CVE-2026-51656 in TOTOLINK
Summary
by MITRE • 08/28/2026
Incorrect access control in the getVpnPassCfg function of TOTOLINK T6 4.1.5cu.748_B20211015 allows unauthenticated attackers to obtain VPN pass-through and WAN ping filter settings via sending a crafted POST request to /cgi-bin/cstecgi.cgi.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability identified in TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015 represents a critical failure in access control mechanisms within the device's web management interface. Specifically, the flaw resides in the getVpnPassCfg function, which is designed to retrieve VPN pass-through and WAN ping filter configuration settings. Under normal operational conditions, such administrative functions should require valid authentication credentials to ensure that only authorized users can view or modify sensitive network configurations. However, due to insufficient validation of user privileges before executing this specific CGI script, the system fails to enforce proper authorization checks. This oversight allows any entity with network connectivity to the router's management interface to bypass security controls entirely.
An unauthenticated attacker can exploit this weakness by sending a crafted POST request directly to the /cgi-bin/cstecgi.cgi endpoint without providing valid session tokens or login credentials. The server processes this request and returns detailed configuration data, including VPN pass-through settings which dictate how virtual private network traffic is handled across the firewall boundaries, as well as WAN ping filter rules that control inbound ICMP requests from external networks. These parameters are critical for maintaining the integrity of a local area network against external threats. By exposing these configurations to unauthenticated parties, the vulnerability effectively strips away layers of defensive security posture provided by the router's default settings and customizations.
The operational impact of this information disclosure is significant, as it provides attackers with valuable intelligence about the target environment. Knowledge of VPN pass-through rules allows an attacker to determine if specific tunneling protocols are permitted through the firewall, potentially facilitating further exploitation or lateral movement within a protected network segment. Similarly, understanding WAN ping filter settings reveals whether the router responds to external probes, which aids in reconnaissance activities and helps attackers map out available attack vectors. This exposure aligns with CWE-284, which classifies improper access control vulnerabilities where insufficient restrictions are applied to unauthorized actions. Furthermore, this behavior is consistent with ATT&CK technique T1078, Valid Accounts, as the attacker effectively gains administrative-level visibility without valid credentials by exploiting a logic flaw rather than stealing actual login data.
Mitigation strategies for this vulnerability primarily involve immediate firmware updates provided by TOTOLINK that patch the access control logic within the getVpnPassCfg function. Until an official patch is applied or installed, network administrators should restrict direct internet-facing exposure of the router's management interface through firewall rules or NAT configurations to limit attack surface area. Additionally, enforcing strong authentication policies and disabling remote administration features where not strictly necessary can reduce the risk of exploitation. Security monitoring tools should be configured to detect anomalous POST requests targeting CGI endpoints with unusual parameters indicative of enumeration attempts. Regular audits of access control implementations across all web-facing services are essential to prevent similar logic flaws from persisting in embedded systems that often lack robust security frameworks compared to general-purpose operating environments.