I assume basic understanding of assembly language and a GDB (How to get started with GNU Project Debugger [GDB]?) basic knowledge. Let's Get Started We'll be using a basic string compare (inelegant) crackme for This tutorial.. crackme :- Code: #include<stdio.h> #define pass "CraxMe001-Explicted" int main(int argc,char **argv) { if(argc != 2) { printf("Usage : %s password\n",argv[0]); return(0); } if(!strcmp(argv[1],pass)) { printf("CraxEd!!!!!\n"); } return(0); } Compiling :- Code: aneesh@aneesh-laptop:~/articles$ gcc crackme.c -o crackme Lets first examine this in GDB.... Code: aneesh@aneesh-laptop:~/articles$ gdb ./crackme GNU gdb (GDB) 7.1-ubuntu Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i486-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... Reading symbols from /home/aneesh/articles/crackme...(no debugging symbols found)...done. (gdb) I am setting the disassmebly flavour to intel because in am comfortable with it but you can also choose the basic att syntax if your confortable.. Now lets disassemble the main function and identify the needed code... Code: (gdb) disassemble main Dump of assembler code for function main: 0x08048454 <+0>: push ebp 0x08048455 <+1>: mov ebp,esp 0x08048457 <+3>: and esp,0xfffffff0 0x0804845a <+6>: sub esp,0x10 0x0804845d <+9>: cmp DWORD PTR [ebp+0x8],0x2 ; The argc check 0x08048461 <+13>: je 0x8048480 <main+44> ; jmp to main+44 is equal 0x08048463 <+15>: mov eax,DWORD PTR [ebp+0xc] 0x08048466 <+18>: mov edx,DWORD PTR [eax] 0x08048468 <+20>: mov eax,0x8048570 0x0804846d <+25>: mov DWORD PTR [esp+0x4],edx 0x08048471 <+29>: mov DWORD PTR [esp],eax 0x08048474 <+32>: call 0x8048364 <printf@plt> ; Print the usage 0x08048479 <+37>: mov eax,0x0 0x0804847e <+42>: jmp 0x80484ad <main+89> 0x08048480 <+44>: mov eax,DWORD PTR [ebp+0xc] ; if jumped here then we passed the argc test thus we have a user entered pass and we are shifting the address of that pass into eax 0x08048483 <+47>: add eax,0x4 0x08048486 <+50>: mov eax,DWORD PTR [eax] ; the eax contains the address of the pass entered by the user 0x08048488 <+52>: mov DWORD PTR [esp+0x4],0x8048585 ;thus most probably this would be the address of the pass in the program lets check 0x08048490 <+60>: mov DWORD PTR [esp],eax 0x08048493 <+63>: call 0x8048384 <strcmp@plt> 0x08048498 <+68>: test eax,eax 0x0804849a <+70>: jne 0x80484a8 <main+84> ---Type <return> to continue, or q <return> to quit--- 0x0804849c <+72>: mov DWORD PTR [esp],0x8048599 0x080484a3 <+79>: call 0x8048374 <puts@plt> 0x080484a8 <+84>: mov eax,0x0 ; The exit code 0x080484ad <+89>: leave ; exit 0x080484ae <+90>: ret The dump looks quite interesting...While glancing over the code we see a call to strcmp @ 0x08048493 with 2 arguments...These arguments must contain the password... I have commented the above dump to make it more understandable for you guyz!!! So now that we have came to know the address of the Pass string lets check the data in it... Lets set a breakpoint at the beginning of the Program.. Code: (gdb) break main Breakpoint 1 at 0x8048457 Lets run the program with some args!! Code: (gdb) run pass The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/aneesh/articles/crackme pass Breakpoint 1, 0x08048457 in main () Lets step the code now Code: (gdb) s Single stepping until exit from function main, which has no line number information. 0x001a1140 in strcmp () from /lib/tls/i686/cmov/libc.so.6 Now that we know where the data exists lets examine the address's memory and get the password... This can be done by 'x' command Lets see how x works Code: (gdb) help x Examine memory: x/FMT ADDRESS. ADDRESS is an expression for the memory address to examine. FMT is a repeat count followed by a format letter and a size letter. Format letters are o(octal), x(hex), d(decimal), u(unsigned decimal), t(binary), f(float), a(address), i(instruction), c(char) and s(string). Size letters are b(byte), h(halfword), w(word), g(giant, 8 bytes). The specified number of objects of the specified size are printed according to the format. Defaults for format and size letters are those previously used. Default count is 1. Default address is following last thing printed with this command or "print". The help page of GDB is quite self-explanatory... So for Examining memory @ address 0x8048585 and display it as a string would be :- Code: (gdb) x/1s 0x8048585 0x8048585: "CraxMe001-Explicted" So … BOOOM!!!! We have successfully managed to get the pass out of this simple program... I hope you all liked it and Most Probably will continue Making more advanced articles on debugging..
