CVE-2023-43645 in OpenFGA
Summary
by MITRE • 09/27/2023
OpenFGA is an authorization/permission engine built for developers and inspired by Google Zanzibar. OpenFGA is vulnerable to a denial of service attack when certain Check calls are executed against authorization models that contain circular relationship definitions. When the call is made, it's possible for the server to exhaust resources and die. Users are advised to upgrade to v1.3.2 and update any offending models. There are no known workarounds for this vulnerability. Note that for models which contained cycles or a relation definition that has the relation itself in its evaluation path, checks and queries that require evaluation will no longer be evaluated on v1.3.2+ and will return errors instead. Users who do not have cyclic models are unaffected.
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Analysis
by VulDB Data Team • 09/27/2023
The vulnerability identified as CVE-2023-43645 affects OpenFGA, an authorization engine designed to provide fine-grained access control for applications and services. This system implements a distributed authorization model inspired by Google Zanzibar, enabling developers to define complex permission relationships between users, objects, and actions. The flaw manifests specifically within the Check operation processing mechanism when dealing with authorization models containing circular relationship definitions. The vulnerability represents a critical denial of service condition that can cause the OpenFGA server to consume excessive computational resources and ultimately terminate its operations. This issue impacts the availability and reliability of authorization services, potentially disrupting access control for applications relying on the system.
The technical root cause of this vulnerability stems from insufficient cycle detection and handling within the Check call evaluation process. When OpenFGA encounters authorization models with circular dependencies in their relationship definitions, the internal evaluation algorithms can enter infinite loops or excessively deep recursion patterns. This occurs because the system attempts to resolve relationships that reference themselves either directly or through a chain of other relationships, creating an evaluation path that never terminates. The absence of proper cycle detection mechanisms means that the server continues processing these problematic calls until system resources are exhausted, leading to service disruption. This flaw aligns with CWE-838, which addresses insufficient input validation leading to denial of service through resource exhaustion, and represents a classic example of improper control flow management in authorization systems.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the integrity of authorization decisions within affected applications. Systems utilizing OpenFGA for access control may experience complete unavailability of their permission checking services when malicious or improperly configured models are processed. This affects not only legitimate users who cannot access resources but also creates potential security risks where unauthorized access might be possible if the system fails to properly evaluate permissions. Organizations relying on OpenFGA for critical access control functions face significant operational risks, including potential data exposure or service outages that could impact business continuity. The vulnerability affects all versions prior to v1.3.2, making it crucial for organizations to assess their current deployment status and implement immediate upgrades.
The remediation strategy for CVE-2023-43645 requires immediate upgrading to OpenFGA version 1.3.2 or later, as this release includes proper cycle detection and handling mechanisms. Organizations must also audit their existing authorization models to identify and correct any circular relationship definitions that could trigger the vulnerability. The updated version implements stricter validation of authorization models and returns explicit errors for models containing cycles rather than allowing resource exhaustion. This approach aligns with defensive programming practices recommended in the MITRE ATT&CK framework for authorization and access control, specifically addressing techniques related to privilege escalation and denial of service. While this solution prevents the immediate vulnerability, it also represents a breaking change for existing models, requiring careful migration planning and testing. Organizations should conduct thorough testing of their authorization models post-upgrade to ensure continued functionality while maintaining proper access control policies. The fix demonstrates the importance of proper input validation and resource management in authorization systems, particularly in distributed environments where denial of service can have cascading effects across dependent services and applications.