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IO SmashtheStack Level01
  • Xin
    Posts: 3,251
    ----[IO SmashtheStack Level01

    Lets navigate to /levels and execute ./level01. We get the following response
     Usage: ./level01 <password>

    So if we type ./level01 awhawfat we get the response "fail"

    Lets open it in gdb to debug it.
    gdb ./level01


    We now disassemble it to view the program flow.
    (gdb) disass main
    Dump of assembler code for function main:
    0x080483f4 <main+0>:    lea    0x4(%esp),%ecx
    0x080483f8 <main+4>:    and    $0xfffffff0,%esp
    0x080483fb <main+7>:    pushl  -0x4(%ecx)
    0x080483fe <main+10>:   push   %ebp
    0x080483ff <main+11>:   mov    %esp,%ebp
    0x08048401 <main+13>:   push   %edi
    0x08048402 <main+14>:   push   %ecx
    0x08048403 <main+15>:   sub    $0x30,%esp
    0x08048406 <main+18>:   mov    %ecx,-0x20(%ebp)
    0x08048409 <main+21>:   movl   $0x80485c8,-0xc(%ebp)
    0x08048410 <main+28>:   mov    -0x20(%ebp),%eax
    0x08048413 <main+31>:   cmpl   $0x2,(%eax)
    0x08048416 <main+34>:   je     0x8048439 <main+69>
    0x08048418 <main+36>:   mov    -0x20(%ebp),%edx
    0x0804841b <main+39>:   mov    0x4(%edx),%eax
    0x0804841e <main+42>:   mov    (%eax),%eax
    0x08048420 <main+44>:   mov    %eax,0x4(%esp)
    0x08048424 <main+48>:   movl   $0x80485d4,(%esp)
    0x0804842b <main+55>:   call   0x804832c <printf@plt>
    0x08048430 <main+60>:   movl   $0x1,-0x1c(%ebp)
    0x08048437 <main+67>:   jmp    0x80484b2 <main+190>
    0x08048439 <main+69>:   mov    -0xc(%ebp),%eax
    ---Type <return> to continue, or q <return> to quit---
    0x0804843c <main+72>:   mov    $0xffffffff,%ecx
    0x08048441 <main+77>:   mov    %eax,-0x24(%ebp)
    0x08048444 <main+80>:   mov    $0x0,%al
    0x08048446 <main+82>:   cld
    0x08048447 <main+83>:   mov    -0x24(%ebp),%edi
    0x0804844a <main+86>:   repnz scas %es:(%edi),%al
    0x0804844c <main+88>:   mov    %ecx,%eax
    0x0804844e <main+90>:   not    %eax
    0x08048450 <main+92>:   lea    -0x1(%eax),%edx
    0x08048453 <main+95>:   mov    -0x20(%ebp),%ecx
    0x08048456 <main+98>:   mov    0x4(%ecx),%eax
    0x08048459 <main+101>:  add    $0x4,%eax
    0x0804845c <main+104>:  mov    (%eax),%ecx
    0x0804845e <main+106>:  mov    %edx,0x8(%esp)
    0x08048462 <main+110>:  mov    -0xc(%ebp),%eax
    0x08048465 <main+113>:  mov    %eax,0x4(%esp)
    0x08048469 <main+117>:  mov    %ecx,(%esp)
    0x0804846c <main+120>:  call   0x804830c <strncmp@plt>
    0x08048471 <main+125>:  test   %eax,%eax
    0x08048473 <main+127>:  jne    0x804849f <main+171>
    0x08048475 <main+129>:  movl   $0x80485ea,(%esp)
    0x0804847c <main+136>:  call   0x80482fc <puts@plt>
    0x08048481 <main+141>:  movl   $0x0,0x8(%esp)


    Look for <strncmp@plt> this is a string compare function obviously comparing our entry with the password. So lets put a breakpoint here. 
    /break *0x0804846c


    Then when we try run we get this
    (gdb) run pass
    Starting program&#58; /levels/level01 pass

    Breakpoint 1, 0x0804846c in main ()
    (gdb)


    Lets now try to examine the values of the addresses further by typing 

    (gdb) i r
    eax            0x80485c8        134514120
    ecx            0xbfffdec3       -1073750333
    edx            0xb      11
    ebx            0x249ff4 2400244
    esp            0xbfffdd00       0xbfffdd00
    ebp            0xbfffdd38       0xbfffdd38
    esi            0x8048510        134513936
    edi            0x80485d4        134514132
    eip            0x804846c        0x804846c &lt;main+120&gt;
    eflags         0x286    &#91; PF SF IF &#93;
    cs             0x73     115
    ss             0x7b     123
    ds             0x7b     123
    es             0x7b     123
    fs             0x0      0
    gs             0x33     51
    /code&#93;

    Standing for info registers&#46;
    We will now examine each address by typing 
    x/s address
    Eg
    &#91;code&#93;(gdb) x/s 0xbfffdec3
    0xbfffdec3&#58;      \"pass\"
    &#91;/code&#93;

    This is what we typed in to run the program, lets now examine other addresses&#46; 

    &#91;code&#93;(gdb) x/s 0x80485c8
    0x80485c8&#58;       \"omgpassword\"


    Here we get the password. So simple run the program
    ./level01 omgpassword
    And you will have completed it.

    Wasnt too hard was it ;)
    Xin
  • Null Set
    Posts: 112
    Too much hassle for such a short challenge, Xinapse. Try "strings level01" :P
  • Xin
    Posts: 3,251
    said:


    Too much hassle for such a short challenge, Xinapse. Try "strings level01" :P



    lol now thats annoying, oh well got the password anyway.
    Xin
  • schumbag
    Posts: 23
    what's the code ./level01 ??
    share that please :)
  • Xin
    Posts: 3,251
    said:


    what's the code ./level01 ??
    share that please :)



    The password? I just pretty much handfed it to you with this surely you can do it :p
    Xin
  • Null Set
    Posts: 112
    said:


    said:


    what's the code ./level01 ??
    share that please :)



    The password? I just pretty much handfed it to you with this surely you can do it :p


    I think he's referring to the source code. I think for Level 1, there is no source code in smashthestack. Try connecting to it using ssh and check for yourself. :)
  • schumbag
    Posts: 23
    i see :LOL maybe this impact my language too bad *ROFL
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