CVE-2026-51768 in TOTOLINK
Summary
by MITRE • 09/01/2026
Incorrect access control in the setElinkQosConfig function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to modify privileged QoS policy on the master device 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 firmware version 4.1.5cu.748_B20211015 represents a critical failure in access control mechanisms within the network device's internal communication architecture. Specifically, the flaw resides in the setElinkQosConfig function, which is responsible for configuring Quality of Service policies on the master device. This component interacts with an MQTT broker service known as cs_broker to receive configuration updates from associated client devices or management interfaces. The core technical deficiency lies in the absence of proper authentication and authorization checks when processing incoming messages directed at this specific endpoint. Consequently, any entity capable of reaching the network interface hosting the MQTT service can interact with the function without providing valid credentials or session tokens. This lack of enforcement allows unauthenticated attackers to inject malicious payloads that manipulate system settings intended for privileged administrative use only.
From a technical perspective, the exploitation vector involves sending a crafted Message Queuing Telemetry Transport message directly to the cs_broker component. MQTT is commonly used in Internet of Things and smart home ecosystems due to its lightweight nature, but it often suffers from weak default security configurations if not properly hardened. In this instance, the setElinkQosConfig function fails to validate the source or identity of the sender before executing configuration changes. This allows an attacker who has network access to the local segment where the router resides to alter QoS policies at will. These policies typically dictate bandwidth allocation, traffic prioritization, and latency management for various devices connected to the network. By modifying these settings maliciously, an adversary can degrade service quality for specific users or applications while potentially boosting performance for their own traffic, effectively creating a denial of service condition for legitimate users or facilitating stealthier data exfiltration by masking it as high-priority traffic.
The operational impact of this vulnerability is significant within the context of home and small office networks where such routers are predominantly deployed. An attacker can disrupt network stability by misconfiguring bandwidth limits, causing latency spikes, or completely blocking internet access for certain devices. Furthermore, since QoS configurations often interact with other security features like parental controls or firewall rules, tampering with these settings may inadvertently disable protective measures. This creates a broader attack surface where subsequent vulnerabilities could be more easily exploited due to the weakened state of the device's defensive posture. The ability to modify privileged configuration without authentication fundamentally breaks the principle of least privilege and undermines trust in the network infrastructure, allowing for persistent disruption or manipulation of network behavior even after reboots if the malicious settings are saved permanently by the firmware logic.
This vulnerability aligns with CWE-269, which describes Improper Privilege Assignment, as well as CWE-862, Missing Authorization, because the system fails to ensure that only authorized actors can perform specific actions. In terms of offensive security frameworks, this behavior is consistent with MITRE ATT&CK technique T1548.003, Abuse Configuration: Misconfiguration, where an attacker alters settings to bypass controls or gain advantage. Additionally, it relates to T1071 Application Layer Protocol, specifically MQTT abuse, as the attack relies on manipulating a standard protocol without proper security extensions like TLS client certificates or strong authentication mechanisms. Remediation requires immediate firmware updates from TOTOLINK that enforce strict access control lists and require valid session tokens for all administrative API calls. Until patches are applied, network administrators should isolate affected devices in separate VLANs with restricted internet access to limit the blast radius of potential exploitation by unauthenticated local attackers.