CVE-2026-51761 in TOTOLINK
Summary
by MITRE • 09/01/2026
Incorrect access control in the updateLanIp function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to refresh the LAN address state 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 network management subsystem. Specifically, the flaw resides in the updateLanIp function, which is responsible for handling and updating Local Area Network address configurations. This component interacts with the cs_broker service, an internal messaging broker that facilitates communication between various system modules using MQTT protocols. The core issue arises from the absence of proper authentication checks when processing incoming messages directed at this specific function. Consequently, any entity capable of reaching the device's network interface and sending a crafted MQTT message can trigger state changes without providing valid credentials or session tokens. This lack of enforcement allows unauthenticated actors to manipulate critical network configuration parameters remotely.
From a technical perspective, the exploitation vector relies on the direct interaction with the cs_broker component via MQTT. MQTT is commonly used in Internet of Things devices for lightweight publish-subscribe messaging due its low overhead and efficiency. However, if the broker does not validate the identity or authorization level of the sender before executing sensitive administrative functions like updating LAN IP settings, it creates a significant security gap. An attacker can craft specific MQTT payloads that mimic legitimate configuration updates but are designed to alter the router's network topology. By sending these crafted messages, an unauthorized user can force the device to refresh its LAN address state, potentially leading to denial of service conditions or facilitating further lateral movement within a local network by changing IP assignments in ways that disrupt normal connectivity or redirect traffic through malicious endpoints.
The operational impact of this vulnerability is substantial for both individual users and enterprise environments relying on TOTOLINK routers. Unauthorized modification of the LAN IP configuration can result in immediate loss of administrative access to the router, effectively locking out legitimate administrators from managing their network infrastructure. Furthermore, if an attacker manipulates the IP settings or associated gateway configurations, they may be able to perform man-in-the-middle attacks against other devices on the local subnet. This could lead to interception of sensitive data, credential harvesting, or injection of malicious content into unencrypted communications flowing through the router. The ability to alter network state without authentication undermines the fundamental trust model of home and small business networking equipment, where physical access is often assumed but remote exploitation via software flaws poses a severe risk.
This vulnerability aligns with CWE-284 Improper Access Control, as it involves failing to restrict actions that should be limited to authorized users or processes. Additionally, given the use of MQTT for command and control-like operations within an IoT device context, this flaw can also be mapped to MITRE ATT&CK techniques related to Command and Scripting Interpreter abuse or specific IoT exploitation patterns where brokers are targeted for unauthorized configuration changes. The lack of authentication on a critical administrative function highlights a common deficiency in embedded systems development where convenience often overrides security during the implementation phase.
To mitigate this risk, users should immediately update their TOTOLINK T6 firmware to the latest available version that addresses this access control flaw if one has been released by the vendor. In cases where patching is not immediately possible or feasible due to legacy constraints, network segmentation strategies can help limit exposure. Placing IoT devices on a separate VLAN isolated from critical corporate or personal data networks reduces the blast radius of such an exploit. Additionally, ensuring that remote management features are disabled unless absolutely necessary minimizes the attack surface presented by internal services like cs_broker. Regular auditing of device configurations and monitoring for unusual MQTT traffic patterns can also aid in early detection of exploitation attempts before significant damage occurs.