CVE-2015-0639 in IOS XE
Summary
by MITRE
The Common Flow Table (CFT) feature in Cisco IOS XE 3.6 and 3.7 before 3.7.1S, 3.8 before 3.8.0S, 3.9 before 3.9.0S, 3.10 before 3.10.0S, 3.11 before 3.11.0S, 3.12 before 3.12.0S, 3.13 before 3.13.0S, 3.14 before 3.14.0S, and 3.15 before 3.15.0S, when MMON or NBAR is enabled, allows remote attackers to cause a denial of service (device reload) via malformed IPv6 packets with IPv4 UDP encapsulation, aka Bug ID CSCua79665.
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Analysis
by VulDB Data Team • 04/16/2022
The vulnerability described in CVE-2015-0639 represents a critical denial of service flaw within Cisco IOS XE software that affects multiple version ranges including 3.6 through 3.15. This issue specifically targets the Common Flow Table functionality when combined with either MMON or NBAR protocol analysis modules. The flaw manifests when devices process malformed IPv6 packets that contain IPv4 UDP encapsulation, creating a condition that can trigger complete device reloads. This vulnerability demonstrates the complexity of network protocol handling where seemingly benign packet structures can cause catastrophic system failures. The attack vector is particularly concerning as it requires no authentication and can be executed remotely, making it accessible to any attacker capable of sending crafted network traffic to affected devices. The vulnerability impacts network infrastructure devices that are critical to maintaining connectivity and service availability, potentially causing widespread disruption when exploited at scale.
The technical root cause of this vulnerability lies in inadequate input validation within the packet processing pipeline of Cisco IOS XE. When the CFT feature is enabled alongside MMON or NBAR, the system attempts to parse and analyze IPv6 packets that contain IPv4 UDP encapsulation. The flaw occurs during the flow table processing logic where malformed packet structures cause memory corruption or state inconsistencies that ultimately lead to system instability. This type of vulnerability falls under CWE-129 Input Validation and CWE-20 Improper Input Validation, as the system fails to properly validate packet contents before processing them through the flow table mechanism. The specific conditions that trigger the vulnerability involve packet structures that do not conform to expected IPv6 or UDP protocol specifications, creating parsing errors that propagate through the system's internal state management. The interaction between the CFT feature and protocol analysis modules creates a complex processing path where input validation occurs at inappropriate layers or fails to account for edge cases in packet construction.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise network availability and reliability. When exploited, the vulnerability forces complete device reloads, requiring network administrators to manually intervene and restore services. This creates cascading effects throughout the network infrastructure as routing tables, neighbor relationships, and other critical network state information are lost during the restart process. The vulnerability affects network devices that are typically considered stable and reliable components of enterprise networks, meaning that successful exploitation can lead to significant business disruption and service outages. Organizations that rely heavily on Cisco IOS XE devices for core network functions face particular risk, as the vulnerability can be exploited to create sustained denial of service conditions that may require extended downtime for recovery. The remote nature of the attack means that devices can be compromised from outside the network perimeter, making traditional network segmentation ineffective as a protective measure.
Mitigation strategies for CVE-2015-0639 should focus on immediate remediation through software updates to affected versions. Cisco released patches for all affected IOS XE versions, with the most critical updates available in the 3.7.1S, 3.8.0S, 3.9.0S, 3.10.0S, 3.11.0S, 3.12.0S, 3.13.0S, 3.14.0S, and 3.15.0S releases. Network administrators should prioritize patching all affected devices and verify that the updates have been successfully applied. In environments where immediate patching is not feasible, temporary workarounds include disabling the CFT feature, or disabling MMON and NBAR protocols when these features are not actively required for network operations. Additionally, network segmentation and access control measures can help limit the attack surface by preventing unauthorized access to devices that are vulnerable to this specific exploit. Monitoring network traffic for malformed IPv6 packets with UDP encapsulation can help detect potential exploitation attempts, though this requires careful configuration to avoid false positives. From a defensive perspective, this vulnerability aligns with ATT&CK technique T1499.004 for Network Denial of Service and T1071.004 for Application Layer Protocol, demonstrating how protocol handling flaws can be leveraged to create persistent service disruption. Organizations should also implement comprehensive network monitoring and incident response procedures to address potential exploitation attempts and ensure rapid recovery from any successful attacks.