The Tenda AC21 is a popular dual-band gigabit wireless router used around the world. But in late 2022, security researchers uncovered a major flaw (CVE-2022-44158) in firmware version V16.03.08.15, and it’s the kind of vulnerability hackers dream about: a buffer overflow in a function named set_device_name. In this post, I’ll break down how this bug works, show you sample code, and explain why it’s dangerous—using plain, simple language.

What is a Buffer Overflow?

Let’s make it simple. A buffer overflow is when a program writes more data to a part of memory (a buffer) than it is supposed to handle. This messes up memory next to the buffer—and if a hacker knows how to do it just right, they can run their own code on your router. That can mean anything: spies on your home network, redirecting your traffic, or launching other attacks.

Where’s the Vulnerability?

The problem happened inside a function called set_device_name. Here’s what Tenda uses this for: you can set a friendly name for devices connected to your router, like "Julia-Laptop" instead of "192.168..105". But this function didn’t check well enough how long your device name is.

Here’s a simplified version of what might be happening behind the scenes (actual code is from reverse-engineered firmware and for instructional purposes):

// This is NOT the actual code, but it's close to how it probably looks

void set_device_name(char* device_name) {
    char buffer[64]; // 64 bytes allocated for your new device name
    strcpy(buffer, device_name); // Uh oh! No length check.
    // ...
}


The crucial mistake? The function directly copies whatever string you give it into a buffer, without checking if it fits. If you tell the router your device’s name is a string that's way longer than 64 characters, it will overwrite other important data in memory.

Attack Scenario

An attacker needs to send a specially crafted HTTP POST request to the router’s web interface, targeting the function behind /goform/setDeviceName. The payload in the POST data for deviceName overflows the buffer.

Exploit Example

Here’s a real-world curl command that shows how an attacker might trigger the bug (don’t run this against devices you don’t own!):

curl -X POST \
     http://<router_ip>/goform/setDeviceName \
     -d "deviceName=$(python3 -c 'print("A"*128)')&macAddr=12:34:56:78:9A:BC"

macAddr is required so the router knows which device to rename.

With careful crafting, an attacker could go further. Instead of just overwriting with random data ("A"s), they could overwrite the memory to inject commands or a small malicious program (a “shellcode”).

Denial of Service: Crash the router, knocking everyone offline.

- Remote Code Execution: Gain control over the router. Change settings, redirect traffic, steal data—or use your router as part of a botnet.

Persistent Backdoors: Install malware so the attacker always has access, even after restarts.

The worst part? If your router’s admin interface is exposed to the internet (some users mistakenly allow this), attackers can exploit it remotely.

How To Protect Yourself

1. Upgrade Firmware: Check for a firmware update from Tenda. New versions should have fixed this bug. Tenda Firmware Update
2. Restrict Access: Never expose your router admin page to the internet. Only access it from your local network.

References

- NVD - CVE-2022-44158
- Exploit Database Entry
- Tenda AC21 Official Firmware
- Original Research (Chinese)

Wrapping Up

Buffer overflows are a classic but dangerous mistake, especially when they show up in devices everyone has at home. If you use the Tenda AC21 router (or even other Tenda routers), it’s worth checking for firmware updates or changing your security settings.

Want to see more router bugs explained in plain language? Let me know! And as always—lock down your network. Stay safe.


Disclaimer:  
All information provided here is for educational purposes only. Do not attempt to exploit devices you do not own or have explicit permission to test.


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Timeline

Published on: 11/21/2022 16:15:00 UTC
Last modified on: 11/22/2022 01:08:00 UTC