Heap Buffer Overflow in BLOB.cpp: How Unchecked memcpy Calls Create Critical Vulnerabilities

Severity: Critical | CWE: CWE-120 (Buffer Copy without Checking Size of Input) | Fixed in: PR – "fix: multiple memcpy calls in blob in BLOB.cpp"

Introduction

Memory safety bugs are among the oldest and most dangerous vulnerability classes in software development. Despite decades of awareness, they continue to appear in production codebases — and when they do, the consequences can be severe. A recently patched vulnerability in BLOB.cpp is a textbook example: multiple memcpy calls that blindly trust attacker-influenced size parameters, opening the door to heap buffer overflows.

If you write C or C++, work with native extensions, or expose low-level memory operations through higher-level APIs (like Python bindings), this post is for you. Understanding how these bugs arise — and how to eliminate them — is a fundamental skill for any developer working close to the metal.

The Vulnerability Explained

What Is a Buffer Overflow?

A buffer overflow occurs when a program writes more data into a memory buffer than the buffer was allocated to hold. The excess data spills into adjacent memory regions, potentially overwriting other data, control structures, or executable code.

In the heap variant (as opposed to a stack overflow), the corrupted memory lives in the dynamically allocated heap region. Heap overflows can be notoriously difficult to detect at runtime, since the crash (if it happens at all) may occur far from the original write operation.

The Vulnerable Code

The vulnerability was identified in three locations across the codebase:

1. source/src/BLOB.cpp (line 43)

// VULNERABLE: No bounds check before copy
void BLOB::setData(uint8_t* buffer, size_t len) {
    memcpy(data, buffer, len);  // len is never validated against allocated size of 'data'
}

Here, memcpy(data, buffer, len) copies len bytes from buffer into data. The problem? There is no check confirming that len is less than or equal to the size of the data buffer. If len exceeds the allocated capacity, memcpy happily writes past the end of the buffer.

2. pcubature.cpp (line 94) and hcubature.cpp (lines 357–401)

Similar patterns appear in the numerical integration routines:

// VULNERABLE: dim is not validated against buffer capacity
memcpy(pts, src, dim * sizeof(double));
memcpy(buf, src, dim * sizeof(double));

These operations copy dim-sized arrays of double values into pts and buf buffers. If dim — which can be influenced through the Python API — is larger than the number of elements the destination buffer was allocated to hold, the result is a heap overflow.

How Could This Be Exploited?

The critical detail here is attacker influence. If any of the size parameters (len, dim, number of points) can be controlled or manipulated by an external party — through a file, a network request, a Python API call, or any other input channel — then the attacker can:

1. Craft an oversized input that causes memcpy to write beyond the buffer boundary.
2. Corrupt adjacent heap metadata or other heap-allocated objects.
3. Achieve arbitrary code execution by overwriting function pointers, vtable entries, or other control-flow-sensitive data on the heap.
4. At minimum, crash the application (Denial of Service).

Real-World Attack Scenario

Imagine this component is exposed through a Python extension module:

import mylib

# Attacker-controlled input: a maliciously crafted blob
malicious_data = b"A" * 999999  # Far larger than expected
mylib.process_blob(malicious_data)

Under the hood, process_blob eventually calls BLOB::setData with len = 999999. If data was only allocated for, say, 1024 bytes, memcpy will write 998,975 bytes of 'A' characters beyond the buffer — corrupting whatever happens to live next on the heap. In a worst-case scenario, a sophisticated attacker can craft this overflow to redirect execution to shellcode or a ROP chain.

The same logic applies to the cubature routines: a Python caller passing an unexpectedly large dim value can trigger the overflow in the numerical integration path.

The Fix

The remediation follows a straightforward but critical principle: always validate size parameters before performing memory copies.

After the Fix

BLOB.cpp (line 43 — fixed)

// FIXED: Validate len before copying
void BLOB::setData(uint8_t* buffer, size_t len) {
    if (len > allocated_size) {
        // Handle error: reject oversized input
        throw std::runtime_error("BLOB::setData: input length exceeds buffer capacity");
    }
    memcpy(data, buffer, len);
}

The fix introduces an explicit bounds check. If len exceeds the allocated size of data, the operation is rejected before memcpy is ever called. The application can then handle the error gracefully rather than silently corrupting memory.

Cubature routines — fixed pattern

// FIXED: Validate dim against buffer capacity before copying
if (dim > max_dim) {
    // Error handling: reject invalid dimension
    return FAILURE;
}
memcpy(pts, src, dim * sizeof(double));

The same pattern applies: check that dim (and the derived byte count) does not exceed the capacity of the destination buffer before performing the copy.

Why This Fix Works

The root cause of CWE-120 is implicit trust in size parameters. The fix eliminates that trust by enforcing an invariant: no copy operation will ever write more bytes than the destination buffer can hold. This is a simple, low-overhead check that completely closes the overflow vector.

Conclusion

The heap buffer overflow in BLOB.cpp is a stark reminder that memory safety requires active vigilance. A single missing bounds check — a few lines of code — can turn a routine data processing function into a critical vulnerability that enables heap corruption and potentially arbitrary code execution.

The fix is conceptually simple: check before you copy. But the discipline to apply that check consistently, especially in performance-sensitive code where developers are sometimes tempted to skip "unnecessary" validation, is what separates secure software from vulnerable software.

Key takeaways:

• ✅ Always validate size parameters before memcpy and similar operations
• ✅ Prefer high-level abstractions (std::vector, std::span) over raw pointer arithmetic
• ✅ Treat external input as untrusted — especially sizes and lengths from language bindings
• ✅ Enable sanitizers during development and testing to catch overflows early
• ✅ Fuzz your parsers and data processors — they are the most common entry points for these bugs
• ✅ Integrate static analysis into CI to catch CWE-120 patterns automatically

Memory safety is not just a C++ problem — it's a software engineering discipline. Whether you're writing Rust, using unsafe blocks carefully, or maintaining legacy C++ code, the principles here apply. Write defensively, validate eagerly, and never trust a size you didn't compute yourself.

This vulnerability was identified and patched by OrbisAI Security. If you're interested in automated security scanning for your codebase, check out their tooling for continuous vulnerability detection.