GDB

GDB is the GNU debugger, which is useful for examining binary applications, exploit development, and reverse engineering compiled programs. To set Intel syntax, add set disassembly-flavor intel to ~/.gdbinit.

A breakpoint pauses execution at a point in the program, like break *_start, while the program can be run with run. disas _start will disassemble the code to see what will run next. info registers shows the state of all the registers. nexti will run the next instruction. To examine the stack, run x/x $esp which displays the value at the esp register in hexadecimal, which points to the top of the stack. To view n number of values, change it to x/<n>x $esp. x/i can examine the value as a instruction, and x/s examines it as a string.

pwndbg

The extension pwndbg helps make GDB more user-friendly with extra functionality.

sudo apt install git
git clone https://github.com/pwndbg/pwndbg
cd pwndbg
./setup.sh

Example

With unknown compiled program:

// Compile with gcc -m32 -o password password.c
 
#include <stdio.h>
#include <string.h>
 
int main() {
    char *password = "ThisIsMyPassword\n";
    char input[151];
 
    puts("Please enter the password:");
    fgets(input, 149, stdin);
 
    if (strcmp(input, password) == 0) {
        puts("Success!");
    } else {
        puts("Fail!");
    }
 
    return(0);
}

Run using {shell icon}gdb ./password.

pwndbg> run 
pwndbg> info functions
pwndbg> break *main
pwndbg> run
pwndbh> next
pwndbg> x/2s 0x565556d0

step and next run the next instruction, but while step will enter a function that is called, next will only execute the function.