CVE-2019-5599 in FreeBSD
Summary
by MITRE
In FreeBSD 12.0-STABLE before r349197 and 12.0-RELEASE before 12.0-RELEASE-p6, a bug in the non-default RACK TCP stack can allow an attacker to cause several linked lists to grow unbounded and cause an expensive list traversal on every packet being processed, leading to resource exhaustion and a denial of service.
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Analysis
by VulDB Data Team • 10/07/2023
The vulnerability identified as CVE-2019-5599 affects the FreeBSD operating system's RACK TCP stack implementation, specifically impacting versions prior to r349197 in the 12.0-STABLE branch and before 12.0-RELEASE-p6 in the 12.0-RELEASE branch. This issue represents a critical flaw in the network stack's handling of TCP connections, where a specific bug in the RACK (Reno-Aware Congestion Control) algorithm creates a condition that allows for unbounded growth of linked lists during normal packet processing operations. The vulnerability falls under the category of resource exhaustion attacks and demonstrates a classic example of a denial of service vulnerability that can be exploited remotely without authentication requirements.
The technical flaw manifests in the RACK TCP stack's improper management of linked list data structures that are used to track TCP segments and their associated metadata. When certain network conditions occur, particularly those involving specific TCP packet sequences and acknowledgment patterns, the linked lists that maintain tracking information for these segments can grow indefinitely without proper bounds checking or cleanup mechanisms. This unbounded growth occurs because the system fails to properly remove outdated or completed segment tracking entries from these lists, causing them to accumulate over time. The impact becomes particularly severe when the system processes packets, as each packet processing operation must traverse these increasingly large linked lists to perform necessary operations such as duplicate detection, retransmission tracking, and congestion control calculations.
The operational impact of this vulnerability is significant and directly translates to a denial of service condition that can severely impact network services running on affected FreeBSD systems. As the linked lists grow unbounded, the time complexity of packet processing increases dramatically, leading to exponential performance degradation. Network services that rely on TCP connectivity become increasingly slow and eventually unresponsive as the system struggles to process each incoming packet through these massive linked list traversals. The resource exhaustion occurs at the kernel level, affecting the system's ability to handle new connections, process existing connections, and maintain overall network throughput. This vulnerability is particularly dangerous because it can be triggered by normal network traffic patterns without requiring any special privileges or authentication, making it an attractive target for attackers seeking to disrupt services.
The vulnerability aligns with CWE-400, which addresses "Uncontrolled Resource Consumption" and specifically relates to the improper handling of data structures that can grow without bounds. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers "Endpoint Denial of Service" and T1595.001, which deals with "Scanning for Information" as attackers may probe systems to identify vulnerable configurations. The RACK TCP stack implementation represents a sophisticated networking feature that was intended to improve congestion control, but the flaw demonstrates how even well-intentioned performance optimizations can introduce critical security vulnerabilities when proper resource management is not implemented. Organizations running FreeBSD systems in production environments should prioritize immediate patching to address this vulnerability, as the denial of service conditions can have cascading effects on network infrastructure and service availability. The patch released with FreeBSD 12.0-RELEASE-p6 specifically addresses the linked list management issue by implementing proper bounds checking and cleanup mechanisms to prevent the unbounded growth of tracking data structures.