A new vulnerability, CVE-2023-4257, was discovered in the Zephyr real-time operating system (RTOS) project, specifically in the WiFi shell code. This bug allows attackers to cause a buffer overflow by sending too long input commands, potentially letting them control or crash the device. If you're a developer working with IoT or embedded devices using Zephyr, understanding and fixing this issue is crucial.

Let's break down what happened, how it works, and what you should do about it—with examples and reference links.

Where The Problem Is

Zephyr is a popular open-source RTOS used in IoT gadgets, industrial controls, and even wearables. The vulnerable code was found in:

/subsys/net/l2/wifi/wifi_shell.c

This file implements shell commands (the part where you type commands into a device for WiFi control). The problem is that the code doesn't check how long the user input is before copying it into a fixed-size buffer.

The Bug: Unchecked Input Length

Here’s a simplified snippet illustrating what went wrong (the real code is more complex but this shows the idea):

void wifi_shell_command_handler(const char *user_input) {
    char buffer[64];
    strcpy(buffer, user_input); // No length check!
    // ... process input
}

In the actual Zephyr source, you’ll find similar patterns around shell commands for WiFi. If user_input is longer than 64 characters, it will overflow buffer. Attackers love this because it lets them write data outside the intended memory area. That can crash the system or (if they're clever) even run their own code.

How Can It Be Exploited?

An attacker with access to the device's command interface (shell prompt) can simply enter an overly long WiFi shell command, like:

wifi connect AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

If the wifi connect handler directly copies the string into a fixed-size buffer (without checking its length), the stack memory gets corrupted.

A proof-of-concept exploit in Python might look like this (assuming you have shell access)

import serial

ser = serial.Serial('/dev/ttyUSB', 115200)
payload = "wifi connect " + "A" * 200 + "\n"
ser.write(payload.encode())
print(ser.readline().decode())

Devices using the WiFi shell interface without extra input validation

If your IoT device exposes a shell (over UART, USB, Telnet, etc.), check your Zephyr version and the source of your shell handlers.

Zephyr fixed the problem by adding proper bounds checking—here’s what a safe fix looks like

void wifi_shell_command_handler(const char *user_input) {
    char buffer[64];
    strncpy(buffer, user_input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\'; // Always null-terminate
    // ... process input
}

Instead of copying blindly, it now only copies what actually fits.

Patch reference:
- Zephyr GitHub commit (example): https://github.com/zephyrproject-rtos/zephyr/pull/63387

Zephyr Security Advisory:

https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-25c9-xxx-4257

CVE page:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-4257

Zephyr release notes (for fixed versions):

https://docs.zephyrproject.org/latest/releases/index.html

Final Thoughts

CVE-2023-4257 is a classic mistake—forgetting to check input length when copying data! If you use Zephyr (or any embedded OS), it's a reminder: always treat external input as untrusted. Catch these mistakes early, and your devices will be safer.

If you're unsure, check your code for old patterns like strcpy and replace them with strncpy (or better, snprintf). Stay secure, and always keep an eye on security advisories!


*This post is exclusive content curated for educational purposes. For further actions, consult Zephyr’s official security team and keep your toolchains up to date!*

Timeline

Published on: 10/13/2023 22:15:10 UTC
Last modified on: 11/14/2023 03:15:09 UTC