CVE-2026-71938 in VigorSwitch G2540xs
Summary
by MITRE • 08/24/2026
Multiple DrayTek VigorSwitch models contain a buffer overflow vulnerability in the switch_lan_gvrp function. The vulnerability is caused by unsafe copying of the portList field into an undersized buffer. A remote attacker can trigger this vulnerability via crafted input, causing a denial of service or potentially executing arbitrary commands. Exploitation requires valid administrative credentials for the device's web management interface.
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Analysis
by VulDB Data Team • 08/24/2026
The identified vulnerability resides within the DrayTek VigorSwitch series and specifically affects the switch_lan_gvrp function, which is responsible for handling Generic VLAN Registration Protocol traffic. This protocol facilitates dynamic VLAN membership registration among switches to ensure consistent network segmentation across a switched environment. The core technical flaw stems from an unsafe memory copy operation where the portList field of incoming GVRP messages is copied into a buffer that is insufficiently sized to accommodate the input data. This classic buffer overflow condition occurs because the application fails to validate or truncate the length of the port list before performing the memcpy operation, allowing crafted packets with oversized payloads to overwrite adjacent memory locations on the stack or heap.
From an operational perspective, this vulnerability presents a significant risk despite requiring authentication for exploitation. Because triggering the flaw necessitates valid administrative credentials for the device's web management interface, it is classified as an authenticated attack vector rather than a remote unauthenticated exploit. However, the presence of such credentials does not eliminate the severity of the issue. If an attacker gains access to these credentials through phishing, credential stuffing, or other means, they can send specially crafted GVRP packets that trigger the buffer overflow. The immediate consequence is typically a denial of service condition, where the switch process crashes or reboots, disrupting network connectivity for all devices connected to the affected ports. In more severe scenarios, depending on the specific memory layout and compiler protections enabled during firmware compilation, an attacker may achieve arbitrary code execution by overwriting return addresses or function pointers with shellcode, thereby gaining full control of the embedded device.
This vulnerability aligns closely with Common Weakness Enumeration identifier CWE-120, which describes buffer copy without checking size limits, a fundamental category of memory safety errors prevalent in C-based network firmware. Furthermore, from an adversary behavior perspective, this exploit technique corresponds to ATT&CK tactic Tactic TA0004 Privilege Escalation and potentially Technique T1059 Command and Scripting Interpreter if code execution is achieved. The requirement for administrative access places the exploitation phase within the lateral movement or persistence phases of a cyber kill chain, where an insider threat or compromised admin account is leveraged to destabilize critical network infrastructure.
Mitigation strategies must prioritize immediate patching by DrayTek firmware updates that address the buffer size validation in the switch_lan_gvrp function. Until patches are applied, administrators should enforce strict access controls on the web management interface, ensuring that only trusted IP addresses can reach the administrative ports and that strong, unique passwords are mandated for all admin accounts. Additionally, network segmentation policies should be reviewed to limit the exposure of management interfaces to untrusted networks. Monitoring tools should be configured to detect anomalous GVRP traffic patterns or repeated authentication failures from single sources as indicators of potential exploitation attempts. Regular vulnerability assessments focused on embedded devices and firmware integrity checks are essential components of a robust defense-in-depth strategy for network infrastructure.