CVE-2025-5621: D-Link DIR-816 Firmware RCE Vulnerability

CVE-2025-5621 Overview

A critical OS command injection vulnerability has been identified in the D-Link DIR-816 router firmware version 1.10CNB05. This vulnerability exists in the qosClassifier function located in the /goform/qosClassifier endpoint. Attackers can exploit this flaw by manipulating the dip_address or sip_address parameters to inject and execute arbitrary operating system commands on the affected device.

The vulnerability is classified under CWE-77 (Command Injection) and CWE-78 (OS Command Injection), indicating improper neutralization of special elements used in OS commands. This flaw allows remote attackers to gain unauthorized control over the affected router without any authentication requirements.

Critical Impact

This OS command injection vulnerability allows unauthenticated remote attackers to execute arbitrary commands on D-Link DIR-816 routers, potentially leading to complete device compromise, network pivoting, and persistent unauthorized access to the network infrastructure.

Affected Products

• D-Link DIR-816 Firmware version 1.10CNB05
• D-Link DIR-816 Hardware

Discovery Timeline

• June 5, 2025 - CVE-2025-5621 published to NVD
• June 6, 2025 - Last updated in NVD database

Technical Details for CVE-2025-5621

Vulnerability Analysis

This vulnerability affects the Quality of Service (QoS) classifier functionality of the D-Link DIR-816 router. The qosClassifier function, accessible via the /goform/qosClassifier web endpoint, fails to properly sanitize user-supplied input in the dip_address and sip_address parameters before incorporating them into system commands.

When a user submits QoS configuration data through the web interface, the router processes the destination IP address (dip_address) and source IP address (sip_address) parameters. Due to insufficient input validation, an attacker can inject shell metacharacters and arbitrary commands that are subsequently executed with the privileges of the web server process, typically root on embedded devices.

It is important to note that this vulnerability affects an end-of-life product that is no longer supported by D-Link. The exploit has been publicly disclosed, increasing the risk of active exploitation in the wild.

Root Cause

The root cause of this vulnerability is the failure to properly sanitize and validate user-supplied input before using it in shell command execution. The qosClassifier function directly incorporates the dip_address and sip_address parameters into OS commands without filtering special characters or command separators such as semicolons, backticks, or pipe characters. This allows attackers to break out of the intended command context and inject arbitrary commands.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker with network access to the router's web management interface can craft a malicious HTTP request to the /goform/qosClassifier endpoint. By including shell metacharacters in the dip_address or sip_address parameters, the attacker can execute arbitrary commands on the underlying operating system.

The attack can be performed remotely from anywhere on the network where the router's management interface is accessible. If the management interface is exposed to the internet, the attack surface extends to any remote attacker.

Detection Methods for CVE-2025-5621

Indicators of Compromise

• Unusual HTTP POST requests to /goform/qosClassifier containing shell metacharacters (;, |, $(), backticks) in the dip_address or sip_address parameters
• Unexpected processes spawning from the router's web server process
• Anomalous outbound network connections from the router to unknown external hosts
• Modified configuration files or new user accounts on the device
• Evidence of command execution artifacts in router logs

Detection Strategies

• Implement network intrusion detection rules to monitor HTTP traffic to D-Link DIR-816 routers for command injection patterns in QoS-related requests
• Deploy web application firewall (WAF) rules to block requests containing shell metacharacters in IP address fields
• Monitor router syslog output for unexpected command execution or error messages indicating shell activity
• Conduct periodic firmware integrity checks to detect unauthorized modifications

Monitoring Recommendations

• Enable verbose logging on the router if supported and forward logs to a central SIEM for analysis
• Monitor for DNS queries or outbound connections to known malicious infrastructure from the router's IP address
• Establish baseline network behavior for the router and alert on deviations such as unusual port scanning or lateral movement attempts
• Regularly audit network traffic for signs of exploitation attempts against router management interfaces

How to Mitigate CVE-2025-5621

Immediate Actions Required

• Immediately disable remote management access to the D-Link DIR-816 if it is exposed to untrusted networks
• Restrict access to the router's web management interface to trusted internal hosts only using firewall rules
• Consider replacing the affected D-Link DIR-816 with a currently supported router model, as this device is end-of-life and will not receive security patches
• Implement network segmentation to limit the potential impact of a compromised router

Patch Information

This vulnerability affects the D-Link DIR-816 firmware version 1.10CNB05, which is an end-of-life product no longer supported by the vendor. D-Link has not released and is not expected to release a security patch for this vulnerability. Users are strongly advised to replace the affected device with a currently supported alternative.

For additional information, refer to the D-Link Official Website and the GitHub Vulnerability Documentation.

Workarounds

• Disable the web management interface entirely and manage the router only through local serial console if available
• Place the router behind an additional firewall that blocks access to the management interface from all untrusted sources
• Implement access control lists (ACLs) on upstream network devices to restrict management traffic to the router
• If possible, configure the router to bind the management interface only to a dedicated management VLAN

# Example iptables rule to restrict access to router management interface
# Run on a firewall protecting the router
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 80 -s <TRUSTED_ADMIN_IP> -j ACCEPT
iptables -A FORWARD -d <ROUTER_IP> -p tcp --dport 80 -j DROP