Heap Buffer Overflow in BLE MIDI: How a Missing Bounds Check Opens the Door to Remote Exploitation

Introduction

Bluetooth Low Energy (BLE) has become the backbone of modern wireless MIDI devices — from smart keyboards to stage controllers. But with wireless connectivity comes an expanded attack surface. Any code that parses data arriving over the air must be treated with the same suspicion as data arriving from the internet.

This post examines a critical heap buffer overflow found in minimidi/source/components/blemidi/blemidi.c — a vulnerability that allowed a remote attacker within Bluetooth range to corrupt heap memory simply by sending a malformed BLE MIDI packet. We'll walk through how the vulnerability works, how it was fixed, and what you can do to write safer embedded C code.

The Vulnerability Explained

What Is a Heap Buffer Overflow?

A heap buffer overflow occurs when a program writes data beyond the end of a dynamically allocated memory region (i.e., memory allocated with malloc, calloc, etc.). Unlike stack overflows, heap overflows are subtler — they don't immediately crash the program in obvious ways. Instead, they silently corrupt adjacent heap metadata or other live allocations, which can lead to:

• Arbitrary code execution (by overwriting function pointers or vtables)
• Denial of service (by corrupting heap management structures)
• Information disclosure (by leaking adjacent memory contents)

The Vulnerable Code

The vulnerability lived in the blemidi_outbuffer_push function, which assembles outgoing BLE MIDI packets. Here's the core of the problem:

// VULNERABLE CODE (before fix)
size_t packet_len = max_header_size + len;  // ⚠️ No overflow check!
uint8_t *packet = malloc(packet_len);
if( packet == NULL ) {
    return -1;
}
// Later: memcpy into packet with offset (+1 or +2)
// If len is attacker-controlled, packet_len can wrap around to a tiny value

The len parameter is derived directly from the incoming BLE MIDI stream — data sent by a remote Bluetooth client. There are two interrelated problems here:

Problem 1: Integer Overflow in Size Calculation

When len is very large (e.g., close to SIZE_MAX), the addition max_header_size + len wraps around to a small value due to integer overflow. For example:

max_header_size = 2
len             = SIZE_MAX  (e.g., 0xFFFFFFFFFFFFFFFF on a 64-bit system)

packet_len = 2 + SIZE_MAX = 1  (integer overflow!)

malloc(1) succeeds and returns a 1-byte buffer. The subsequent memcpy then writes len bytes of attacker data into that 1-byte buffer — a massive heap overflow.

Problem 2: No Bounds Validation on Attacker-Supplied Length

Even without integer overflow, there was no check that len actually fits within the allocated buffer after accounting for the header offset. The memcpy with a fixed offset (+1 or +2) could write past the end of the allocation.

A Secondary Issue: Redundant sizeof(uint8_t)

A secondary (non-security-critical but worth noting) issue was also present in the prepare buffer allocation:

// BEFORE: redundant sizeof(uint8_t) — always 1, but misleading
prepare_write_env->prepare_buf = (uint8_t *)malloc(PREPARE_BUF_MAX_SIZE * sizeof(uint8_t));

// AFTER: clean and correct
prepare_write_env->prepare_buf = (uint8_t *)malloc(PREPARE_BUF_MAX_SIZE);

While sizeof(uint8_t) is always 1 and this doesn't introduce a bug, it's a code quality issue that could mislead future maintainers into thinking a different element size might be appropriate.

Attack Scenario

Here's how an attacker could exploit this vulnerability in the real world:

1. Attacker pairs or connects to a BLE MIDI device (many devices allow unauthenticated connections).
2. Attacker crafts a malformed BLE MIDI packet with a len field set to SIZE_MAX - 1 or another large value.
3. The vulnerable code computes packet_len = max_header_size + len, which wraps to a tiny value.
4. malloc allocates a tiny buffer (e.g., 1 byte).
5. The subsequent memcpy writes gigabytes of attacker-controlled data into heap memory, corrupting adjacent allocations.
6. Depending on the heap layout, this can lead to remote code execution or a device crash.

No physical access required. An attacker within Bluetooth range (typically 10–100 meters) can trigger this.

The Fix

What Changed

The fix adds a single but crucial integer overflow guard before the size calculation:

// FIXED CODE
if( len > SIZE_MAX - max_header_size )
    return -1; // integer overflow guard
size_t packet_len = max_header_size + len;
uint8_t *packet = malloc(packet_len);

Let's break down why this works:

The Guard Condition Explained

if( len > SIZE_MAX - max_header_size )

This checks whether adding max_header_size to len would exceed SIZE_MAX. By rearranging the arithmetic to avoid the overflow itself (comparing len against SIZE_MAX - max_header_size rather than checking len + max_header_size > SIZE_MAX), the check is safe from the very overflow it's trying to prevent.

If len is too large, the function returns -1 immediately — the packet is rejected before any memory allocation occurs.

Before and After

// BEFORE — vulnerable to integer overflow → heap overflow
size_t packet_len = max_header_size + len;
uint8_t *packet = malloc(packet_len);
if( packet == NULL ) {
    return -1;
}

// AFTER — safe: overflow guard prevents malformed allocation
if( len > SIZE_MAX - max_header_size )
    return -1; // integer overflow guard
size_t packet_len = max_header_size + len;
uint8_t *packet = malloc(packet_len);
if( packet == NULL ) {
    return -1;
}

Why This Fix Is Correct

The fix follows the "validate before use" principle. By checking the arithmetic before it happens, we ensure:

1. packet_len always accurately represents the intended allocation size.
2. malloc receives a meaningful size value.
3. Subsequent memcpy operations stay within the allocated region.

This is a minimal, targeted fix — it adds two lines and changes nothing about the happy path for valid packets.

Prevention & Best Practices

1. Always Validate Attacker-Controlled Lengths Before Arithmetic

Any length, size, or count value that originates from an external source (network, Bluetooth, file, user input) must be validated before it participates in arithmetic used to size a buffer.

// Pattern: safe size addition
if (b > SIZE_MAX - a) {
    // handle overflow
    return -1;
}
size_t total = a + b;

// Pattern: safe size multiplication
if (count > SIZE_MAX / element_size) {
    // handle overflow
    return -1;
}
size_t total = count * element_size;

2. Use Safe Integer Libraries

For complex arithmetic, consider using safe integer libraries:

• C: safe-iop, or compiler builtins like __builtin_add_overflow (GCC/Clang)
• C++: std::numeric_limits checks, or the SafeInt library
• Rust: Rust's default integer arithmetic panics on overflow in debug mode and uses wrapping semantics in release mode — consider using checked_add, saturating_add, etc.

// Using GCC/Clang builtin — clean and expressive
size_t packet_len;
if (__builtin_add_overflow(max_header_size, len, &packet_len)) {
    return -1; // overflow detected
}
uint8_t *packet = malloc(packet_len);

3. Apply the Principle of Least Privilege to BLE Connections

If your BLE MIDI device doesn't need to accept connections from arbitrary devices, enforce pairing and authentication at the BLE layer. Reducing who can send you data reduces your attack surface.

4. Enable Compiler and Runtime Protections

For embedded targets, enable as many of these as your toolchain supports:

5. Fuzz Your Parsers

BLE packet parsers are excellent candidates for fuzzing. Tools like AFL++ or libFuzzer can automatically generate malformed inputs that trigger edge cases like this one — often finding bugs that code review misses.

6. Relevant Security Standards and References

• CWE-122: Heap-based Buffer Overflow
• CWE-190: Integer Overflow or Wraparound
• CWE-20: Improper Input Validation
• OWASP: Buffer Overflow
• SEI CERT C Coding Standard: INT30-C — Ensure unsigned integer operations do not wrap

Conclusion

This vulnerability is a textbook example of why all externally supplied data must be treated as hostile. A single missing bounds check — two lines of code — was enough to allow a remote attacker within Bluetooth range to corrupt heap memory on an embedded device.

The fix is elegant in its simplicity: check that the arithmetic is safe before performing it. No complex refactoring, no new dependencies, no API changes — just a guard that ensures the code only proceeds when the inputs make sense.

Key Takeaways

• Integer overflow can turn a large len into a tiny allocation — always guard size arithmetic involving external values.
• BLE devices are network-connected devices — their parsers deserve the same scrutiny as web application input handlers.
• Two lines of code can make the difference between a secure device and a remotely exploitable one.
• Fuzz your parsers — automated tools are highly effective at finding these issues before attackers do.

Security in embedded and IoT systems is often treated as an afterthought, but devices like BLE MIDI controllers are increasingly deployed in professional environments where reliability and security matter. Treating every byte that arrives over the air as potentially malicious is the mindset that keeps users safe.

This vulnerability was identified and fixed by OrbisAI Security. Automated security scanning and remediation helps teams catch critical issues before they reach production.