CVE-2025-6916 is a critical vulnerability identified in the TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015. The flaw exists in the Form_Login function within the /formLoginAuth.htm endpoint. Specifically, the vulnerability arises from improper handling of the authCode and goURL parameters, which leads to missing authentication controls. This means an attacker can manipulate these parameters to bypass the authentication mechanism entirely. The vulnerability requires the attacker to be on the local network (AV:A - Attack Vector: Adjacent), but no privileges or user interaction are required to exploit it. The CVSS 4.0 score of 8.7 (high severity) reflects the ease of exploitation combined with the significant impact on confidentiality, integrity, and availability. Exploiting this vulnerability could allow an attacker to gain unauthorized access to the router's administrative interface, potentially enabling them to change configurations, intercept or redirect network traffic, or deploy further attacks within the network. Although no public exploits are currently known in the wild, the exploit details have been disclosed, increasing the risk of active exploitation. The vulnerability does not require authentication or user interaction, making it particularly dangerous in environments where the local network is accessible to untrusted users or devices. Since the flaw is in a widely used consumer-grade router, the scope of affected systems could be broad, especially in home and small office networks using this specific TOTOLINK T6 firmware version.

For European organizations, this vulnerability poses a significant risk, especially for small and medium enterprises (SMEs) and home office setups that rely on TOTOLINK T6 routers. Unauthorized access to the router could lead to interception of sensitive communications, manipulation of DNS settings to redirect users to malicious sites, or complete network compromise. This could result in data breaches, loss of confidentiality, and disruption of business operations. Given the local network attack vector, organizations with less segmented or poorly secured internal networks are particularly vulnerable. The impact is heightened in environments where remote work is prevalent, and employees connect personal or less-secure devices to corporate networks via such routers. Additionally, compromised routers could be used as footholds for lateral movement or as part of botnets, amplifying the threat landscape. The lack of a patch at the time of disclosure further increases exposure, necessitating immediate mitigation efforts.

1. Network Segmentation: Isolate TOTOLINK T6 routers on separate VLANs or network segments to limit access to trusted devices only. 2. Access Control: Restrict local network access to the router’s management interface by MAC address filtering or IP whitelisting where possible. 3. Disable Remote Management: Ensure remote management features are disabled to prevent external exploitation. 4. Firmware Updates: Monitor TOTOLINK’s official channels for firmware updates addressing this vulnerability and apply patches promptly once available. 5. Network Monitoring: Implement network intrusion detection systems (NIDS) to detect unusual access patterns or unauthorized login attempts on the router. 6. Replace Vulnerable Devices: For critical environments, consider replacing TOTOLINK T6 routers with devices from vendors with a stronger security track record until a patch is released. 7. User Awareness: Educate users about the risks of connecting untrusted devices to the local network and encourage strong network access policies. 8. Incident Response Preparation: Prepare to isolate and remediate affected devices quickly if exploitation is detected.