TL;DR
This guide shows you how to exploit a simple buffer overflow in MIPS assembly to redirect execution to an arbitrary address (0x0040xxxx). We’ll cover identifying the vulnerability, crafting the payload, and running it.
Prerequisites
- Basic understanding of MIPS assembly.
- A vulnerable program compiled for a MIPS emulator (e.g., MARS).
- MARS or another MIPS simulator.
1. Identify the Vulnerability
The core idea is to overwrite the return address on the stack with your target address. This usually happens when a program copies user input into a buffer without checking its size.
- Examine the code: Look for functions like
strcpy,gets, or custom copy routines that don’t perform bounds checks. - Find the buffer: Identify the memory location where user input is stored.
- Locate the return address: Determine where the return address is saved on the stack relative to the buffer. This often requires debugging and examining the call frame.
2. Crafting the Payload
The payload consists of a series of bytes designed to overwrite the return address with your target address.
Step 1: Determine the Offset
You need to know how many bytes you need to write before reaching the return address on the stack. Use debugging tools (MARS’s debugger is suitable) to find this offset. Fill the buffer with a known pattern (e.g., ‘A’ characters) and observe where the return address gets overwritten.
Step 2: Construct the Payload
The payload will have the following structure:
[Padding] + [Target Address]- Padding: Fill the buffer with enough bytes to reach the return address.
- Target Address: The address you want execution to jump to (0x0040xxxx). Important: Ensure this address is valid and contains executable code in your program.
For example, if the offset is 20 bytes and your target address is 0x00401000, the payload might look like this (assuming little-endian MIPS):
"A" * 20 + "x00x10x40x00"3. Running the Exploit
Step 1: Provide Input
Feed the crafted payload as input to the vulnerable program.
Step 2: Debugging (MARS)
- Set a breakpoint: Set a breakpoint at the target address (0x0040xxxx).
- Run the program: Execute the program in MARS.
- Observe execution: If everything goes correctly, execution should jump to your target address when the vulnerable function returns.
4. Example Scenario
Let’s assume a simple program with a buffer overflow vulnerability:
.text
.globl main
main:
# Allocate space on the stack for the buffer
buffer: .space 64
# Copy user input into the buffer (vulnerable function)
la $a0, buffer
li $a1, 128 # Input size - larger than buffer!
jal gets_wrapper
# Return from main
jr $ra
.text
gets_wrapper:
sw $fp, 0($sp)
move $fp, $sp
subu $sp, $sp, 4
sw $ra, 0($sp)
la $a0, buffer
li $a1, 128
jal gets
lw $ra, 0($sp)
move $sp, $fp
lw $fp, 0($sp)
jr $ra
In this example, the gets_wrapper function calls gets with a size larger than the buffer allocated on the stack. This creates a classic buffer overflow vulnerability.
5. Important Considerations
- Address Space Layout Randomization (ASLR): ASLR randomizes memory addresses, making it harder to predict the target address. You may need techniques like information leaks or return-oriented programming (ROP) to bypass ASLR.
- Non-Executable Stack: Some systems prevent code execution from the stack. ROP can also help with this.
- Stack Canaries: Stack canaries are values placed on the stack to detect buffer overflows. Overwriting a canary will cause the program to terminate.