Buffer Overflow Exploitation Walkthrough

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Buffer overflow remains one of the most critical vulnerabilities in system security and exploit development. In this guide, I’ll walk through a 32-bit reverse engineering challenge that involves exploiting both buffer overflow and format string vulnerabilities to retrieve a hidden flag.

📌 Prerequisites

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Before diving in, make sure you are comfortable with:

• Using gdb for debugging, breakpoints, and piping payloads.
• Understanding stack operations: how addresses are pushed, popped, and swapped during function calls.
• Basics of assembly language for analyzing disassembled code.
• Familiarity with 32-bit environments (skills here transfer easily to 64-bit).

Buffer Overflow Basics

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A buffer overflow happens when more data is written to a buffer than it can handle, causing memory corruption. This often allows attackers to overwrite function return addresses and take control of execution flow.

In this challenge, the vulnerable function strcpy is the primary entry point for the overflow. By carefully crafting the payload, we can control program behavior.

Step 1: Function Enumeration

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We start by listing all functions using:

info functions

On disassembling the main function, we spot a string copy vulnerability leading to a buffer overflow.

Step 2: Analyzing Key Functions

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validateArgs

Checks whether two arguments are provided:

howYouDoing

Contains both a strcpy (classic buffer overflow candidate) and a printf (format string vulnerability).

whatIsDeadMayNeverDie

Acts as the first exploitation target. It depends on a global variable update to progress further.

We locate the address of this global variable:

Step 3: Exploiting the Format String Vulnerability

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1. The program accepts two arguments:

• Arg1 → vulnerable to both buffer overflow and format string bugs

• Arg2 → vulnerable only to buffer overflow

We exploit Arg1’s format string bug to overwrite the global variable with 39 (0x27). Then, we craft a buffer overflow in Arg2 to overwrite the return address, redirecting execution to whatIsDeadMayNeverDie.

From the main function, buffer size = 256 bytes.

• 256 + 4 → corrupts ebp

• Next 4 bytes → overwrite RIP with 0x08048548 (address of whatIsDeadMayNeverDie)

At this stage, the global variable is successfully updated.

Step 4: Chaining Exploits to Trigger the Flag

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Disassembling giveMeTheFlagAlready shows it takes one argument:

0x4(%ebp) → return address

0x8(%ebp) → function argument

We prepare a chained payload:

1. Use format string exploit to update the global variable.

2. Overflow buffer with 260 NOPs.

3. Append function addresses:

• First → whatIsDeadMayNeverDie

• Second → giveMeTheFlagAlready

• Third → global variable address (argument for flag function).

Step 5: Final Payload Execution

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Run the crafted payload:

./Final_Q1.bin $(printf “\x6c\xa0\x04\x08”)-%33u-%1\$n `cat payload`

🎉 Success! The flag is revealed.

Format String Exploit Example Explained

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In this case, the format string vulnerability was leveraged to overwrite a global variable. By controlling the %n specifier inside printf, we directly modified memory. This kind of attack is extremely powerful because it bypasses conventional buffer size limits.

Pairing this with buffer overflow gave us full control over execution flow.

Real-World Relevance

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While this example is part of a CTF challenge, buffer overflows and format string vulnerabilities are still relevant today:

• Exploit research in legacy 32-bit applications

• Reverse engineering malware or binary protections

• Studying defense techniques like stack canaries, ASLR, and DEP

• Strengthening offensive security skills for penetration testing

Key Takeaways

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• Buffer overflows often pair with format string vulnerabilities for powerful exploits.

• Understanding stack layout and offsets is essential for payload crafting.

• Tools like gdb and objdump are indispensable for exploit development.

• These strategies form the foundation for modern exploit research and CTF challenges.

Mastering buffer overflow exploitation not only strengthens reverse engineering skills but also builds the mindset needed for both offensive security research and defensive mitigations.