Home routers are the first line of defense for many families and small businesses. But sometimes, even these digital gatekeepers have cracks. In early 2022, security researchers found a critical flaw in the ASUS RT-AC56U router—specifically, a heap-based buffer overflow vulnerability, tracked as CVE-2022-25596. What makes this flaw dangerous is its ease of exploitation: an attacker on your local network can run code on your router without needing to log in first.

In this post, we'll explain how CVE-2022-25596 works, show you where the bug lives in the code, share insights from official references, and demonstrate how a determined attacker could exploit it on real hardware.

Access Vector: LAN (any device on the local network)

- Impact: Unauthenticated attackers can execute arbitrary code, change settings, or brick the router.

Understanding the Bug

The problem starts in the configuration handler—a service built into the ASUS RT-AC56U firmware. When you want to configure the router (change internet settings, reset passwords, etc.), your browser submits encrypted parameters to the device.

The router then tries to decrypt those parameters. But here's the catch: it doesn't properly check if the data is too long for the assigned buffer. If a user-supplied value is longer than the buffer, data will overflow into the next area of memory—the 'heap'. This bug is classic in C/C++ programming and often leads to code execution.

Here's a simplified version of the vulnerable code, based on public analysis

void handle_config_request(char *req_data) {
    char decrypted_param[512];
    int decrypted_len;

    // Decrypt user data WITHOUT proper length check
    decrypted_len = decrypt_param(req_data, decrypted_param);

    // Assume decrypted_len is always < 512, but it could actually be larger!
    // ... process decrypted_param ...
}

In the real firmware, decrypt_param doesn't check if the decrypted data will fit in the buffer decrypted_param, which is fixed at 512 bytes. If you send encrypted garbage that decrypts to more than 512 bytes, it overflows memory past decrypted_param—letting the attacker hijack the router's control flow.

Exploit Walkthrough

Is exploitation easy?
On the LAN—yes. All an attacker needs is to be connected to your Wi-Fi or LAN.

Prepare a malicious request:

The attacker crafts a POST request to the router's configuration endpoint. The 'decryption required' parameter is filled with data that, after decryption, is larger than 512 bytes.

2. Overwrite heap metadata/code pointers:
The overflowed data is carefully arranged to overwrite critical structures, like pointers used by the heap manager.

Trigger malicious code execution:

Once the router processes the request, its control flow can be redirected to attacker-controlled data, running whatever code the attacker wants.

Gain control:

Now, the attacker has a foothold on your router. They can run commands, redirect traffic, or brick the device.

Sample Exploit Skeleton (Python)

import requests

router_ip = "192.168.1.1"
url = f"http://{router_ip}/apply.cgi";

# Craft an overlong encrypted string
encrypted_param = "A"*1024  # More than 512 bytes when decrypted

payload = {
    "action_mode": "apply",
    "flag": "1",
    "decryption_required": encrypted_param  # this triggers the heap overflow
}

response = requests.post(url, data=payload)
print(f"Exploit sent, status: {response.status_code}")

Note: For a real attack, encrypted_param would need to be carefully crafted binary data, not just 'A's. But the core idea is the same.

Official References and Further Reading

- NVD entry: CVE-2022-25596
- ASUS Security Advisory: ASUS Security Center
- VulDB: https://vuldb.com/?id.221288
- Firmware Exploit Report: https://github.com/peanuts62/CVE-2022-25596 (unofficial PoC)

Disable configuration from Wi-Fi if possible

If your router is EOL (no updates):
Consider replacing it. This vulnerability illustrates why old routers can be a real risk.

Conclusion

CVE-2022-25596 is a rare example of an unauthenticated, local network code execution bug in a popular home router. It’s a reminder: if you don’t keep your router updated, someone on your Wi-Fi could take it over—no password required. Check your device, update now, and keep your home network safe!


_Readers are welcome to ask for more details (or a breakdown of other router bugs) in the comments below!_

Timeline

Published on: 04/07/2022 19:15:00 UTC
Last modified on: 04/14/2022 20:27:00 UTC