Critical Buffer Overflow in Windows USB HID: How One Byte Can Compromise Your System

Severity: 🔴 Critical | CVE Type: Buffer Overflow (CWE-122) | Fixed In: Windows USB HID Host Library

Introduction

There's a saying in security circles: "It only takes one byte to ruin your day." That saying has never been more literal than in the vulnerability we're discussing today — a heap buffer overflow in the Windows USB HID (Human Interface Device) host library where a single off-by-one error in a memcpy call could corrupt heap metadata and open the door to arbitrary code execution.

This vulnerability was discovered in Win/lib/src/usb_hidhost_impl.cpp and affects code that processes data from USB HID devices — the same category of devices that includes keyboards, mice, gamepads, and countless specialty input devices. Because this code runs with elevated trust and interacts directly with hardware-reported data, the attack surface is significant.

If you're a developer who works with native C/C++ code, hardware interfaces, or low-level Windows APIs, this post is essential reading. Even if you don't, the lessons here apply broadly to any code that copies memory using externally-controlled size values.

The Vulnerability Explained

What Is a Heap Buffer Overflow?

A heap buffer overflow occurs when a program writes data beyond the boundaries of a buffer that was dynamically allocated on the heap. Unlike stack overflows (which famously overwrite return addresses), heap overflows corrupt adjacent heap memory — which often contains allocator metadata, other buffers, or object pointers.

Modern heap allocators like Windows' Low Fragmentation Heap (LFH) store bookkeeping information in structures adjacent to allocated blocks. Overwriting even a single byte of this metadata can cause the allocator to behave incorrectly on the next allocation or deallocation — a condition that skilled attackers can turn into reliable code execution primitives.

The Vulnerable Code

The vulnerability centered on four memcpy calls that used sizes derived from HID device-reported capabilities without first validating that the destination buffer was large enough to hold the data.

The two most critical instances:

Instance 1 — The Off-By-One at Line 318:

// VULNERABLE: vpTempBuf was allocated as `vCaps.FeatureReportByteLength` bytes
// Writing at offset +1 means we can overflow by at least 1 byte
memcpy(&vpTempBuf[1], pBuf, min(BufSize, vCaps.FeatureReportByteLength));

Here's the subtle but devastating problem: vpTempBuf is allocated with vCaps.FeatureReportByteLength bytes. The memcpy writes starting at vpTempBuf[1] — one byte into the buffer — but uses vCaps.FeatureReportByteLength as the maximum copy size. This means the write can extend to vpTempBuf[1 + FeatureReportByteLength - 1], which is one byte past the end of the allocated buffer.

Allocated buffer:  [0][1][2][3]...[N-1]
Write starts at:      [1]
Write ends at:           [2][3]...[N-1][N]  ← ONE BYTE PAST THE END
                                             ↑ HEAP CORRUPTION

Instance 2 — No Guard at All at Line 232:

// VULNERABLE: `len` comes from device-reported data, no bounds check whatsoever
memcpy(destBuf, srcBuf, len);

This instance is arguably worse: len is sourced directly from HID device capabilities (vTempBufSize) with no min() guard against the actual destination buffer size. An attacker controlling a malicious USB HID device could report an arbitrarily large len value and trigger a massive heap overflow.

Why Is This Exploitable?

The key detail that elevates this from a bug to a critical vulnerability is where the size values come from: the HID device itself.

HID devices report their capabilities (including buffer sizes like FeatureReportByteLength) through USB descriptors. A malicious USB device — or a legitimate device with manipulated firmware — can report whatever values it wants. This means:

1. A rogue USB device plugged into a machine could report a FeatureReportByteLength larger than what the host actually allocates.
2. The memcpy would then write beyond the buffer boundary.
3. Heap metadata corruption could lead to a crash, or with careful crafting, to arbitrary code execution.

Real-World Attack Scenario

Imagine a malicious USB device disguised as a standard keyboard or game controller. When plugged in:

1. The device reports a FeatureReportByteLength of, say, 512 bytes.
2. The host allocates a buffer based on this value.
3. The device then sends a feature report that triggers the vulnerable memcpy at line 318.
4. Because of the +1 offset, the copy overflows by one byte into the adjacent heap chunk's metadata.
5. On the next heap operation, the corrupted metadata causes a controlled write to an attacker-chosen address.
6. Code execution is achieved — potentially at the privilege level of the process handling USB input.

This class of attack is known as a "BadUSB" style attack and has been demonstrated in real-world scenarios. Physical access to a USB port is the only prerequisite.

The Fix

What Changed

The fix addressed all four vulnerable memcpy calls in Win/lib/src/usb_hidhost_impl.cpp by implementing proper bounds validation before each copy operation.

The core principle of the fix:

Never trust externally-supplied size values. Always validate that the number of bytes to be copied does not exceed the capacity of the destination buffer — accounting for any offsets applied to the destination pointer.

Fixed Instance 1 — Accounting for the Offset:

// BEFORE (vulnerable):
memcpy(&vpTempBuf[1], pBuf, min(BufSize, vCaps.FeatureReportByteLength));

// AFTER (safe):
// The destination starts at offset 1, so available space is (bufferSize - 1)
size_t availableSpace = vCaps.FeatureReportByteLength - 1;
size_t copySize = min({BufSize, availableSpace, vCaps.FeatureReportByteLength - 1});
memcpy(&vpTempBuf[1], pBuf, copySize);

The critical insight: when your destination pointer is buffer + offset, your available space is bufferSize - offset, not bufferSize. The fix correctly subtracts the offset from the available space calculation.

Fixed Instance 2 — Adding the Missing Guard:

// BEFORE (vulnerable):
memcpy(destBuf, srcBuf, len);

// AFTER (safe):
size_t safeLen = min(len, destBufSize);
memcpy(destBuf, srcBuf, safeLen);

A min() guard ensures that no matter what value a device reports for len, the copy can never exceed the actual allocated buffer size.

The Broader Security Improvement

Beyond the individual fixes, this patch reinforces a critical security boundary: hardware-reported values must be treated as untrusted input. Just as web developers sanitize user input before using it in SQL queries, systems programmers must validate device-reported sizes before using them in memory operations.

This is especially important for USB and HID code because:
- USB devices can be emulated by software (e.g., USB/IP, virtual machines)
- Physical USB devices can have their firmware modified
- The attack requires no network access — just physical proximity

Prevention & Best Practices

1. Never Trust Externally-Sourced Size Values

Any size value that originates from outside your program — user input, network packets, file data, hardware descriptors — must be validated before use in memory operations.

// ❌ Dangerous pattern
memcpy(dest, src, external_size);

// ✅ Safe pattern
if (external_size > dest_capacity) {
    // Handle error: reject, truncate, or log
    return ERROR_BUFFER_OVERFLOW;
}
memcpy(dest, src, external_size);

2. Account for Pointer Arithmetic in Bounds Checks

When writing to buffer + offset, your available capacity is buffer_size - offset, not buffer_size.

// ❌ Dangerous: ignores the offset
memcpy(&buf[offset], src, buf_size);

// ✅ Safe: accounts for the offset
if (offset >= buf_size) return ERROR_INVALID_OFFSET;
size_t available = buf_size - offset;
memcpy(&buf[offset], src, min(src_size, available));

3. Prefer Safe Memory Functions

Modern C and C++ offer safer alternatives to raw memcpy:

// C11: memcpy_s (bounds-checking version)
errno_t result = memcpy_s(dest, destSize, src, count);
if (result != 0) { /* handle error */ }

// C++: std::copy with iterators (compile-time or runtime bounds)
std::copy_n(src, std::min(count, destSize), dest);

// Windows-specific: RtlCopyMemory with explicit size validation

4. Use Static Analysis Tools

Several tools can catch these vulnerabilities automatically:

5. Use SAL Annotations on Windows

Microsoft's Source Annotation Language (SAL) lets you annotate buffer parameters so the compiler and static analysis tools can verify correct usage:

// Annotate your functions to express buffer size contracts
void ProcessHidReport(
    _Out_writes_bytes_(bufSize) BYTE* buffer,
    _In_ size_t bufSize,
    _In_reads_bytes_(dataLen) const BYTE* data,
    _In_ size_t dataLen
);

6. Apply Defense in Depth

Even with correct bounds checking, consider additional mitigations:

• Enable Heap Integrity Checking: Windows' heap manager has optional integrity checks that can detect corruption earlier.
• Use Control Flow Guard (CFG): Helps prevent exploitation of heap corruption for code execution.
• ASLR + DEP: Ensure these are enabled for all modules to raise the exploitation bar.
• Fuzz Testing: Use fuzzers like libFuzzer or AFL++ to feed malformed HID descriptors to your parsing code.

7. Security Standards Reference

This vulnerability maps to several well-known security standards:

• CWE-122: Heap-based Buffer Overflow
• CWE-131: Incorrect Calculation of Buffer Size
• CWE-20: Improper Input Validation
• OWASP: A03:2021 – Injection (untrusted data driving memory operations)
• CERT C: Rule MEM35-C (Allocate sufficient memory for an object) and ARR38-C (Guarantee that library functions do not form invalid pointers)

Conclusion

This vulnerability is a textbook example of why trusting external data is dangerous — and why the consequences can be severe even when the bug seems small. A single byte of overflow was enough to create a potential path to arbitrary code execution via heap metadata corruption.

The key takeaways from this fix:

1. Treat hardware-reported values as untrusted input — just like you'd treat user-supplied data in a web application.
2. Always account for pointer offsets when calculating available buffer space — &buf[1] has bufSize - 1 bytes available, not bufSize.
3. Add explicit size guards to every memcpy that uses an externally-derived size.
4. Use static analysis and fuzzing to catch these issues before they reach production.
5. Prefer safe memory APIs (memcpy_s, bounded iterators) over raw memcpy where possible.

Memory safety bugs in hardware interface code are particularly dangerous because they often run with elevated privileges and interact with devices that can be controlled by attackers. As USB-based attacks become more sophisticated and accessible, hardening the code that processes USB data is not optional — it's essential.

The fix here was relatively straightforward once the vulnerability was identified: validate sizes, account for offsets, and never assume that a device is telling the truth about its capabilities. These are small changes that make an enormous difference in security posture.

This vulnerability was discovered and fixed by the OrbisAI Security automated scanning system. If you're interested in automated security scanning for your codebase, visit orbisappsec.com.

Further Reading:
- MSDN: HID Architecture
- CWE-122: Heap-based Buffer Overflow
- CERT C Coding Standard: MEM35-C
- BadUSB: On Accessories that Turn Evil
- Microsoft SDL: Banned Functions