CVE-2013-6981 in IOS XE
Summary
by MITRE
Cisco IOS XE 3.7S(.1) and earlier allows remote attackers to cause a denial of service (Packet Processor crash) via fragmented MPLS IP packets, aka Bug ID CSCul00709.
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Analysis
by VulDB Data Team • 06/04/2021
Cisco IOS XE software versions 3.7S and earlier contain a critical vulnerability that enables remote attackers to induce a denial of service condition through carefully crafted fragmented MPLS IP packets. This vulnerability specifically affects the Packet Processor component within the network operating system, causing it to crash and subsequently disrupting network services. The flaw manifests when the system processes fragmented MPLS packets that contain IP headers, creating a condition where the packet processor cannot properly handle the malformed data structure. This issue represents a classic buffer overflow scenario where the system fails to properly validate packet fragmentation sequences, leading to memory corruption and subsequent system instability. The vulnerability impacts network infrastructure devices running affected IOS XE versions, potentially affecting routers, switches, and other network equipment that rely on MPLS forwarding capabilities.
The technical implementation of this vulnerability stems from inadequate input validation within the MPLS packet processing module. When the Packet Processor receives fragmented MPLS IP packets, it fails to properly verify the integrity of the fragment headers and their relationship to the complete packet structure. This validation gap allows attackers to craft packets with malformed fragment offsets or overlapping segments that cause the processor to attempt invalid memory operations. The vulnerability aligns with CWE-129, which describes improper validation of input ranges, and CWE-125, which covers out-of-bounds read conditions. The specific nature of the flaw demonstrates how improper handling of network protocol fragmentation can lead to system crashes, particularly when dealing with complex multiprotocol label switching implementations. Attackers can exploit this by sending specifically crafted fragmented packets that trigger the buffer overflow condition in the packet processing engine.
The operational impact of this vulnerability extends beyond simple service disruption, as it can affect network availability and reliability across enterprise and service provider networks. When the Packet Processor crashes, it typically requires a system reboot to restore normal operations, resulting in network downtime that can last from several minutes to hours depending on the device configuration and recovery procedures. This denial of service condition affects routing stability and can potentially cascade through network infrastructure, especially in environments where MPLS traffic is prevalent. The vulnerability affects devices that utilize the MPLS forwarding engine, which is commonly implemented in backbone networks, service provider environments, and enterprise core networks where traffic engineering and quality of service are critical. Organizations with extensive MPLS deployments face significant risk, as this vulnerability can be exploited remotely without authentication, making it particularly dangerous in unsecured network environments.
Mitigation strategies for this vulnerability require immediate patching of affected devices with Cisco's security updates and software releases. Network administrators should prioritize updating all IOS XE devices to versions that contain the fix for CSCul00709, which typically involves applying the relevant security patch or upgrading to a newer software release that addresses the packet processing validation flaw. Organizations should also implement network segmentation and access control measures to limit exposure, particularly in environments where MPLS traffic flows through untrusted network segments. Monitoring network traffic for unusual fragmentation patterns and implementing intrusion detection systems can help identify potential exploitation attempts. The vulnerability demonstrates the importance of proper input validation in network protocol implementations and aligns with ATT&CK technique T1499.002, which covers network denial of service attacks through fragmentation. Additionally, organizations should consider implementing traffic filtering rules that limit the acceptance of fragmented packets or restrict MPLS traffic from untrusted sources to reduce the attack surface. Regular security assessments and vulnerability scanning should be conducted to identify other potential weaknesses in network infrastructure that could be exploited in similar manners.