CVE-2026-51683 in TOTOLINK
Summary
by MITRE • 08/31/2026
Incorrect access control in the setLanCfg function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to alter LAN network configuration 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 setLanCfg function of TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015 represents a critical failure in access control mechanisms, specifically classified under CWE-284 Improper Access Control. This flaw permits unauthenticated remote attackers to modify the local area network configuration of the device without requiring valid credentials or session tokens. The attack vector involves sending a specially crafted HTTP POST request directly to the /cgi-bin/cstecgi.cgi endpoint, which serves as the primary interface for executing administrative commands on the router's embedded web server. By bypassing authentication checks that should be enforced before processing configuration changes, an attacker can gain unauthorized control over fundamental network parameters such as IP address assignments, subnet masks, gateway settings, and DNS servers.
From a technical perspective, this vulnerability stems from insufficient validation of user privileges within the CGI script handling LAN configuration updates. In secure implementations, any request intended to modify system-level configurations must be preceded by an authenticated session or verified administrative token. However, in this specific firmware version, the setLanCfg function processes incoming POST data without verifying whether the requester has been properly authenticated through the router's login mechanism. This oversight allows external entities on the same network segment or potentially over the internet if port forwarding is misconfigured to expose the management interface, to inject malicious configuration payloads that alter how the device routes traffic and assigns addresses within the local network.
The operational impact of this vulnerability is severe, as it effectively compromises the integrity and availability of the entire network infrastructure protected by the router. An attacker who successfully exploits this flaw can redirect DNS queries to malicious servers, facilitating phishing attacks or man-in-the-middle operations against connected devices. Additionally, altering IP configuration settings can cause denial-of-service conditions for legitimate users by assigning invalid gateways or subnet masks that prevent normal internet connectivity. Furthermore, changing network parameters may allow the attacker to position themselves as a central point of interception for all traffic passing through the router, enabling eavesdropping on sensitive communications such as banking details, personal messages, and corporate data transmitted over the compromised network segment.
This type of vulnerability aligns with MITRE ATT&CK technique T1548.002 Abuse Configuration: Default Settings, where attackers exploit default or improperly secured configurations to gain initial access or escalate privileges. It also relates to CWE-798 Use of Hard-coded Credentials if the lack of authentication is tied to hardcoded bypass logic in the firmware codebase. The ability to reconfigure network settings remotely without authorization undermines the foundational trust model of home and small business networking environments, where routers are often assumed to be secure gatekeepers between internal private networks and public internet resources.
Mitigation strategies must prioritize immediate patching by updating the router firmware to a version that corrects this access control flaw. If an update is not immediately available, network administrators should restrict access to the management interface /cgi-bin/cstecgi.cgi using firewall rules or VLAN segmentation to ensure only trusted administrative workstations can reach it. Disabling remote administration features entirely and ensuring that the web interface is not exposed to untrusted networks are essential defensive measures. Additionally, implementing strong, unique passwords for all administrator accounts adds a layer of defense-in-depth, although in this specific case, authentication appears to be completely bypassed rather than merely weak, making patching the only reliable long-term solution. Regular security audits and monitoring of configuration change logs can also help detect unauthorized modifications early.