CVE-2026-51723 in TOTOLINK
Summary
by MITRE • 08/31/2026
Incorrect access control in the UploadCustomModule function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to install a custom CGI module via sending a crafted POST request to /cgi-bin/cstecgi.cgi.
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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 the device's web management interface. The core of this issue resides in the UploadCustomModule function, which is exposed through the endpoint /cgi-bin/cstecgi.cgi. This component is designed to allow administrators to upload and install custom Common Gateway Interface modules, thereby extending the router's functionality. However, due to insufficient authentication checks or flawed authorization logic within this specific function, the system fails to verify whether the requesting entity possesses valid administrative credentials before processing the request. Consequently, any unauthenticated attacker who can reach the device over a network connection can interact with this endpoint without providing valid login information.
From a technical perspective, an adversary exploits this flaw by constructing and sending a specially crafted HTTP POST request directly to the /cgi-bin/cstecgi.cgi URL. The payload typically includes binary data or script files intended for execution as CGI modules on the router's embedded Linux system. Because the server does not enforce strict access controls at this stage of the transaction, it accepts the upload and proceeds with the installation process. This bypasses standard security boundaries that are meant to restrict such high-privilege operations exclusively to authenticated users. The ability to inject arbitrary code into a privileged execution context effectively grants the attacker full control over the underlying operating system running on the router hardware.
The operational impact of this vulnerability is severe, as it leads directly to remote code execution with root-level privileges. Once the custom CGI module is installed and executed, an attacker can run arbitrary commands on the device. This compromises the confidentiality, integrity, and availability of the network infrastructure protected by the router. Attackers may use this access to intercept traffic, modify routing tables, install persistent backdoors for future exploitation, or pivot into internal networks that rely on the compromised device as a gateway. Given that routers often serve as the primary security perimeter for home and small business networks, compromising one unit can expose all connected devices to further attacks, including data theft and lateral movement within the local area network.
This vulnerability aligns with CWE-284, which describes Improper Access Control, specifically where access control is not properly enforced before performing a sensitive operation. In terms of offensive security frameworks, this exploit maps to MITRE ATT&CK technique T1505.003, known as Web Shell: Custom Command and Scripting Interpreter, as the attacker utilizes custom scripts uploaded via CGI interfaces to maintain persistent access and execute commands on the target system. The lack of authentication for a critical administrative function also reflects weaknesses in session management and privilege separation principles outlined in general secure coding standards.
To mitigate this risk, immediate action is required from both users and vendors. Users should check their router's firmware version and apply any available updates provided by TOTOLINK that address this specific access control flaw. If an update is not yet released, isolating the device on a separate network segment with strict firewall rules can limit exposure to untrusted networks until remediation occurs. For administrators managing multiple devices, implementing network-level authentication proxies or restricting management interface access to trusted IP addresses only can provide temporary defense-in-depth measures. Ultimately, vendors must enforce rigorous input validation and mandatory authentication checks for all administrative endpoints, particularly those involving file uploads and system configuration changes, to prevent unauthorized code execution in embedded systems.