Bluetooth is woven into our everyday devices—from smartphones to wireless headphones, it’s everywhere. However, lurking beneath the convenience are security risks. One particularly severe vulnerability discovered in 2023, tracked as CVE-2023-40129, highlights how critical security checks can sometimes go missing, opening doors to remote attacks. In this exclusive long-read, we break down what happened, how it works, and how attackers could exploit it with real code snippets.

What is CVE-2023-40129?

CVE-2023-40129 is a heap buffer overflow vulnerability in the build_read_multi_rsp function, found inside the file gatt_sr.cc. This file is a part of Bluetooth implementations found on many popular Android devices (primarily in the Android Open Source Project). If an attacker is within Bluetooth range (no user interaction required), they can exploit this to achieve code execution on the vulnerable device—often leading to a full compromise with the privileges of the Bluetooth process.

Vulnerability Type: Heap buffer overflow (out-of-bounds write)

- Impact: Remote (adjacent/proximal) code execution

How Heap Buffer Overflows Happen

A buffer overflow occurs when a program writes more data to a block of memory (the "buffer") than it was allocated to hold. If the extra data is written past the buffer's boundary on the heap, it’s called a heap buffer overflow. This kind of memory corruption can be manipulated by hackers to change the flow of execution, inject code, or crash the app.

Let's look at a simplified snippet that resembles what was found in gatt_sr.cc

// Pseudocode similar to what’s in build_read_multi_rsp

uint8_t* buffer = (uint8_t*)malloc(max_response_size);
size_t offset = ;

for (int i = ; i < num_attributes; i++) {
    size_t len = attr_list[i].length;
    // Missing check: does offset + len > max_response_size ?
    memcpy(buffer + offset, attr_list[i].value, len);
    offset += len;
}

It fails to check if offset + len will overflow the allocated buffer.

- If num_attributes or attr_list[i].length is manipulated (through malicious Bluetooth packets), it can write beyond buffer, corrupting heap memory.

Remote attacker: The attacker doesn’t need to trick the user—just be within Bluetooth range.

- Send crafted requests: The attacker constructs a special Bluetooth packet that, when processed, makes the server copy more data than the buffer is supposed to hold.
- Corrupt heap: Overflows can overwrite metadata, function pointers, or control data, leading to arbitrary code execution.

Pair and establish a GATT connection (possible in some cases without PIN on older devices).

3. Send a crafted Read Multiple Characteristic Values request with carefully chosen attribute counts and sizes.

Proof of Concept (Exploit-Style Pseudocode)

While ethical considerations prevent full RCE exploit release, here’s a safe reproduction for educational purposes (do NOT use this maliciously):

# Example of sending an oversized Read Multiple request with Python and bluepy (Linux)

from bluepy.btle import Peripheral

mac_addr = "XX:XX:XX:XX:XX:XX"  # Replace with victim's Bluetooth MAC
p = Peripheral(mac_addr)

# The following is a conceptual example:
# Attempt to trigger the bug by requesting many large attributes at once.

attr_handles = [x0001, x0002, x0003, x0004, x0005] * 100  # Exaggerated count

# No standard API to send raw multi-read requests, so low-level Bluetooth stack tools or fuzzers would do this in practice.

# Example "fuzzer" sends deep custom packets via L2CAP, not shown for brevity.

*Practical Bluetooth exploit frameworks: bleah, btproxy*

References & Further Reading

- CVE-2023-40129 Detail Page - MITRE
- Android Security Bulletin – September 2023
- AOSP bug tracker: A-286241462 (Login Required)
- Official Patch Commit

How Was It Fixed?

Developers patched this by adding proper bounds checking before copying attribute data into the buffer.

Sample Fix

if (offset + len > max_response_size) {
    // Truncate len or abort operation to prevent overflow
    break;
}
memcpy(buffer + offset, attr_list[i].value, len);
offset += len;

Don’t leave Bluetooth on needlessly, especially in public places.

- Vendors: If you maintain an embedded device with custom Bluetooth stacks, review and apply the fix.

Conclusion

CVE-2023-40129 reminds us that even "simple" programming mistakes can have severe consequences—especially in software as complex and widespread as Bluetooth stacks. If you haven’t already, update your device. If you build IoT products or custom Android ROMs, audit your code for similar risks. Bluetooth attacks are invisible, and attackers don’t need physical access or user interaction.

Timeline

Published on: 10/27/2023 21:15:08 UTC
Last modified on: 10/30/2023 17:14:25 UTC