Principles and Defense of Buffer Overflow Attacks

Principles and Defense of Buffer Overflow Attacks

1. Problem Description

Buffer Overflow is a common memory attack technique in cybersecurity. By inputting data that exceeds the expected length into a program, an attacker can overwrite adjacent memory regions, thereby altering the program's execution flow (e.g., executing malicious code or triggering a program crash). Such vulnerabilities often appear in languages like C/C++ that do not automatically check array bounds.

2. Root Principle

  • Memory Layout: During program execution, the stack in memory is used to store local variables, function parameters, and return addresses.
  • Overflow Mechanism: If the program does not validate the length of input data, when an attacker submits excessively long data, it can overwrite critical memory areas such as the return address. For example: 
    void vulnerable_function(char* input) {
    char buffer[8];  // Allocate an 8-byte buffer on the stack
    strcpy(buffer, input); // No length check on input, may cause overflow
}
    If input exceeds 8 bytes, the excess data will overwrite the return address adjacent to buffer on the stack.

3. Attack Steps (Taking Stack Overflow as an Example)

  1. Detect Vulnerability: The attacker inputs excessively long data (e.g., "AAAABBBBCCCC...") into the target program and observes whether the program crashes.
  2. Locate Return Address: Determine the offset of the return address on the stack using debugging tools (e.g., GDB). For example, after inputting "AAAABBBBCCCCDDDD", it is found that DDDD overwrites the return address.
  3. Construct Payload:
    • Place malicious code (Shellcode) into the input data.
    • Overwrite the return address with the starting address of the malicious code (or use instructions like JMP ESP to redirect execution).
  4. Trigger the Attack: When the program executes the return instruction of vulnerable_function, it will jump to and execute the malicious code.

4. Defense Measures

  1. Code Level:
    • Use safe functions (e.g., replace strcpy with strncpy, sprintf with snprintf).
    • Compiler options: Enable stack protection (GCC's -fstack-protector), Address Space Layout Randomization (ASLR), and Data Execution Prevention (DEP/NX).
  2. System Level:
    • Enable ASLR: Randomize memory addresses to increase prediction difficulty.
    • DEP/NX: Mark the stack and heap as non-executable to prevent malicious code execution.
  3. Testing and Auditing:
    • Use fuzzing tools to automatically discover overflow vulnerabilities.
    • Employ static code analysis tools (e.g., Clang Static Analyzer) to detect potential risks.

5. Example Demonstration (Simplified Version)

Assume the following vulnerable code exists:

#include <string.h>
void main(int argc, char* argv[]) {
    char buffer[4];
    strcpy(buffer, argv[1]); // Overflow occurs when inputting excessively long parameters
}

The attacker executes:

./program AAAAAAAA\xef\xbe\xad\xde  # Overwrite the return address with a specific address

The program crashes and jumps to 0xdeadbeef (which must be adjusted to an actual valid address).

Through the above steps, the principles, exploitation methods, and defense mechanisms of buffer overflows are systematically presented.