CVE-2026-51724 in TOTOLINK
Summary
by MITRE • 08/31/2026
Incorrect access control in the delSmartQosCfg function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to remove Smart QoS rules via sending a crafted POST request to /cgi-bin/cstecgi.cgi.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 08/31/2026
The vulnerability identified in the TOTOLINK T6 router, specifically within firmware version 4.1.5cu.748_B20211015, represents a critical failure in access control mechanisms governing network configuration management. The flaw resides in the delSmartQosCfg function, which is responsible for deleting Quality of Service (QoS) rules that prioritize specific types of internet traffic to ensure optimal performance for latency-sensitive applications such as VoIP or online gaming. This component fails to enforce proper authentication checks before executing administrative commands, allowing any entity with network access to interact directly with the underlying CGI interface without providing valid credentials.
Technically, the exploitation vector involves sending a specifically crafted HTTP POST request to the endpoint /cgi-bin/cstecgi.cgi. By targeting this specific function within the Common Gateway Interface script, an attacker can bypass the expected authentication gatekeeper logic that should validate user sessions or administrative privileges before processing configuration changes. The system incorrectly assumes that requests directed at this particular CGI handler are authorized, thereby permitting unauthenticated actors to modify critical network settings. This type of error is a classic example of broken access control where the software does not properly verify whether the authenticated user has authority to perform the requested action on specific resources or functions.
The operational impact of this vulnerability is significant for both home and small business networks relying on QoS features. An attacker who exploits this flaw can remove existing Smart QoS rules, effectively neutralizing any traffic prioritization configured by the network administrator. This degradation in service quality can lead to increased latency, packet loss, or jitter for critical applications that depend on bandwidth reservation. Furthermore, the ability to manipulate these settings without authorization undermines the integrity of the network configuration management process, potentially leading to denial-of-service conditions for specific services or creating opportunities for further exploitation by masking malicious traffic patterns within normal data flows due to altered prioritization rules.
From a classification perspective, this vulnerability aligns with CWE-269, which denotes Improper Privilege Control, as it allows an actor to perform actions that should be restricted to privileged users. It also relates closely to CWE-862, Missing Authorization, because the system fails to enforce access control policies on specific resources or functions. In terms of offensive security frameworks, this activity maps to MITRE ATT&CK technique T1546.003, specifically Event Triggered Execution via COM Hijacking or similar configuration manipulation tactics where an attacker modifies system settings to maintain persistence or disrupt operations, although in this context it is more accurately described as unauthorized modification of network configurations leading to service degradation rather than persistent backdoor installation.
Mitigation strategies for this vulnerability primarily involve immediate firmware updates provided by the vendor if a patch has been released to address the authentication bypass in the delSmartQosCfg function. In environments where updating is not immediately feasible, administrators should restrict access to the router's management interface through network segmentation or firewall rules that limit exposure of the /cgi-bin/cstecgi.cgi endpoint to trusted internal IP addresses only. Additionally, disabling remote administration features if they are not required can reduce the attack surface available to external attackers. Regular auditing of configuration changes and monitoring for unusual POST requests to CGI endpoints can also aid in early detection of exploitation attempts.