CVE-2018-15840 in TL-WR840N
Summary
by MITRE
TP-Link TL-WR840N devices allow remote attackers to cause a denial of service (networking outage) via fragmented packets, as demonstrated by an "nmap -f" command.
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Analysis
by VulDB Data Team • 08/17/2023
The TP-Link TL-WR840N router series represents a widely deployed consumer-grade networking device that has been identified with a critical vulnerability affecting its packet processing capabilities. This vulnerability manifests when the device receives fragmented network packets, specifically those generated through tools like nmap with the -f flag that creates small fragmented packets. The flaw stems from inadequate handling of IP fragmentation within the router's network stack, creating a condition where legitimate network traffic can be disrupted through carefully crafted packet sequences. The vulnerability impacts the device's ability to maintain stable network connectivity and can result in complete networking outages for connected devices, effectively rendering the router unusable until manual intervention occurs through device reboot or power cycling.
The technical root cause of this vulnerability lies in the router's failure to properly validate and process fragmented IP packets during the reassembly phase of network communication. When the device receives fragmented packets, it attempts to reconstruct them for processing but encounters a condition that causes the networking subsystem to become unresponsive or crash entirely. This behavior aligns with common patterns found in buffer overflow conditions or improper state management within network protocol handlers, though the specific implementation flaw remains within the realm of packet reassembly logic rather than memory corruption. The vulnerability is particularly concerning because it can be exploited remotely without requiring authentication, making it accessible to any attacker with network access to the affected device. The nmap -f command specifically creates packets with small fragments that can overwhelm the router's reassembly buffer or trigger a processing loop that consumes all available system resources.
The operational impact of this vulnerability extends beyond simple network disruption to encompass broader security implications for organizations and individuals relying on these devices. When exploited successfully, the denial of service condition can persist for extended periods until the device is manually rebooted, potentially causing business disruption for enterprises or inconvenience for home users. The attack surface is particularly broad since the vulnerability can be triggered from any network location where the device is accessible, including public networks or through compromised devices within the same local network segment. Network administrators may not immediately recognize the cause of the outage, as the symptoms appear as a complete network failure rather than specific error conditions. This vulnerability also demonstrates the importance of proper network protocol implementation and the risks associated with insufficient input validation in embedded network devices, which often operate with limited resources and simplified security models compared to enterprise-grade systems.
Mitigation strategies for this vulnerability should focus on both immediate defensive measures and long-term architectural improvements. Network administrators should consider implementing network segmentation to limit exposure of vulnerable devices to untrusted networks, while also applying firmware updates when available from TP-Link or other security vendors. The use of network monitoring tools can help detect anomalous packet patterns that may indicate exploitation attempts, though the vulnerability itself does not generate clear audit trails. Device-level mitigations include disabling unnecessary network services and implementing access controls to limit who can send packets to the vulnerable device. Organizations should also consider implementing intrusion detection systems that can identify and block fragmented packet traffic patterns that are commonly associated with this type of attack. From a security standards perspective, this vulnerability relates to CWE-129 and CWE-131 which address improper validation of length of input data and improper handling of buffer boundaries, respectively. The attack pattern aligns with ATT&CK technique T1499 which covers network denial of service attacks and T1071 which involves application layer protocol usage for command and control communications. Given the embedded nature of these devices and their typically limited update mechanisms, administrators should also consider replacing vulnerable units with newer models that have better security implementations or implementing network-level firewalls to filter problematic packet patterns before they reach the affected devices.