Analysis Summary

A newly disclosed security flaw, CVE-2025-46176, exposes thousands of D-Link DIR-605L (v2.13B01) and DIR-816L (v2.06B01) routers to unauthenticated remote code execution due to hardcoded Telnet credentials embedded in the firmware.

Security researchers discovered this vulnerability by analyzing the router firmware using tools like binwalk, which revealed SquashFS file systems containing insecure Telnet initialization scripts. These scripts launch Telnet services using the command /usr/sbin/telnetd -l /bin/sh -u Alphanetworks:$image_sign, where $image_sign pulls plaintext credentials (e.g., Wj5eH%JC) from ./etc/alpha_config/image_sign. Since these credentials are embedded and unchangeable by end users, the flaw introduces a serious security loophole.

The vulnerability, categorized under CWE-77 (Improper Neutralization of Special Elements used in a Command), allows attackers with simple network access to port 23/TCP to bypass authentication and gain administrative shell access via Telnet. An attacker can initiate a connection using telnet 192.168.0[.]1, then input the hardcoded username Alphanetworks and the corresponding password, leading to full system compromise. This enables malicious actors to modify router configurations, deploy malware, or pivot into internal networks, posing a serious threat to both home and small business users.

Despite the vulnerability receiving a CVSS score of (medium severity), the Exploit Prediction Scoring System (EPSS) indicates a relatively low likelihood (0.04%) of active exploitation. Nonetheless, the persistence of the hardcoded credentials poses a long-term risk, particularly because both affected models reached End-of-Life (EOL) on November 17, 2023, and therefore will not receive official security patches. D-Link has acknowledged the flaw but advises users to disable Telnet services, restrict WAN access, and monitor for updates, though none are expected due to the EOL status.

To mitigate the threat, users are urged to block port 23 using firewall rules, disable Telnet via administrative settings if possible, and replace these routers with supported, actively maintained models. The incident underscores the broader issue of legacy IoT device vulnerabilities, particularly the risks posed by embedded, hardcoded credentials that cannot be modified or removed. Security professionals can detect vulnerable devices by scanning for open Telnet ports using tools like nmap -p 23 <target_IP>. Given the complete lack of patchability and ongoing exposure, retirement of these devices is the only reliable long-term mitigation.

Impact

• Code Execution
• Security Bypass
• Gain Access

Indicators of Compromise

CVE

• CVE-2025-46176

Affected Vendors

Remediation

• Refer to the D-Link Website for patch, upgrade, or suggested workaround information.
• Access the router’s administrative interface and disable the Telnet service if the option is available.
• Use firewall rules to block inbound and outbound traffic on port 23, preventing unauthorized Telnet access.
• Ensure that remote (WAN-side) access to router management services is disabled or strictly limited to trusted IP addresses.
• Retire and replace DIR-605L and DIR-816L routers with supported, up-to-date models that receive ongoing security updates.
• Check D-Link's official support page periodically in case unofficial updates or advisories are released (though not expected for EOL models).
• Use tools like nmap -p 23 <target_IP> to detect active Telnet services and monitor for unauthorized access or suspicious activity.
• If replacement is not immediately possible, segment vulnerable routers from sensitive internal networks using VLANs or DMZ configurations.