Blog | G5 Cyber Security

ESP Register Change: Buffer Overflow Explained

G5 Cyber Security

TL;DR

A buffer overflow overwrites memory beyond the allocated space for a variable. This can corrupt other data, including the stack pointer (ESP register in x86 architecture). When an access violation occurs, the ESP register’s value has likely changed because the overflow corrupted return addresses or other critical stack information.

Understanding Buffer Overflows

A buffer overflow happens when a program tries to write more data into a memory area (the ‘buffer’) than it can hold. This is a common security vulnerability, and understanding why the ESP register changes after an access violation is key to debugging these issues.

Why the ESP Register Changes

  1. The Stack: The stack is a region of memory used for storing local variables, function arguments, and return addresses. The ESP (Extended Segment Pointer) register points to the top of the stack.
  2. Function Calls: When a function is called, its return address (the location in code to go back to after the function finishes) is pushed onto the stack. Local variables are also allocated space on the stack.
  3. Overflowing the Buffer: If you write beyond the bounds of a buffer located on the stack, you can overwrite data that’s higher up in memory – including the return address.
  4. Access Violation: When the function tries to ‘return’, it attempts to jump to the corrupted return address. This usually leads to an access violation (segmentation fault) because the overwritten address is invalid or points to non-executable memory.
  5. ESP Corruption: The overflow can also directly overwrite the ESP register itself, or data used to calculate its value. When this happens, the program’s stack becomes inconsistent, leading to unpredictable behaviour and the access violation.

Debugging Steps

  1. Identify the Vulnerable Code: Use a debugger (like GDB on Linux or WinDbg on Windows) to find where the buffer overflow is occurring. Look for code that copies data into a fixed-size buffer without checking the input length.
  2. Set Breakpoints: Set breakpoints before and after the vulnerable copy operation.
  3. Inspect Memory: Use the debugger’s memory inspection tools to examine the contents of the stack around the buffer. Look for overwritten data, especially the return address. 
    x/20xw $esp

    (GDB example – displays 20 words in hexadecimal starting from ESP)

  4. Step Through Execution: Step through the code line by line to see exactly when and how the buffer is being overflowed.
  5. Examine the Call Stack: After the access violation, examine the call stack (using 
    bt

    in GDB) to see where the program was trying to return from. The corrupted return address will be visible here.

  6. Check for Stack Alignment Issues: Sometimes overflows can disrupt stack alignment which causes issues when returning.

Example Scenario (C Code)

#include <stdio.h>
#include <string.h>

int main() {
  char buffer[10];
  char input[] = "This is a very long string";

  strcpy(buffer, input); // Vulnerable: No bounds checking!
  printf("%sn", buffer);
  return 0;
}

In this example, strcpy copies the contents of input into buffer without checking its size. Since input is longer than 10 characters, it overflows buffer, potentially corrupting the stack and leading to an access violation when the function returns.

Preventing Buffer Overflows

Exit mobile version