CVE-2015-4293 in IOS XE
Summary
by MITRE
The packet-reassembly implementation in Cisco IOS XE 3.13S and earlier allows remote attackers to cause a denial of service (CPU consumption or packet loss) via fragmented (1) IPv4 or (2) IPv6 packets that trigger ATTN-3-SYNC_TIMEOUT errors after reassembly failures, aka Bug ID CSCuo37957.
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Analysis
by VulDB Data Team • 06/07/2022
The vulnerability described in CVE-2015-4293 represents a critical denial of service flaw within Cisco IOS XE software versions 3.13S and earlier. This issue specifically targets the packet reassembly functionality that processes fragmented IPv4 and IPv6 packets, creating a condition where maliciously crafted network traffic can trigger system instability. The vulnerability operates through the ATTN-3-SYNC_TIMEOUT error mechanism, which occurs when the system encounters reassembly failures during the processing of fragmented packets. This flaw demonstrates a fundamental weakness in how the router handles fragmented network traffic, potentially allowing remote attackers to exploit the system without requiring authentication or privileged access. The impact extends beyond simple service interruption to include complete system resource exhaustion, making it particularly dangerous in network infrastructure environments where availability is paramount. The vulnerability affects devices running Cisco IOS XE software, which represents a significant portion of enterprise network infrastructure, amplifying the potential impact across multiple organizational boundaries.
The technical implementation of this vulnerability stems from improper handling of fragmented packet reassembly processes within the network operating system. When the system receives fragmented IPv4 or IPv6 packets, it attempts to reconstruct the original packets through a reassembly process that involves maintaining state information and synchronization mechanisms. The flaw manifests when the reassembly process encounters specific patterns of fragmented packets that cause the system to enter a timeout state, triggering the ATTN-3-SYNC_TIMEOUT error. This error condition leads to excessive CPU utilization as the system attempts to process the malformed reassembly requests, ultimately resulting in either complete CPU exhaustion or packet loss throughout the network device. The vulnerability operates at the network layer where the system's packet processing engine fails to properly validate or handle edge cases in fragmented packet reassembly, creating a resource consumption attack vector that can be executed remotely.
The operational impact of this vulnerability creates substantial risk for network infrastructure administrators, as it allows attackers to cause complete denial of service conditions without requiring any form of authentication or privileged access. The vulnerability can be exploited by sending specifically crafted fragmented packets to the affected Cisco IOS XE devices, causing them to consume excessive CPU resources or drop packets entirely. This condition affects network availability and can potentially disrupt business operations, particularly in environments where network reliability is critical. The system behavior manifests as either sustained high CPU usage that degrades overall performance or complete packet loss, both of which result in network service degradation. Organizations using affected Cisco IOS XE versions face the risk of unauthorized parties disrupting their network services, potentially causing cascading failures in network infrastructure that could affect multiple connected systems and services.
Mitigation strategies for CVE-2015-4293 require immediate attention from network administrators, with the most effective approach being the deployment of Cisco IOS XE software patches that address the packet reassembly implementation flaw. Organizations should prioritize upgrading their affected devices to versions that contain the necessary fixes, as this represents the most direct solution to the vulnerability. Network segmentation and access control measures can provide additional protection by limiting the attack surface and preventing unauthorized access to vulnerable network devices. Implementing traffic filtering rules that identify and drop suspicious fragmented packets can also reduce the risk of exploitation, though this approach does not address the root cause. Monitoring systems should be configured to detect unusual CPU utilization patterns or packet loss conditions that may indicate exploitation attempts. The vulnerability aligns with CWE-129, which describes improper validation of input boundaries, and represents a specific instance of improper input validation that leads to resource exhaustion. From an ATT&CK framework perspective, this vulnerability maps to the denial of service technique, potentially enabling further attacks through network disruption that could facilitate other exploitation vectors. Organizations should also consider implementing network intrusion detection systems that can identify patterns associated with this specific vulnerability, providing early warning capabilities for potential exploitation attempts.