Heap Buffer Overflow in Mach-O Parser: How Unchecked memcpy Calls Create Critical Attack Vectors

Severity: 🔴 Critical | CWE: CWE-120 (Buffer Copy Without Checking Size of Input) | Component: archo.cpp

Introduction

Memory corruption vulnerabilities have been the backbone of some of the most devastating exploits in computing history — from the Morris Worm to modern browser sandbox escapes. Yet despite decades of awareness, the humble memcpy call continues to be a source of critical security bugs, especially in code that parses attacker-controlled binary formats.

This post covers a recently patched heap buffer overflow in archo.cpp, a C++ file responsible for parsing and manipulating Mach-O binaries — the executable format used by macOS and iOS. The vulnerability (V-001) was rated critical because it allows a maliciously crafted binary file to corrupt heap memory, which in the right context can lead to arbitrary code execution.

If you write C or C++ code that processes binary file formats, reads user-supplied data into fixed-size buffers, or works with memcpy — this post is for you.

The Vulnerability Explained

What Is a Heap Buffer Overflow?

A buffer overflow occurs when a program writes more data into a buffer than the buffer was allocated to hold. When this happens on the heap (dynamically allocated memory), the overflow corrupts adjacent heap metadata or other live objects, potentially giving an attacker control over program execution.

The classic pattern looks like this:

// Dangerous: no bounds check
char* buffer = (char*)malloc(expected_size);
memcpy(buffer, attacker_controlled_data, attacker_controlled_size); // 💥

If attacker_controlled_size is larger than expected_size, you've just written past the end of your allocation.

The Specific Flaw in archo.cpp

The vulnerability existed in two locations within the Mach-O parsing logic:

Location 1 — Code Signing Blob (Line 568)

// VULNERABLE CODE (illustrative)
memcpy(m_pBase + m_uCodeLength,
       strCodeSignBlob.data(),
       strCodeSignBlob.size()); // ❌ No check: m_uCodeLength + size > allocated capacity?

Here, strCodeSignBlob is data read directly from a Mach-O binary's code-signing section. The code copies strCodeSignBlob.size() bytes into m_pBase at offset m_uCodeLength. The critical missing piece: there is no verification that m_uCodeLength + strCodeSignBlob.size() fits within the allocated size of m_pBase.

An attacker who controls the Mach-O file can set the code-signing blob to an arbitrarily large size, causing the memcpy to write well beyond the heap allocation.

Location 2 — Dylib File Data (Line 681)

// VULNERABLE CODE (illustrative)
memcpy(pDylibFile,
       strDylibFile.data(),
       strDylibFile.size()); // ❌ No bounds validation on pDylibFile capacity

Similarly, dylib (dynamic library) content is copied into pDylibFile without validating that the destination buffer is large enough to hold the incoming data. Since dylib files are also attacker-controllable inputs, this is a textbook heap buffer overflow via attacker-supplied file content.

How Could This Be Exploited?

The attack scenario is straightforward for anyone who can supply a file to the parsing pipeline:

┌─────────────────────────────────────────────────────┐
│  ATTACK FLOW                                        │
│                                                     │
│  1. Attacker crafts a malicious Mach-O/.ipa file   │
│     with an oversized code-sign blob or dylib       │
│                                                     │
│  2. Victim (or automated pipeline) passes the       │
│     file to the signing/processing tool             │
│                                                     │
│  3. archo.cpp parses the file and calls memcpy      │
│     without bounds checking                         │
│                                                     │
│  4. Heap memory is corrupted beyond the buffer      │
│                                                     │
│  5. Depending on heap layout:                       │
│     - Crash (Denial of Service)                     │
│     - Heap metadata corruption                      │
│     - Adjacent object overwrite → code execution   │
└─────────────────────────────────────────────────────┘

Real-World Impact

The severity of heap buffer overflows depends heavily on context, but the potential consequences include:

• Arbitrary Code Execution (ACE): By carefully controlling heap layout, an attacker can overwrite function pointers, vtable entries, or heap metadata to redirect execution flow.
• Privilege Escalation: If the parsing tool runs with elevated privileges (common in signing pipelines and build systems), ACE translates directly to privilege escalation.
• Supply Chain Attacks: Tools like app signers are often integrated into CI/CD pipelines. A malicious IPA or dylib submitted to an automated signing service could compromise the build infrastructure itself.
• Denial of Service: Even without full exploitation, the overflow will typically crash the process — disrupting signing workflows and potentially bricking automated pipelines.

The Fix

What Changed

The fix adds explicit bounds validation before each memcpy call, ensuring that the number of bytes to be copied never exceeds the available capacity of the destination buffer.

Fix for the Code Signing Blob

// BEFORE (vulnerable)
memcpy(m_pBase + m_uCodeLength,
       strCodeSignBlob.data(),
       strCodeSignBlob.size());

// AFTER (safe)
size_t available = m_uAllocatedSize - m_uCodeLength;
if (strCodeSignBlob.size() > available) {
    // Handle error: reject the malformed input
    return false; // or throw, or resize safely
}
memcpy(m_pBase + m_uCodeLength,
       strCodeSignBlob.data(),
       strCodeSignBlob.size()); // ✅ Safe: bounds verified

Fix for the Dylib File Copy

// BEFORE (vulnerable)
memcpy(pDylibFile,
       strDylibFile.data(),
       strDylibFile.size());

// AFTER (safe)
if (strDylibFile.size() > pDylibFileCapacity) {
    // Handle error: reject oversized input
    return false;
}
memcpy(pDylibFile,
       strDylibFile.data(),
       strDylibFile.size()); // ✅ Safe: bounds verified

Why This Works

The fix enforces a fundamental invariant: the amount of data written can never exceed the space available. By checking source_size <= (allocated_size - offset) before every copy, the code ensures that:

1. The write stays within the allocated heap region.
2. Attacker-controlled sizes cannot influence memory beyond the intended buffer.
3. Malformed inputs are rejected gracefully rather than silently corrupting memory.

A Note on Integer Overflow in Bounds Checks

One subtle pitfall when writing these checks: integer overflow in the bounds check itself.

// ⚠️ STILL DANGEROUS if m_uCodeLength is attacker-influenced
if (m_uCodeLength + strCodeSignBlob.size() <= m_uAllocatedSize) { ... }
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//  This addition can overflow if both values are large!

The safer pattern avoids addition entirely:

// ✅ SAFE: subtraction-based check avoids overflow
if (strCodeSignBlob.size() <= m_uAllocatedSize - m_uCodeLength) { ... }
// (assuming m_uCodeLength <= m_uAllocatedSize is already guaranteed)

This is a common gotcha that has bitten many experienced developers — always be suspicious of bounds checks that use addition.

Conclusion

This vulnerability is a textbook example of why binary format parsers deserve the highest level of security scrutiny. Every field in a Mach-O file — every size, every offset, every length — is ultimately attacker-controlled data. The moment any of those values flow into a memcpy call without validation, you have a potential critical vulnerability.

The key takeaways from this fix:

1. Never trust sizes from binary files. Validate before you allocate, and validate again before you copy.
2. Bounds checks must account for integer overflow. Prefer subtraction-based checks over addition-based ones.
3. Use modern C++ abstractions (std::vector, std::span) to let the standard library manage buffer safety.
4. Instrument your parsers with ASan and fuzz them — these tools are specifically designed to find exactly this class of bug.
5. Integrate static analysis into CI/CD to catch dangerous patterns like untrusted data flowing to memcpy.

Memory safety bugs are not inevitable. With the right tools, practices, and code review culture, they can be systematically eliminated. The patch here is a small change — a few bounds checks — but it closes the door on a critical attack vector that could have compromised entire signing pipelines.

Write code like every byte of your input is adversarial. Because sometimes, it is.

This vulnerability was identified and fixed by the OrbisAI Security automated security scanning system. Automated scanning, combined with human review, is a powerful combination for catching critical issues before they reach production.