Heap Spraying Explained

TL;DR

Heap spraying is a technique used in exploit development to increase the reliability of exploits by filling the heap with predictable data, making it easier to land shellcode or other payloads at known addresses. This guide explains how it works and provides basic examples.

What is Heap Spraying?

When an exploit runs, it often needs to execute code (shellcode) in a specific location in memory. However, the exact address of this location can be unpredictable due to Address Space Layout Randomisation (ASLR). Heap spraying aims to overcome this by filling a large portion of the heap with copies of your shellcode or other useful data. This increases the probability that when an exploit tries to jump to a calculated address, it will land within one of these sprayed blocks.

How Does Heap Spraying Work?

1. Allocate Memory: The exploit allocates a large amount of memory on the heap.
2. Fill with Payload: This allocated memory is then filled with copies of your shellcode or other data you want to execute. Typically, this involves repeating a pattern containing your payload many times.
3. Trigger Execution: The exploit attempts to redirect execution flow to an address within the sprayed heap region. Because so much memory has been populated with the same data, there’s a higher chance of hitting a valid copy of your payload.

Basic Heap Spraying Example (Conceptual)

This example demonstrates the core idea using Python and assumes you have some way to trigger heap allocation within an application.

Step 1: Define Your Payload

First, define the shellcode or data you want to spray. For simplicity, let’s use a simple string.

payload = "x48x31xc0x50x68//shx68/binx89xe3x50x53x89xe1xb0x0bxcdx80" # Example shellcode (executes /bin/sh)

Step 2: Create the Spray Pattern

Create a repeating pattern containing your payload. The size of this pattern is important; it needs to be large enough to fill a significant portion of the heap.

spray_size = 1024 * 1024 # 1MB spray size
payload_length = len(payload)
spray_pattern = payload * (spray_size // payload_length) + payload[:spray_size % payload_length]

Step 3: Allocate and Fill the Heap

This is where it gets application-specific. You need to find a way to allocate memory within the target application and fill it with your spray pattern. The following example shows how you might do this in a hypothetical scenario using a vulnerable function.

# Hypothetical vulnerable function that allocates heap space
def allocate_heap_space(size):
    # In reality, this would involve interacting with the target application's API or memory.
    return spray_pattern[:size] # Simulate allocation and filling

# Allocate a large chunk of heap space
sprayed_memory = allocate_heap_space(spray_size)

Step 4: Trigger Execution

The final step is to trigger execution at an address within the sprayed heap region. This usually involves overwriting a function pointer or other control data with an address in the sprayed memory.

This part is highly dependent on the specific vulnerability being exploited and requires detailed knowledge of the target application’s architecture and memory layout.

Important Considerations

• ASLR: ASLR makes heap addresses unpredictable. Heap spraying increases the probability of success but doesn’t guarantee it.
• Heap Fragmentation: Heap fragmentation can reduce the effectiveness of heap spraying if large contiguous blocks of memory are not available.
• Application-Specific Techniques: The best way to perform heap spraying depends heavily on the target application and its memory management system.
• Security Measures: Modern operating systems and applications often include security measures (e.g., DEP, ASLR) that make heap spraying more difficult.

Further Research

• ROPgadget: https://github.com/todayisgreatday/ropgadget (Finds useful gadgets for Return-Oriented Programming)
• pwntools: https://docs.pwntools.com/ (A Python library for exploit development)