CVE-2018-5391 in Linux
Summary
by MITRE
The Linux kernel, versions 3.9+, is vulnerable to a denial of service attack with low rates of specially modified packets targeting IP fragment re-assembly. An attacker may cause a denial of service condition by sending specially crafted IP fragments. Various vulnerabilities in IP fragmentation have been discovered and fixed over the years. The current vulnerability (CVE-2018-5391) became exploitable in the Linux kernel with the increase of the IP fragment reassembly queue size.
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Analysis
by VulDB Data Team • 01/27/2025
The vulnerability identified as CVE-2018-5391 represents a critical denial of service weakness in the Linux kernel affecting versions 3.9 and later. This flaw specifically targets the IP fragment reassembly mechanism, which is a fundamental component of network packet processing in operating systems. The vulnerability emerged from the increased IP fragment reassembly queue size that was introduced in newer kernel versions, creating an exploitable condition that allows attackers to consume system resources excessively. The underlying issue stems from inadequate resource management during the reassembly process of fragmented IP packets, where the kernel's handling of overlapping or malformed fragments can be manipulated to trigger excessive memory allocation and processing overhead.
The technical exploitation of CVE-2018-5391 occurs through the careful crafting of IP fragments that cause the kernel's reassembly queue to grow uncontrollably. When the kernel receives specially modified packets, it attempts to reassemble them according to standard IP protocols, but the malicious fragments are designed to either create overlapping ranges or to exploit the queue management logic in ways that cause the kernel to allocate increasingly more memory resources. This vulnerability is classified under CWE-400 as an excessive resource consumption issue, where the attacker can force the system into a state where it exhausts available memory or processing power through seemingly benign network traffic. The attack requires only a low rate of packet transmission, making it particularly dangerous as it can be executed from a single source or even multiple sources without requiring high bandwidth or sophisticated attack infrastructure.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise entire network infrastructure, especially in environments where Linux systems serve as network gateways, routers, or firewalls. When exploited successfully, the vulnerability can cause systems to become unresponsive or crash entirely, leading to complete denial of network services for the affected hosts. Network administrators may observe symptoms including system slowdowns, memory exhaustion, or complete system hangs that require manual intervention to restore normal operation. The vulnerability particularly affects systems with limited resources or those that handle high volumes of network traffic, as the resource consumption can escalate rapidly. According to ATT&CK framework techniques, this vulnerability maps to T1499.004 which covers network denial of service attacks, and T1595.001 which addresses network infiltration through protocol manipulation. The exploitability of this vulnerability also demonstrates the importance of proper input validation and resource management in kernel space operations, as the attack vector leverages legitimate network protocols to achieve malicious outcomes.
Mitigation strategies for CVE-2018-5391 require both immediate kernel updates and network-level protections. The primary solution involves upgrading to kernel versions that contain the specific fixes implemented by the Linux kernel development team, typically those released after the vulnerability disclosure. System administrators should also implement network-level filtering to limit the number of fragments that can be processed per second or to drop suspicious fragment patterns entirely. Additional protective measures include monitoring network traffic for unusual fragment patterns, implementing rate limiting on incoming packets, and configuring firewalls to block or restrict fragment reassembly processing. Organizations should also consider implementing intrusion detection systems that can identify the characteristic packet patterns associated with this attack, and establish automated response procedures that can isolate affected systems before complete service disruption occurs. The vulnerability underscores the critical importance of maintaining up-to-date kernel versions and implementing proper network segmentation to limit the potential impact of such attacks on critical infrastructure components.