CVE-2025-5901 is a critical buffer overflow vulnerability identified in the TOTOLINK T10 router, specifically affecting version 4.1.8cu.5207. The flaw exists in the UploadCustomModule function within the /cgi-bin/cstecgi.cgi POST request handler. An attacker can exploit this vulnerability by manipulating the 'File' argument in the POST request, causing a buffer overflow condition. This type of vulnerability can lead to arbitrary code execution, potentially allowing an attacker to gain control over the affected device remotely without requiring user interaction or prior authentication. The vulnerability is remotely exploitable over the network, which significantly increases the attack surface. The CVSS v4.0 score of 8.7 (high severity) reflects the ease of exploitation (network attack vector, low attack complexity, no privileges or user interaction required) and the high impact on confidentiality, integrity, and availability. Although no public exploits are currently known to be in the wild, the disclosure of the exploit code increases the risk of active exploitation. The TOTOLINK T10 router is commonly used in small to medium-sized enterprise and residential environments, making this vulnerability a significant concern for network security. The lack of available patches at the time of disclosure further exacerbates the risk.

For European organizations, this vulnerability poses a substantial risk to network infrastructure security. Exploitation could lead to unauthorized access to internal networks, interception or manipulation of sensitive data, and disruption of network services. Given that routers like the TOTOLINK T10 often serve as the primary gateway to the internet, a compromised device could be used as a foothold for lateral movement within corporate networks or as a launchpad for further attacks such as data exfiltration or ransomware deployment. The impact is particularly critical for organizations handling sensitive personal data under GDPR regulations, as a breach could result in significant legal and financial consequences. Additionally, the disruption of network availability could affect business continuity, especially for SMEs relying on these devices for daily operations. The remote and unauthenticated nature of the exploit increases the urgency for mitigation in environments where these routers are deployed.

Immediate mitigation steps include isolating affected TOTOLINK T10 devices from critical network segments and restricting remote access to the router's management interface. Network administrators should implement strict firewall rules to block unsolicited inbound traffic targeting the /cgi-bin/cstecgi.cgi endpoint. Monitoring network traffic for unusual POST requests to this CGI path can help detect attempted exploitation. Since no official patches are currently available, organizations should consider temporary replacement of vulnerable devices with models from vendors that provide timely security updates. Additionally, deploying network intrusion detection/prevention systems (IDS/IPS) with signatures for this vulnerability can help prevent exploitation attempts. Organizations should also ensure that router firmware is regularly updated once a patch is released and verify the authenticity of firmware updates to avoid supply chain attacks. Finally, conducting internal audits to inventory all TOTOLINK T10 devices and verifying their firmware versions is essential to assess exposure.