CVE-2013-4563 in Linux
Summary
by MITRE
The udp6_ufo_fragment function in net/ipv6/udp_offload.c in the Linux kernel through 3.12, when UDP Fragmentation Offload (UFO) is enabled, does not properly perform a certain size comparison before inserting a fragment header, which allows remote attackers to cause a denial of service (panic) via a large IPv6 UDP packet, as demonstrated by use of the Token Bucket Filter (TBF) queueing discipline.
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Analysis
by VulDB Data Team • 06/02/2021
The vulnerability identified as CVE-2013-4563 represents a critical denial of service flaw within the Linux kernel's IPv6 UDP handling mechanism. This issue specifically affects kernel versions through 3.12 and stems from improper size validation within the udp6_ufo_fragment function located in the net/ipv6/udp_offload.c file. The flaw manifests when UDP Fragmentation Offload (UFO) is enabled, a feature designed to improve network performance by offloading packet fragmentation tasks from the CPU to network hardware. The vulnerability operates by failing to properly validate packet sizes before inserting fragment headers, creating a condition where maliciously crafted IPv6 UDP packets can trigger system instability.
The technical exploitation of this vulnerability occurs through the manipulation of IPv6 UDP packets that exceed normal size thresholds when processed through the UFO mechanism. When a large IPv6 UDP packet is received with UFO enabled, the kernel's udp6_ufo_fragment function attempts to fragment the packet but neglects to perform adequate size comparison checks before inserting fragment headers. This omission creates a scenario where the kernel's memory management system receives malformed fragment header data, leading to a kernel panic and complete system crash. The vulnerability is particularly dangerous because it can be triggered remotely without requiring authentication, making it an attractive target for denial of service attacks against network infrastructure.
The operational impact of CVE-2013-4563 extends beyond simple service disruption to potentially compromising entire network systems that rely on Linux-based infrastructure. Network administrators and system operators face significant risk when their systems have UFO enabled, as a single malicious packet can bring down critical services such as web servers, DNS systems, or network routers. The vulnerability affects systems using the Token Bucket Filter (TBF) queueing discipline, which is commonly deployed in traffic shaping scenarios, amplifying the potential attack surface. Organizations running Linux kernels version 3.12 or earlier with UFO enabled are particularly vulnerable, as the flaw exists in the core networking stack that handles all IPv6 UDP traffic.
This vulnerability aligns with CWE-129, which describes improper validation of input ranges, and represents a classic example of inadequate bounds checking in kernel space code. From an adversarial perspective, the flaw maps to ATT&CK technique T1499.004, which covers network denial of service attacks through exploitation of kernel vulnerabilities. The attack vector requires only network access to send malicious packets, making it easily executable from remote locations. The exploitability factor is high due to the minimal requirements for triggering the condition and the severe impact on system availability. System administrators should consider this vulnerability as part of their broader security posture assessment, particularly in environments where IPv6 networking is active and UFO functionality is enabled.
Mitigation strategies for CVE-2013-4563 primarily focus on kernel version upgrades to patched releases, with the most effective solution being the deployment of Linux kernel versions 3.13 or later where the vulnerability has been addressed. Organizations should also consider disabling UDP Fragmentation Offload functionality on affected systems through the use of sysctl parameters or by modifying network interface configurations. Additionally, network segmentation and intrusion detection systems can help identify and block suspicious traffic patterns that might indicate exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date kernel versions and implementing proper network monitoring to detect anomalous packet behavior that could indicate exploitation attempts. Security teams should also review their network configurations to ensure that UFO is disabled on systems where it is not required, as this provides an additional layer of protection against such kernel-level vulnerabilities.