Windows Buffer Overflow: Shellcode Guide

TL;DR

This guide shows you how to create and inject simple shellcode into a vulnerable Windows application to gain command execution. It’s for learning purposes only – exploiting systems without permission is illegal.

Prerequisites

• A vulnerable Windows application (e.g., one with an unsafe string copy function).
• A debugger like x64dbg or WinDbg.
• Basic understanding of assembly language.
• Python installed (for shellcode generation).

Step 1: Shellcode Generation

We’ll create shellcode that spawns a command prompt. Metasploit is a common tool, but we can also generate it manually with Python.

Using Python

import sys

# Shellcode to execute cmd.exe
shellcode = b"x31xc0x50x68x2fx2fx73x68x68x2fx62x69x6ex89xe3x50x53x89xe1xb0x0bxcdx80"

# Print the shellcode in a format suitable for injection
sys.stdout.buffer.write(shellcode)

Save this as shellcode.py and run it: python shellcode.py > shellcode.bin. This creates a file named shellcode.bin containing our shellcode.

Step 2: Identifying the Vulnerability

1. Locate the vulnerable function: Use a disassembler (like IDA Pro or Ghidra) to find functions that copy strings without bounds checking (e.g., strcpy, strcat, sprintf).
2. Find the buffer: Identify the memory location where the input string is copied.
3. Determine the offset: Calculate how many bytes you need to overwrite to reach the return address on the stack. This is crucial for redirecting execution to your shellcode. Use a debugger and breakpoints within the vulnerable function.

Step 3: Injecting Shellcode

There are several ways to inject shellcode:

• Direct Injection: If you have control over the input string, you can directly include your shellcode followed by a NOP sled (a series of 0x90 bytes) and then the address of your shellcode.
• Stack Overflow: Overwrite the return address on the stack with the address of your shellcode. This is the classic buffer overflow technique.

Example (Stack Overflow)

Assuming you’ve found that the offset to overwrite the return address is 20 bytes, and your shellcode starts at address 0x74400000:

# Input string = "A" * 20 + b"x00x00x00x00" + b"x00x00x00x00" + struct.pack("

Note: The exact address (0x74400000 in this example) will vary depending on the application and its memory layout.

Step 4: Bypassing Data Execution Prevention (DEP)

DEP prevents shellcode from executing directly from data sections like the stack. Common bypass techniques include:

• ROP (Return-Oriented Programming): Chain together existing code snippets in the application to achieve your desired functionality. This is more complex but effective.
• SEH (Structured Exception Handling) Overwrite: Overwrite exception handlers to redirect execution.

Bypassing DEP is beyond the scope of this basic guide, as it requires a deeper understanding of assembly and application internals.

Step 5: Debugging and Verification

1. Set breakpoints: Set a breakpoint at the return address you've overwritten.
2. Run the application: Execute the vulnerable application with your crafted input.
3. Verify execution: Check if the debugger stops at your shellcode and that it executes correctly (e.g., spawns a command prompt).

Important Considerations

• Address Space Layout Randomization (ASLR): ASLR randomizes memory addresses, making it harder to predict the location of your shellcode. You may need to leak an address or use techniques like ROP to bypass ASLR.
• Antivirus software: Antivirus programs can detect and block shellcode injection attempts.