CVE-2023-6563 in keycloak
Summary
by MITRE • 12/14/2023
An unconstrained memory consumption vulnerability was discovered in Keycloak. It can be triggered in environments which have millions of offline tokens (> 500,000 users with each having at least 2 saved sessions). If an attacker creates two or more user sessions and then open the "consents" tab of the admin User Interface, the UI attempts to load a huge number of offline client sessions leading to excessive memory and CPU consumption which could potentially crash the entire system.
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Analysis
by VulDB Data Team • 10/09/2025
The vulnerability identified as CVE-2023-6563 represents a critical memory exhaustion flaw within the Keycloak identity and access management platform that demonstrates a classic example of insufficient resource management in web applications. This issue manifests when the administrative user interface attempts to display consent information for users who have accumulated substantial offline session data, creating a scenario where the system's memory consumption becomes unbounded and potentially leads to complete system failure. The vulnerability specifically impacts environments where Keycloak manages large user populations with extensive session persistence requirements, making it particularly dangerous in enterprise deployments where thousands of concurrent users maintain active sessions across multiple applications.
The technical root cause of this vulnerability lies in the lack of proper bounds checking and resource limiting within the admin console's consent management functionality. When administrators navigate to the consents tab for a user account, the system attempts to enumerate and load all associated offline client sessions without implementing any form of pagination, filtering, or memory consumption limits. This behavior directly aligns with CWE-770, which addresses the allocation of resources without proper bounds checking, and represents a failure in implementing the principle of least privilege for resource consumption. The flaw is particularly insidious because it operates within the administrative interface, which typically has elevated privileges and access to sensitive data, making it a prime target for both accidental system crashes and deliberate denial-of-service attacks.
The operational impact of this vulnerability extends beyond simple system instability to encompass potential business disruption and security implications. In environments with over 500,000 users maintaining at least two sessions each, the memory consumption can escalate rapidly to the point where the application server becomes unresponsive or crashes entirely. Attackers can exploit this by creating multiple user sessions and then accessing the consents interface, causing the system to consume excessive CPU cycles and memory resources. This creates a denial-of-service condition that can render the entire Keycloak instance unavailable to legitimate users, potentially affecting authentication services for all connected applications. The vulnerability also demonstrates characteristics of ATT&CK technique T1499.004, which involves resource exhaustion attacks targeting availability.
Mitigation strategies for CVE-2023-6563 should focus on implementing both immediate defensive measures and long-term architectural improvements. Organizations should implement strict session management policies that limit the number of concurrent sessions per user and establish memory consumption thresholds for administrative operations. The implementation of pagination and lazy loading mechanisms within the consent display functionality would prevent the loading of excessive session data at once. Additionally, deploying monitoring and alerting systems that can detect unusual memory consumption patterns would provide early warning of potential exploitation attempts. The fix should also include proper input validation and resource limiting for administrative interfaces, ensuring that operations requiring substantial data processing are bounded and do not consume uncontrolled system resources. Organizations should also consider implementing rate limiting and access controls for administrative functions to prevent unauthorized users from triggering the vulnerability through automated means, while maintaining proper audit logging to track all administrative activities that could potentially trigger this memory exhaustion condition.