CVE-2021-21005 in FL SWITCH SMCS
Summary
by MITRE • 06/26/2021
In Phoenix Contact FL SWITCH SMCS series products in multiple versions if an attacker sends a hand-crafted TCP-Packet with the Urgent-Flag set and the Urgent-Pointer set to 0, the network stack will crash. The device needs to be rebooted afterwards.
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Analysis
by VulDB Data Team • 07/03/2021
The vulnerability identified as CVE-2021-21005 affects Phoenix Contact FL SWITCH SMCS series industrial network devices across multiple firmware versions, representing a critical network stack flaw that can lead to denial of service conditions. This vulnerability specifically targets the TCP protocol implementation within these industrial switching devices, which are commonly deployed in critical infrastructure environments including industrial control systems and smart grid applications. The affected devices operate within the operational technology (OT) domain where network reliability is paramount for continuous operations and safety systems.
The technical flaw manifests when the network stack processes specially crafted TCP packets containing the urgent flag set to one and the urgent pointer field explicitly set to zero. This particular packet construction triggers a buffer overflow or memory corruption condition within the device's TCP processing routine, causing the network stack to become unstable and eventually crash. The vulnerability stems from inadequate input validation and memory management within the TCP implementation, allowing malformed packet data to corrupt critical system resources. According to CWE classification, this corresponds to CWE-121: Stack-based Buffer Overflow, as the malformed packet data leads to memory corruption in the network stack's processing functions. The specific conditions required for exploitation involve sending a TCP packet with the urgent flag set to 1 and urgent pointer set to 0, which is a valid TCP packet construction but triggers undefined behavior in the vulnerable implementation.
The operational impact of this vulnerability extends beyond simple network disruption, as it creates a persistent denial of service condition that requires manual intervention to restore functionality. When the network stack crashes, the affected device becomes completely non-functional for network communications until a manual reboot is performed, potentially leading to extended downtime in critical infrastructure environments. This vulnerability particularly affects industrial environments where network availability is critical for process control, safety systems, and real-time operations. The attack vector is straightforward and requires only network access to send the malicious packet, making it an attractive target for adversaries seeking to disrupt industrial operations. From an ATT&CK framework perspective, this vulnerability maps to T1498: Network Denial of Service, as it enables an attacker to cause a denial of service condition through network packet manipulation. The vulnerability also aligns with T1566: Phishing, as attackers could potentially use this flaw as part of a broader attack campaign to disrupt network communications before deploying additional malicious payloads.
Mitigation strategies for this vulnerability should focus on both immediate defensive measures and long-term architectural improvements. Network administrators should implement ingress filtering and TCP packet validation at network boundaries to prevent malformed packets from reaching vulnerable devices. The most effective immediate solution involves applying firmware updates from Phoenix Contact that address the TCP stack implementation flaw, which typically include enhanced input validation and proper memory management routines. Network segmentation and access control measures should be implemented to limit network exposure of vulnerable devices, reducing the attack surface available to potential adversaries. Additionally, implementing network monitoring solutions that can detect unusual packet patterns or malformed TCP traffic can provide early warning of exploitation attempts. Organizations should also consider implementing redundant network paths and failover mechanisms to minimize the impact of service disruption. The vulnerability highlights the importance of secure coding practices in industrial network equipment and the need for comprehensive security testing of network stack implementations in OT environments. Regular security assessments and vulnerability management programs should include specific testing for TCP stack vulnerabilities in industrial network equipment, as these devices often operate in environments where traditional network security controls may be insufficient.