Buffer Overflow Attack Explanation

Overview

A buffer overflow occurs when an attacker writes more data into a memory buffer than it can hold. This excess data overflows into adjacent memory, potentially altering the program's behavior.

Normal Application Behavior

• Developers typically perform bounds checking to prevent memory overflows.
• For example, a buffer may be restricted to 8 bytes of input.
• This ensures the application functions correctly and securely.

Attacker Strategy

Attackers probe applications to find areas vulnerable to buffer overflows. Once found, they try to:

• Alter the behavior of the application.
• Gain unauthorized access or elevated privileges.

⚠️ Exploiting buffer overflows is not trivial. Incorrect attempts may crash the system or application. The attacker’s goal is to find a repeatable and advantageous overflow.

Example: Gaining Elevated Privileges

Memory Layout

• Variable A: 8 bytes, initialized with all 0s.
• Variable B: 2 bytes, initial value: 1979.

Variable B's Role

• If < 2000: guest/user access.
• If ≥ 24000: administrator access.

Vulnerability Discovery

Although Variable B is not editable via the application, Variable A has a buffer overflow vulnerability. Writing 9 bytes to Variable A causes:

• First 8 bytes → fill Variable A.
• 9th byte → overflows into Variable B.

Attack Execution

The attacker writes the word "excessive" (9 characters) into Variable A:

• Characters 1-8 → stored in Variable A.
• Character 9 ("e", hex 0x65, decimal 101) → overflows into Variable B.

Result

The overflow changes Variable B’s value to 25,856, which grants administrator access.

Conclusion

This example demonstrates how a crafted buffer overflow can:

• Exploit memory management vulnerabilities.
• Bypass normal application restrictions.
• Grant unauthorized elevated privileges to an attacker.