CVE-2026-16103 in Keycloak
Summary
by MITRE • 07/17/2026
A flaw was found in the keycloak-services component of Keycloak. This issue is an incomplete fix for CVE-2026-9798, where brute-force protection checks were added to the Client-Initiated Backchannel Authentication (CIBA) initiation handler but were omitted from the token redemption handler. This allows an attacker with valid client credentials to obtain access and refresh tokens for a user account that has been locked due to brute-force protection, provided the authentication request was started before the lockout occurred and was approved by the user.
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Analysis
by VulDB Data Team • 07/17/2026
This vulnerability exists within the keycloak-services component of Keycloak, representing a critical security flaw that undermines the system's authentication protections. The issue stems from an incomplete remediation effort for CVE-2026-9798, where developers initially implemented brute-force protection mechanisms but failed to apply them consistently across all relevant authentication pathways. The specific weakness lies in the Client-Initiated Backchannel Authentication (CIBA) implementation, where protections were correctly added to the initiation handler but omitted from the token redemption handler, creating a significant security gap that attackers can exploit.
The technical flaw manifests when an attacker possesses valid client credentials and attempts to authenticate a user account that has already been locked out due to brute-force protection mechanisms. This scenario creates a temporal window vulnerability where authentication requests initiated before the account lockout occurs can still be redeemed for valid tokens if the user approves the authentication request. The vulnerability operates through the CIBA flow's two-phase process, where the initial authentication request is validated against brute-force protections but the subsequent token redemption phase lacks these safeguards, allowing attackers to bypass account lockout measures.
The operational impact of this vulnerability is substantial as it directly undermines the effectiveness of Keycloak's brute-force protection mechanisms, potentially enabling unauthorized access to user accounts even after they have been locked due to suspicious authentication attempts. Attackers can leverage this weakness to obtain legitimate access tokens and refresh tokens for accounts that should be protected, effectively nullifying the security controls designed to prevent credential stuffing and automated attack scenarios. This creates a scenario where attackers can systematically work around account lockout policies without triggering additional detection mechanisms.
Organizations utilizing Keycloak deployments are strongly advised to implement immediate mitigations including applying the latest available patches that properly address both the initiation and redemption handlers in the CIBA flow. The vulnerability aligns with CWE-305 authentication bypass weaknesses and represents a specific ATT&CK technique under T1110 credential access, where adversaries exploit incomplete security controls to maintain persistent access. Additional defensive measures should include monitoring for unusual authentication patterns, implementing additional rate limiting controls beyond brute-force protections, and ensuring that all authentication pathways within the CIBA implementation receive consistent security treatment to prevent similar issues from arising in future releases.