CVE-2026-51756 in TOTOLINK
Summary
by MITRE • 09/01/2026
Incorrect access control in the meshSlaveUpgfw function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to start firmware flashing using existing upgrade files 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 the meshSlaveUpgfw function of TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015 represents a critical failure in access control mechanisms within the device's mesh networking subsystem. This flaw specifically affects the cs_broker component, which is responsible for handling MQTT messages related to network management and configuration updates. The core issue lies in the absence of proper authentication or authorization checks when processing firmware upgrade requests initiated through this function. Consequently, any entity capable of communicating with the router over the local area network can exploit this weakness without needing valid credentials. This lack of security enforcement allows unauthenticated attackers to interact directly with the internal broker service that manages device updates, bypassing standard administrative controls designed to prevent unauthorized modifications to the system software.
From a technical perspective, the exploitation vector involves sending a specifically crafted MQTT message to the cs_broker component. MQTT is a lightweight publish-subscribe based networking protocol commonly used in Internet of Things devices for efficient data transmission over limited bandwidth networks. In this context, the router utilizes MQTT to facilitate communication between mesh nodes and potentially with external management interfaces. The vulnerability arises because the meshSlaveUpgfw function does not verify the identity or privileges of the sender before proceeding with a firmware flashing operation using existing upgrade files. This design oversight means that an attacker who can reach the device on its network interface, whether through wired Ethernet or wireless connections depending on how MQTT is exposed, can trigger the update process remotely. The ability to initiate this process without authentication highlights a fundamental misconfiguration in the security architecture of the embedded web server and broker services running on the device.
The operational impact of this vulnerability is severe, primarily due to the nature of firmware flashing operations. An attacker who successfully exploits this flaw can force the router to reflash its firmware using maliciously prepared or existing upgrade files. This capability effectively allows for a denial-of-service attack if the injected file is corrupted or incompatible, rendering the device inoperable and requiring physical access to recover it through recovery modes. More critically, if an attacker provides a custom-built firmware image containing backdoors, rootkits, or other malicious code, they gain persistent remote control over the network infrastructure. This compromises not only the router itself but also all devices connected behind it, potentially leading to data interception, lateral movement within the local network, and use of the compromised device as part of a botnet for further attacks against external targets.
This vulnerability aligns with CWE-284 Improper Access Control, specifically reflecting failures in authentication and authorization processes where access rights are not properly enforced before performing sensitive operations. It also maps to MITRE ATT&CK techniques related to Initial Access via valid accounts or services that lack proper security controls, as well as Defense Evasion through the manipulation of system updates. The exploitation path demonstrates how IoT devices often prioritize functionality over security, leaving management interfaces accessible without sufficient safeguards against unauthorized modification.
Mitigation strategies for this vulnerability focus on network segmentation and access restriction. Users should ensure that MQTT ports used by the cs_broker are not exposed to untrusted networks or the internet. If possible, restricting access to these services to only trusted IP addresses within the local subnet can prevent remote exploitation. Additionally, keeping firmware updated is crucial as vendors may release patches addressing such access control flaws in subsequent releases. Administrators should also disable unnecessary management protocols and features if they are not actively used for mesh networking operations. Implementing strong administrative passwords does not mitigate this specific flaw since authentication is bypassed entirely during the MQTT message processing phase, making network-level controls the primary defense mechanism against exploitation until a software patch is available.