CVE-2018-6923 in FreeBSD
Summary
by MITRE
In FreeBSD before 11.1-STABLE, 11.2-RELEASE-p2, 11.1-RELEASE-p13, ip fragment reassembly code is vulnerable to a denial of service due to excessive system resource consumption. This issue can allow a remote attacker who is able to send an arbitrary ip fragments to cause the machine to consume excessive resources.
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Analysis
by VulDB Data Team • 05/02/2023
The vulnerability identified as CVE-2018-6923 represents a critical denial of service weakness in the FreeBSD operating system's IP fragment reassembly mechanism. This flaw exists in versions prior to 11.1-STABLE, 11.2-RELEASE-p2, and 11.1-RELEASE-p13, where the kernel's handling of fragmented IP packets becomes susceptible to resource exhaustion attacks. The issue stems from inadequate validation and processing of fragmented packets during the reassembly phase, creating a pathway for malicious actors to consume system resources at an unsustainable rate.
The technical root cause of this vulnerability lies within the kernel's IP fragment reassembly code which fails to properly limit or validate the memory allocation and processing resources consumed during the reconstruction of fragmented packets. When an attacker sends carefully crafted fragmented IP packets, the system's reassembly mechanism becomes overwhelmed with memory allocations and processing overhead, leading to system instability and potential complete resource exhaustion. This weakness operates at the network layer where the kernel must maintain tracking information for each fragment set, including sequence numbers, packet boundaries, and reassembly buffers. The lack of proper resource limits in this process creates an opportunity for attackers to exploit the system's memory management through a series of carefully constructed fragments that consume increasing amounts of kernel memory and processing cycles.
From an operational perspective, this vulnerability presents a significant threat to network infrastructure and services hosted on FreeBSD systems. Remote attackers can leverage this weakness to perform denial of service attacks against target systems without requiring authentication or elevated privileges. The impact extends beyond simple service disruption as excessive resource consumption can lead to system crashes, complete service unavailability, and potential compromise of system stability. Network administrators face the challenge of defending against attacks that can be launched from anywhere on the internet, making this vulnerability particularly dangerous for publicly accessible servers and network equipment. The vulnerability's exploitation does not require sophisticated techniques or deep system knowledge, making it accessible to attackers with basic networking skills.
The mitigation strategies for CVE-2018-6923 primarily involve upgrading to patched versions of FreeBSD that contain improved resource management and validation controls for IP fragment reassembly. System administrators should prioritize applying the relevant security patches and updates provided by the FreeBSD project, which typically include enhanced memory allocation limits and improved validation routines. Network-level protections such as firewall rules that limit fragment processing or implement rate limiting can provide additional defense in depth, though these measures may impact legitimate network traffic. The vulnerability aligns with CWE-400, which covers unchecked resource allocation, and relates to ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also implement monitoring solutions that can detect unusual network fragment processing patterns and alert on potential exploitation attempts, as the vulnerability can be difficult to distinguish from legitimate high-traffic scenarios without proper detection mechanisms in place.