CVE-2019-6474 in DHCP
Summary
by MITRE
A missing check on incoming client requests can be exploited to cause a situation where the Kea server's lease storage contains leases which are rejected as invalid when the server tries to load leases from storage on restart. If the number of such leases exceeds a hard-coded limit in the Kea code, a server trying to restart will conclude that there is a problem with its lease store and give up. Versions affected: 1.4.0 to 1.5.0, 1.6.0-beta1, and 1.6.0-beta2
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Analysis
by VulDB Data Team • 01/16/2024
The vulnerability identified as CVE-2019-6474 represents a critical flaw in the Kea DHCP server software that affects versions ranging from 1.4.0 through 1.5.0 and specific beta releases of version 1.6.0. This issue stems from inadequate validation mechanisms within the server's lease handling process, creating a scenario where malformed or unauthorized client requests can corrupt the lease database. The vulnerability operates at the intersection of configuration management and data integrity validation, where the absence of proper input sanitization allows malicious or malformed requests to persist in the system's storage layer.
The technical implementation of this vulnerability manifests through a missing validation check that should occur during client request processing. When the Kea server receives incoming requests, it fails to properly validate the legitimacy of lease information before storing it in its persistent database. This oversight creates a condition where invalid lease entries can accumulate within the storage system, particularly when the server encounters requests that should be rejected based on standard DHCP protocol compliance or security parameters. The flaw specifically affects the server's ability to maintain data consistency between its operational memory and persistent storage layers.
During server restart operations, the Kea software attempts to load all stored leases from its database to resume normal operations. However, when the system encounters leases that were stored despite failing validation checks, it triggers an internal error handling mechanism. The server contains a hard-coded limit that determines the maximum number of invalid leases it will tolerate during this loading process. When this threshold is exceeded, the server's restart procedure terminates prematurely, effectively causing a denial of service condition where the DHCP service becomes unavailable until manual intervention occurs.
This vulnerability directly maps to CWE-20, which describes "Improper Input Validation," and demonstrates how insufficient validation can lead to cascading failures within system components. The operational impact extends beyond simple service disruption to encompass potential network availability issues that could affect critical infrastructure relying on DHCP services for IP address allocation. The ATT&CK framework categorizes this issue under privilege escalation and denial of service tactics, as attackers could exploit this weakness to disrupt network services while potentially gaining insights into the server's internal state through the error conditions generated.
The mitigation strategy for CVE-2019-6474 requires immediate deployment of patched versions of the Kea server software, specifically versions that implement proper validation checks for incoming client requests. Organizations should also consider implementing network segmentation and access controls to limit exposure to potentially malicious requests. Additionally, system administrators should monitor lease database integrity regularly and establish automated alerting mechanisms for unusual lease storage patterns. The fix addresses the root cause by implementing proper validation logic that prevents invalid lease entries from being stored in the first place, thereby eliminating the conditions that lead to the hard-coded limit overflow during server restart operations.