CVE-2018-6922 in FreeBSD
Summary
by MITRE
One of the data structures that holds TCP segments in all versions of FreeBSD prior to 11.2-RELEASE-p1, 11.1-RELEASE-p12, and 10.4-RELEASE-p10 uses an inefficient algorithm to reassemble the data. This causes the CPU time spent on segment processing to grow linearly with the number of segments in the reassembly queue. An attacker who has the ability to send TCP traffic to a victim system can degrade the victim system's network performance and/or consume excessive CPU by exploiting the inefficiency of TCP reassembly handling, with relatively small bandwidth cost.
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Analysis
by VulDB Data Team • 05/01/2023
The vulnerability described in CVE-2018-6922 represents a significant performance degradation issue within the FreeBSD operating system's TCP reassembly mechanism. This flaw affects multiple versions of FreeBSD including 11.2-RELEASE-p1, 11.1-RELEASE-p12, and 10.4-RELEASE-p10, and demonstrates a classic case of inefficient algorithmic implementation that can be exploited for denial of service attacks. The core issue lies in how the kernel handles TCP segment reassembly, where the data structure employed for managing these segments suffers from poor algorithmic complexity that scales linearly with the number of segments in the reassembly queue. This design flaw creates a fundamental weakness in the system's ability to handle network traffic efficiently, particularly under conditions where multiple TCP segments are being processed simultaneously.
The technical implementation of this vulnerability stems from an inefficient algorithm that governs how TCP segments are reassembled within the kernel's network stack. When TCP segments arrive at a FreeBSD system, they must be properly reassembled to reconstruct the original data stream before being delivered to the application layer. The flawed reassembly mechanism causes the CPU processing time required to handle these segments to increase linearly with each additional segment queued for reassembly. This means that as more segments accumulate in the reassembly queue, the system's CPU utilization grows proportionally, creating a quadratic growth pattern in processing overhead. The vulnerability specifically targets the data structure used for maintaining TCP segment information, where the algorithmic approach to segment ordering and reassembly becomes increasingly inefficient as the queue length increases.
From an operational perspective, this vulnerability presents a substantial risk to network performance and system availability. An attacker can exploit this weakness by sending carefully crafted TCP traffic to a target system, causing the reassembly queue to grow continuously while the CPU time required to process each segment increases linearly. This creates a resource exhaustion scenario where the victim system's network performance degrades significantly, potentially leading to complete service unavailability. The attack requires relatively minimal bandwidth to be effective, making it particularly dangerous as it can be executed with modest network resources while still causing substantial system impact. The linear scaling behavior means that even a moderate number of segments can cause dramatic performance degradation, and the attack can be sustained over time to maintain the degradation effect.
The security implications of CVE-2018-6922 align with several ATT&CK framework techniques including privilege escalation through resource exhaustion and denial of service attacks. This vulnerability represents a classic example of a computationally expensive operation that can be exploited to consume system resources without requiring elevated privileges. The issue maps to CWE-124 in the CWE database, which categorizes weaknesses related to the improper handling of resource consumption in network protocols. Organizations affected by this vulnerability should implement immediate mitigations including applying the appropriate FreeBSD security patches, monitoring network traffic for unusual patterns that might indicate exploitation attempts, and implementing rate limiting mechanisms to prevent excessive segment accumulation in the reassembly queue. Additionally, system administrators should consider implementing network segmentation and access controls to limit the potential impact of such attacks while the permanent patches are deployed.
The remediation approach for this vulnerability requires careful consideration of system stability and operational requirements. FreeBSD administrators must upgrade to the patched versions mentioned in the CVE description to eliminate the vulnerable code path entirely. The security patches typically address the underlying algorithmic inefficiency by implementing more optimal data structures and processing approaches for TCP reassembly. Organizations should conduct thorough testing of the updated systems to ensure that network functionality remains intact while addressing the performance degradation issues. Network monitoring should be enhanced to detect unusual TCP segment patterns that might indicate exploitation attempts, and baseline performance metrics should be established to quickly identify when systems are being impacted by similar vulnerabilities. The vulnerability serves as a reminder of the critical importance of algorithmic efficiency in system-level code, particularly in network stack implementations where resource consumption directly impacts system availability and performance.