was discovered where the new Standard Token Exchange V2 feature does not check for this certificate. This allows an attacker with stolen client credentials to obtain a standard, unrestricted token that bypasses these security protections.
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability identified in Keycloak relates to a critical failure in the implementation of mutual Transport Layer Security (mTLS) holder-of-key binding within its Standard Token Exchange V2 feature. In secure identity and access management architectures, mTLS with holder-of-key binding serves as a robust mechanism for ensuring that an issued OAuth 2.0 or OpenID Connect token is cryptographically bound to the specific client certificate used during authentication. This process typically involves validating that the public key in the presented X.509 certificate matches the one associated with the authorized client, thereby preventing unauthorized parties from replaying stolen tokens even if they possess valid client credentials such as secrets or private keys alone. The flaw arises because the token exchange endpoint fails to enforce this validation step when processing requests for new access tokens via the V2 standard flow. Consequently, an attacker who has compromised a client secret can request a new token without presenting the required mTLS certificate, effectively bypassing the intended security controls that restrict token usage to specific hardware or software identities.
From a technical perspective, this defect represents a breakdown in the principle of least privilege and proper cryptographic binding. When a malicious actor obtains valid client credentials through phishing, malware infection, or configuration exposure, they can interact with the token endpoint directly. Because the server does not verify that the request originates from an entity holding the corresponding private key for the expected certificate, it issues a standard bearer-style access token rather than one bound to a specific identity proof. This unrestricted token behaves like a traditional password-based credential set, allowing the attacker to authenticate as the compromised client against downstream services or APIs without needing further authentication factors. The absence of this check undermines the security model designed for high-assurance environments where certificate-bound tokens are essential for preventing lateral movement and unauthorized access after an initial compromise.
The operational impact of this vulnerability is severe, particularly in enterprise deployments that rely on Keycloak to manage sensitive integrations between microservices or external partners using mTLS. An attacker leveraging this flaw can achieve full impersonation of the compromised client application. This enables actions such as accessing protected resources, exfiltrating data, modifying configurations, or initiating further attacks against other systems trusting tokens issued by that client identity. Since the resulting token appears valid and unrestricted to relying parties, detection is difficult without deep inspection of certificate validation logs at the authorization server level. The risk is amplified in scenarios where clients are granted broad scopes or administrative privileges, as the attacker gains immediate access equivalent to the original compromised entity.
This issue aligns with CWE-287, which describes Improper Authentication, specifically regarding the failure to verify identity credentials properly during token issuance. It also relates to CWE-345, Insufficient Verification of Data Authenticity, as the server fails to validate the cryptographic proof associated with the client's identity. In terms of MITRE ATT&CK framework tactics, this vulnerability facilitates Initial Access and Privilege Escalation by allowing credential theft to translate directly into functional access without additional barriers. It also supports Defense Evasion because the resulting token does not carry indicators that it was obtained through an unauthorized exchange method lacking mTLS proof.
Mitigation strategies should focus on immediate configuration adjustments and long-term architectural improvements. Administrators must ensure that their Keycloak instances are updated to versions where this validation logic has been corrected, as patching is the primary remediation path. In environments running unpatched software, organizations can implement reverse proxy configurations or API gateways in front of Keycloak to enforce mTLS certificate verification before requests reach the token endpoint. Additionally, reviewing client registrations for unnecessary permissions and enforcing strict scope limitations can reduce the blast radius if a credential is compromised. Regular auditing of authentication logs for anomalies in token issuance patterns may also help detect exploitation attempts earlier. Ultimately, maintaining rigorous adherence to mutual TLS standards across all identity flows is essential to preserving the integrity of zero-trust security models dependent on cryptographic binding.