Zephyr RTOS is a popular, lightweight operating system designed for resource-limited devices like sensors and wearables. Its IEEE 802.15.4 implementation is widely used for low-power wireless communications, especially in the context of IoT. In September 2023, a potential buffer overflow vulnerability — CVE-2023-4263 — was found in Zephyr's nRF 802.15.4 radio driver. This post takes an exclusive, simple-language look at what happened, how the bug works, how an attacker might exploit it, and how you can stay safe.

What’s this Bug All About?

A buffer overflow happens when a program writes more data to a block of memory, or buffer, than it was intended to hold. This can corrupt data, crash programs, or (in the worst case) let an attacker run their own malicious code.

With CVE-2023-4263, there was a flawed boundary check in Zephyr’s driver code used for managing data received over the nRF IEEE 802.15.4 radio. If a specially crafted frame arrived, data could overflow a buffer allocated for incoming packets.

The Code at Fault

Here’s a simplified version of the buggy code, as found in the Zephyr repository (source ref):

#define RX_BUFF_SIZE 256

void ieee802154_nrf5_rx()
{
    uint8_t rx_buffer[RX_BUFF_SIZE];
    uint16_t length;

    // Imagine this length comes from a header of the incoming packet
    length = get_frame_length();
    
    // Vulnerable copy! No check if length > RX_BUFF_SIZE
    memcpy(rx_buffer, radio_rx_data, length);
    
    // ...process rx_buffer...
}

The problem: get_frame_length() could return a value bigger than RX_BUFF_SIZE (256). If so, memcpy would overwrite memory after rx_buffer.

How Can This Be Exploited?

If an attacker is in range of the radio, they can send a custom-crafted IEEE 802.15.4 frame. By forging the header to say, for example, “this frame has 500 bytes” (when rx_buffer is only 256 bytes), Zephyr copies 500 bytes — splattering bytes beyond the edge of rx_buffer. This can:

Proof of Concept: Exploiting the Bug

Here’s a conceptual demonstration in Python of how someone could craft a payload to trigger the overflow. (You can't run this on your PC, but gives an idea.)

# Bogus 802.15.4 packet: header saying length is 500, payload is 500 bytes of 'A's
length = 500
packet = bytes([length]) + b'A' * length

# Send this packet over the air with a radio tool like HackRF, 
# or another 802.15.4 device in transmit mode.
send_packet(packet)

When the Zephyr-powered device receives this, it blindly copies all 500 bytes into a 256-byte buffer, leading to overflow.

All Zephyr applications using the nRF 802.15.4 driver before the patch

- Devices running Zephyr in environments where attackers can transmit 802.15.4 packets (typical for IoT!)

The fix is as simple as adding a boundary check before copying

// The new, safe way
if (length <= RX_BUFF_SIZE) {
    memcpy(rx_buffer, radio_rx_data, length);
} else {
    // Handle error: frame too big
}

Or, use memcpy_s, memmove_s, or similar safe buffer copy functions when available.

Where to Read More?

- Zephyr Security Advisory: GHSA-cw3j-9jf8-3gpw
- NVD CVE Entry
- Zephyr IEEE 802.15.4 Stack Docs

Conclusion

CVE-2023-4263 is a classic buffer overflow that could have a big impact on resource-constrained IoT devices using Zephyr's nRF 802.15.4 radio driver. By understanding the details of this issue, checking your code, and updating to patched versions, you can help keep your devices and users safe.


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Timeline

Published on: 10/13/2023 21:15:51 UTC
Last modified on: 11/14/2023 03:15:10 UTC