A heap-based buffer overflow occurs when a program writes more data to a dynamically allocated memory buffer than it can hold, overwriting adjacent heap…
A heap-based buffer overflow occurs when a program writes more data to a dynamically allocated memory buffer than it can hold, overwriting adjacent heap memory. Unlike stack-based overflows, heap overflows corrupt data structures and metadata rather than return addresses, but can still lead to arbitrary code execution, denial of service, or information disclosure.
Heap memory is allocated at runtime using functions like malloc() or new, and is managed by the heap allocator. When a program copies user-controlled data into a heap buffer without validating the input length, it can write past the buffer's boundary. The overflow corrupts adjacent heap objects—other allocated buffers, metadata pointers, or allocator structures—depending on memory layout. Unlike stack overflows, exploiting heap overflows requires understanding heap internals and is often more complex, but the consequences can be equally severe.
Heap overflows can lead to arbitrary code execution by corrupting function pointers or allocator metadata, allowing an attacker to redirect execution flow. They can also cause denial of service by corrupting critical data structures, or leak sensitive information from adjacent memory. The impact depends on what data is adjacent to the overflowed buffer and how the heap allocator responds to corruption.
import ctypes
def process_user_data(user_input):
# Allocate a fixed-size buffer
buffer = ctypes.create_string_buffer(16)
# Copy user input without length validation
ctypes.memmove(buffer, user_input.encode(), len(user_input))
return buffer.valueWhy it's vulnerable:
The code allocates a 16-byte buffer but copies the entire user input without checking its length. If user_input exceeds 16 bytes, the write overflows the buffer boundary and corrupts adjacent heap memory.
import ctypes
def process_user_data(user_input, max_length=16):
# Allocate a fixed-size buffer
buffer = ctypes.create_string_buffer(max_length)
# Validate input length before copying
input_bytes = user_input.encode()
if len(input_bytes) > max_length - 1:
raise ValueError(f"Input exceeds maximum length of {max_length - 1}")
ctypes.memmove(buffer, input_bytes, len(input_bytes))
return buffer.value<?php
function process_user_data($user_input) {
// Simulate a fixed-size buffer (16 bytes)
$buffer = str_pad("", 16, "\x00");
// Unsafe: copy user input without length check
$buffer = substr_replace($buffer, $user_input, 0);
return $buffer;
}
$data = $_GET['data'];
process_user_data($data);
?>Why it's vulnerable:
The code treats a string as a fixed-size buffer and overwrites it with user input of arbitrary length. In languages with manual memory management (C/C++), this would overflow the heap; in PHP, it demonstrates the conceptual flaw of unbounded writes.
<?php
function process_user_data($user_input, $max_length = 16) {
// Validate input length
if (strlen($user_input) > $max_length) {
throw new Exception("Input exceeds maximum length of {$max_length}");
}
// Safe: copy only validated length
$buffer = str_pad($user_input, $max_length, "\x00");
return $buffer;
}
$data = isset($_GET['data']) ? $_GET['data'] : '';
process_user_data($data);
?>strncpy() instead of strcpy(), snprintf() instead of sprintf(), or language-level bounds checking.
std::string in C++, or native strings in Python/PHP) over manual buffers.
Heap overflows may manifest as unexpected crashes or segmentation faults in production, particularly when processing large or specially crafted input. You may also observe memory corruption errors in logs, or unusual behavior in features that process user-supplied data (file uploads, form fields, API parameters). If a security scanner or fuzzer reports a heap overflow, investigate the affected code path immediately.
