CVE-2026-51763 in TOTOLINK
Summary
by MITRE • 09/01/2026
Incorrect access control in the freeStaClient function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to forcibly disconnect wireless clients via sending a crafted MQTT message to the cs_broker component.
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Analysis
by VulDB Data Team • 09/01/2026
The vulnerability identified in TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015 represents a critical failure in access control mechanisms within the device's internal communication architecture. Specifically, the flaw resides in the freeStaClient function, which is responsible for managing and terminating connections of wireless clients associated with the router. This component interacts with an MQTT broker service known as cs_broker to relay state changes and commands regarding connected devices. The core technical deficiency lies in the absence of proper authentication or authorization checks when processing requests directed at this specific function. Consequently, any entity capable of reaching the network interface hosting the MQTT broker can issue commands that manipulate client connection states without providing valid credentials or tokens.
From a technical perspective, an unauthenticated attacker who has gained access to the local area network segment where the router resides can exploit this weakness by crafting and transmitting malicious MQTT messages. These crafted packets are designed to invoke the freeStaClient function with parameters intended to forcibly disconnect specific wireless clients from the network. The vulnerability stems from insufficient validation of the source or authority of these internal API calls, allowing external actors to bypass security boundaries that should restrict such administrative actions to authenticated management interfaces only. This type of flaw is characteristic of improper access control vulnerabilities where service endpoints are exposed without adequate safeguards against unauthorized invocation.
The operational impact of this vulnerability is significant for network integrity and availability. By forcing wireless clients to disconnect, an attacker can effectively launch a localized denial-of-service attack against specific users or devices on the network. This disruption prevents legitimate users from maintaining their internet connectivity, leading to service degradation and potential data loss if active sessions are abruptly terminated. Furthermore, this capability could be leveraged as part of a broader attack chain. For instance, an attacker might use forced disconnections to trigger reconnection attempts that expose other vulnerabilities during the authentication handshake process or to disrupt security monitoring systems relying on continuous client presence for threat detection.
This vulnerability aligns with CWE-284, which classifies improper access control issues where actors are able to bypass intended restrictions. In terms of offensive cybersecurity frameworks, this exploit maps directly to MITRE ATT&CK technique T1531, Access Denial via Resource Exhaustion or other means, specifically through the disruption of service availability for targeted endpoints. It also relates to T1078, Valid Accounts, in scenarios where attackers might use compromised credentials if available, but more critically highlights risks associated with unauthenticated access vectors common in IoT device management interfaces.
Mitigation strategies must focus on hardening the MQTT broker configuration and reinforcing internal API security. The most effective immediate remedy is for users to update their router firmware to a version that patches this specific flaw, as vendors typically address such issues by implementing strict authentication requirements for all administrative functions exposed via internal services like cs_broker. Network segmentation can also serve as a defensive measure; placing the management interface of IoT devices on a separate VLAN or subnet limits exposure to untrusted network segments. Additionally, deploying intrusion detection systems that monitor MQTT traffic patterns may help identify and block attempts to exploit this specific vector by detecting anomalous message structures targeting client disconnection functions.