CVE-2025-29521 is a security vulnerability affecting the D-Link DSL-7740C router running firmware version DSL7740C.V6.TR069.20211230. The issue arises from insecure default credentials configured for the Administrator account. This misconfiguration allows an attacker to perform a brute-force attack remotely over the network without requiring any prior authentication or user interaction. The attacker can systematically attempt multiple password guesses to gain administrative access to the device. Once administrative privileges are obtained, the attacker can potentially modify device configurations, intercept or redirect network traffic, or use the compromised device as a foothold for further attacks within the network. The vulnerability is classified under CWE-1392, which relates to insecure default credentials. The CVSS v3.1 base score is 5.3 (medium severity), reflecting that the attack vector is network-based (AV:N), with low attack complexity (AC:L), no privileges required (PR:N), and no user interaction needed (UI:N). The impact on confidentiality is limited (C:L), with no impact on integrity or availability. There are no known public exploits in the wild at the time of publication, and no patches have been officially released. The affected firmware version is specifically identified, but no broader version range is provided.

For European organizations, this vulnerability poses a moderate risk primarily to those using the D-Link DSL-7740C router with the specified firmware version. Successful exploitation could lead to unauthorized administrative access, enabling attackers to manipulate network settings, potentially intercept sensitive communications, or pivot to internal systems. This could compromise confidentiality of data traversing the network and undermine trust in network infrastructure. While the vulnerability does not directly impact integrity or availability, the administrative control gained could be leveraged for further malicious activities, including deploying malware or establishing persistent access. Organizations in sectors with high data sensitivity or regulatory requirements (e.g., finance, healthcare, government) could face compliance risks and reputational damage if exploited. The lack of required authentication and user interaction increases the likelihood of automated brute-force attacks, especially if devices are exposed to the internet or poorly segmented internal networks.

1. Immediate verification and change of default administrator credentials on all affected D-Link DSL-7740C devices to strong, unique passwords. 2. Restrict remote administrative access to trusted IP addresses or disable remote management entirely if not needed. 3. Implement network segmentation to isolate management interfaces from general user networks and the internet. 4. Monitor device logs for repeated failed login attempts indicative of brute-force attacks. 5. Where possible, upgrade firmware to a version that addresses this vulnerability once available from D-Link. 6. Employ network-level protections such as intrusion detection/prevention systems (IDS/IPS) to detect and block brute-force attempts targeting router management interfaces. 7. Conduct regular audits of network devices to ensure no default credentials remain in use and that security best practices are enforced. 8. Educate network administrators about the risks of default credentials and the importance of secure configuration management.