Zombie Routers & Ghost Commands: The Resurrection of D-Link Exploits (CVE-2026-0625)

There is a special place in hell reserved for End-of-Life (EOL) SOHO routers. They sit in closets and dusty shelves, blinking away, routing packets, and serving as the perfect, unmonitored beachhead for attackers. CVE-2026-0625 is the latest reminder that just because a vendor stops supporting a device, the internet doesn't stop finding ways to break it.

This vulnerability targets the dnscfg.cgi endpoint on several legacy D-Link DSL gateway models. Historically, this specific endpoint was the playground for "DNSChanger" malware—nuisance attacks that swapped your DNS settings to redirect Google searches to sketchy ad farms. But as it turns out, the rabbit hole goes deeper.

Researchers discovered that this wasn't just a logic flaw allowing configuration changes; it was a full-blown OS Command Injection. By manipulating the parameters meant for IP addresses, an attacker can trick the router into executing shell commands with root privileges. It's the difference between a vandal spray-painting your garage door and a burglar moving into your guest room.

The vulnerability stems from a classic sin in embedded development: passing unsanitized user input directly to a system shell. The component in question is the Common Gateway Interface (CGI) script responsible for handling DNS configuration updates, typically named dnscfg.cgi or similar depending on the firmware revision.

When a user (or an automated bot) sends a request to update the primary or secondary DNS servers, the router takes those values and needs to apply them to the system configuration. In a well-architected system, this would happen via specific API calls or by writing to a configuration file parser. In the wild west of legacy firmware, however, it's often done by constructing a shell command string and executing it.

The logic flaw is blindingly simple: the application assumes the input will be an IP address (e.g., 8.8.8.8). It performs no validation to ensure the input is actually an IP address. It just takes whatever string you give it, pastes it into a command buffer, and hands it off to the OS. If that string contains shell metacharacters, the OS obediently interprets them.

While we don't have the exact source code for every specific firmware version, the pattern in these devices is remarkably consistent. Based on the behavior and reverse engineering of similar firmware, the vulnerable C code looks conceptually like this:

// Conceptual representation of the vulnerable logic
void handle_dns_update() {
    char command_buffer[512];
    char *primary_dns = get_cgi_param("dnsPrimary");
    char *secondary_dns = get_cgi_param("dnsSecondary");
 
    // VULNERABILITY: No regex check for IP format. 
    // No sanitization of ';', '&', '|', or '$'.
    
    // The developer intends to run: set_dns 8.8.8.8 8.8.4.4
    sprintf(command_buffer, "/bin/set_dns %s %s", primary_dns, secondary_dns);
    
    // EXECUTION: The shell executes the string.
    system(command_buffer);
}

This uses sprintf to concatenate the strings and system() to execute them. The system() function essentially spawns /bin/sh -c [command].

If the primary_dns variable contains 8.8.8.8; reboot, the resulting command string becomes /bin/set_dns 8.8.8.8; reboot. The shell sees the semicolon command separator, executes the DNS tool (which might error out or succeed), and then immediately executes reboot. We have achieved control flow hijacking of the shell itself.

Exploiting this is trivially easy, but there are some nuances regarding the embedded environment. These routers often run stripped-down versions of Linux with BusyBox. They might also have rudimentary web servers that dislike spaces in URL parameters, or the injection point might be inside a quoted string.

To ensure reliability, attackers typically avoid spaces. In a standard Bash shell, spaces are arguments. In an injection payload, a space might break the HTTP request or be URL-encoded to %20, which the shell might not interpret as a literal space depending on how the decoding happens before the system() call.

The universal workaround for this in the embedded world is the ${IFS} (Internal Field Separator) environment variable. By default, ${IFS} contains space, tab, and newline. So, cat${IFS}/etc/passwd is executed by the shell exactly like cat /etc/passwd.

The Attack Chain:

1. Target: http://<router-ip>/dnscfg.cgi
2. Method: GET (or POST)
3. Payload: Inject into dnsPrimary.

Proof of Concept:

GET /dnscfg.cgi?dnsPrimary=8.8.8.8;telnetd${IFS}-p${IFS}1337&dnsSecondary=8.8.4.4&dnsDynamic=0&dnsRefresh=1 HTTP/1.1
Host: 192.168.0.1
Connection: keep-alive

This request tells the router to set the DNS, and then immediately start a Telnet daemon on port 1337. The attacker can then connect to the router on port 1337 and enjoy a root shell. No authentication tokens, no cookies, no complex memory corruption—just a semicolon and some bad coding practices.

You might ask, "Who cares about a 10-year-old router?" Botnet operators. That's who.

When a device like this is compromised, it isn't just about the owner's data. These devices are recruited into massive botnets (like Mirai variants or Mozi). They are used to:

• Launch DDoS attacks: Amplifying traffic against high-value targets.
• Proxy Malicious Traffic: Hiding the origin of attacks against other networks.
• Man-in-the-Middle (MitM): Since the vulnerability is in the DNS configuration, the attacker can silently switch the DNS server to a malicious one. This allows them to intercept banking traffic, steal credentials for other services, or inject ads into every device connected to that WiFi network.

The "Critical" 9.3 CVSS score is justified because the attack complexity is Low, privileges required are None, and the impact on Confidentiality, Integrity, and Availability is High.

Here is the hard truth: There is no patch for most of these devices.

D-Link has officially listed the DSL-2640B, DSL-2780B, and DSL-526B as End of Life (EOL) and End of Service (EOS). They will not be fixing this. If you are using these devices, you are driving a car with no brakes on a highway full of semi-trucks.

Remediation Steps:

1. The only real fix: Unplug the device. Smash it with a hammer. Buy a modern router that receives automatic security updates.
2. If you absolutely cannot replace it: You must ensure the management interface is not exposed to the WAN. Go into the administration settings and disable "Remote Management". However, this only protects you from internet-based attacks; a malware-infected laptop on your LAN can still exploit the router from the inside.

For the DSL-2740R, a firmware update (v1.17) reportedly exists, but finding a trusted download source for hardware this old is an adventure in itself.