Rodolfo Bandeira

Rodolfo Bandeira

software, electronics, security, devops, thoughts

Cracking binary files using VIM and some Linux tools

Hi there!

This post shows a simple example of how to crack a binary file. We compile a C file that when compiled will ask for a password. We dissasemble the file, change the assembly instruction that validates the password so any password is considered valid.

On my last post I wrote a simple crack to bypass a binary file. Today I’m gonna give you another example but at this time instead of using a simple unique number as a password, we’re going to change our binary in order of that every password-key entered will validade as true.

Our source file is: cracking-tutorial-2.c

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

int main (int argc, char *argv[]) {
    int i=0;

    if (argc < 2){
        printf("Error, please type the password key \n");
        return 0;
    }

    if (strcmp(argv[1], "my-secret-key") == 0) { 
        printf("Congratulations \n");
    } else {
        printf("Try again \n");
    }

    return 0;
}

Now, let’s compile and then run the objdump tool to inspect the internal functions and behavior.

objdump -d cracking-tutorial-2 > /tmp/dump.txt

cat /tmp/dump.txt

cracking-tutorial-2:     file format elf64-x86-64


Disassembly of section .init:

0000000000400418 <_init>:
  400418:   48 83 ec 08             sub    $0x8,%rsp
  40041c:   48 8b 05 d5 0b 20 00    mov    0x200bd5(%rip),%rax        # 600ff8 <_DYNAMIC+0x1d0>
  400423:   48 85 c0                test   %rax,%rax
  400426:   74 05                   je     40042d <_init+0x15>
  400428:   e8 53 00 00 00          callq  400480 <__gmon_start__@plt>
  40042d:   48 83 c4 08             add    $0x8,%rsp
  400431:   c3                      retq   

Disassembly of section .plt:

0000000000400440 <puts@plt-0x10>:
  400440:   ff 35 c2 0b 20 00       pushq  0x200bc2(%rip)        # 601008 <_GLOBAL_OFFSET_TABLE_+0x8>
  400446:   ff 25 c4 0b 20 00       jmpq   *0x200bc4(%rip)        # 601010 <_GLOBAL_OFFSET_TABLE_+0x10>
  40044c:   0f 1f 40 00             nopl   0x0(%rax)

0000000000400450 <puts@plt>:
  400450:   ff 25 c2 0b 20 00       jmpq   *0x200bc2(%rip)        # 601018 <_GLOBAL_OFFSET_TABLE_+0x18>
  400456:   68 00 00 00 00          pushq  $0x0
  40045b:   e9 e0 ff ff ff          jmpq   400440 <_init+0x28>

0000000000400460 <__libc_start_main@plt>:
  400460:   ff 25 ba 0b 20 00       jmpq   *0x200bba(%rip)        # 601020 <_GLOBAL_OFFSET_TABLE_+0x20>
  400466:   68 01 00 00 00          pushq  $0x1
  40046b:   e9 d0 ff ff ff          jmpq   400440 <_init+0x28>

0000000000400470 <strcmp@plt>:
  400470:   ff 25 b2 0b 20 00       jmpq   *0x200bb2(%rip)        # 601028 <_GLOBAL_OFFSET_TABLE_+0x28>
  400476:   68 02 00 00 00          pushq  $0x2
  40047b:   e9 c0 ff ff ff          jmpq   400440 <_init+0x28>

0000000000400480 <__gmon_start__@plt>:
  400480:   ff 25 aa 0b 20 00       jmpq   *0x200baa(%rip)        # 601030 <_GLOBAL_OFFSET_TABLE_+0x30>
  400486:   68 03 00 00 00          pushq  $0x3
  40048b:   e9 b0 ff ff ff          jmpq   400440 <_init+0x28>

Disassembly of section .text:

0000000000400490 <_start>:
  400490:   31 ed                   xor    %ebp,%ebp
  400492:   49 89 d1                mov    %rdx,%r9
  400495:   5e                      pop    %rsi
  400496:   48 89 e2                mov    %rsp,%rdx
  400499:   48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
  40049d:   50                      push   %rax
  40049e:   54                      push   %rsp
  40049f:   49 c7 c0 60 06 40 00    mov    $0x400660,%r8
  4004a6:   48 c7 c1 f0 05 40 00    mov    $0x4005f0,%rcx
  4004ad:   48 c7 c7 7d 05 40 00    mov    $0x40057d,%rdi
  4004b4:   e8 a7 ff ff ff          callq  400460 <__libc_start_main@plt>
  4004b9:   f4                      hlt    
  4004ba:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)

00000000004004c0 <deregister_tm_clones>:
  4004c0:   b8 4f 10 60 00          mov    $0x60104f,%eax
  4004c5:   55                      push   %rbp
  4004c6:   48 2d 48 10 60 00       sub    $0x601048,%rax
  4004cc:   48 83 f8 0e             cmp    $0xe,%rax
  4004d0:   48 89 e5                mov    %rsp,%rbp
  4004d3:   77 02                   ja     4004d7 <deregister_tm_clones+0x17>
  4004d5:   5d                      pop    %rbp
  4004d6:   c3                      retq   
  4004d7:   b8 00 00 00 00          mov    $0x0,%eax
  4004dc:   48 85 c0                test   %rax,%rax
  4004df:   74 f4                   je     4004d5 <deregister_tm_clones+0x15>
  4004e1:   5d                      pop    %rbp
  4004e2:   bf 48 10 60 00          mov    $0x601048,%edi
  4004e7:   ff e0                   jmpq   *%rax
  4004e9:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)

00000000004004f0 <register_tm_clones>:
  4004f0:   b8 48 10 60 00          mov    $0x601048,%eax
  4004f5:   55                      push   %rbp
  4004f6:   48 2d 48 10 60 00       sub    $0x601048,%rax
  4004fc:   48 c1 f8 03             sar    $0x3,%rax
  400500:   48 89 e5                mov    %rsp,%rbp
  400503:   48 89 c2                mov    %rax,%rdx
  400506:   48 c1 ea 3f             shr    $0x3f,%rdx
  40050a:   48 01 d0                add    %rdx,%rax
  40050d:   48 d1 f8                sar    %rax
  400510:   75 02                   jne    400514 <register_tm_clones+0x24>
  400512:   5d                      pop    %rbp
  400513:   c3                      retq   
  400514:   ba 00 00 00 00          mov    $0x0,%edx
  400519:   48 85 d2                test   %rdx,%rdx
  40051c:   74 f4                   je     400512 <register_tm_clones+0x22>
  40051e:   5d                      pop    %rbp
  40051f:   48 89 c6                mov    %rax,%rsi
  400522:   bf 48 10 60 00          mov    $0x601048,%edi
  400527:   ff e2                   jmpq   *%rdx
  400529:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)

0000000000400530 <__do_global_dtors_aux>:
  400530:   80 3d 11 0b 20 00 00    cmpb   $0x0,0x200b11(%rip)        # 601048 <__TMC_END__>
  400537:   75 11                   jne    40054a <__do_global_dtors_aux+0x1a>
  400539:   55                      push   %rbp
  40053a:   48 89 e5                mov    %rsp,%rbp
  40053d:   e8 7e ff ff ff          callq  4004c0 <deregister_tm_clones>
  400542:   5d                      pop    %rbp
  400543:   c6 05 fe 0a 20 00 01    movb   $0x1,0x200afe(%rip)        # 601048 <__TMC_END__>
  40054a:   f3 c3                   repz retq 
  40054c:   0f 1f 40 00             nopl   0x0(%rax)

0000000000400550 <frame_dummy>:
  400550:   48 83 3d c8 08 20 00    cmpq   $0x0,0x2008c8(%rip)        # 600e20 <__JCR_END__>
  400557:   00 
  400558:   74 1e                   je     400578 <frame_dummy+0x28>
  40055a:   b8 00 00 00 00          mov    $0x0,%eax
  40055f:   48 85 c0                test   %rax,%rax
  400562:   74 14                   je     400578 <frame_dummy+0x28>
  400564:   55                      push   %rbp
  400565:   bf 20 0e 60 00          mov    $0x600e20,%edi
  40056a:   48 89 e5                mov    %rsp,%rbp
  40056d:   ff d0                   callq  *%rax
  40056f:   5d                      pop    %rbp
  400570:   e9 7b ff ff ff          jmpq   4004f0 <register_tm_clones>
  400575:   0f 1f 00                nopl   (%rax)
  400578:   e9 73 ff ff ff          jmpq   4004f0 <register_tm_clones>

000000000040057d <main>:
  40057d:   55                      push   %rbp
  40057e:   48 89 e5                mov    %rsp,%rbp
  400581:   48 83 ec 20             sub    $0x20,%rsp
  400585:   89 7d ec                mov    %edi,-0x14(%rbp)
  400588:   48 89 75 e0             mov    %rsi,-0x20(%rbp)
  40058c:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
  400593:   83 7d ec 01             cmpl   $0x1,-0x14(%rbp)
  400597:   7f 11                   jg     4005aa <main+0x2d>
  400599:   bf 78 06 40 00          mov    $0x400678,%edi
  40059e:   e8 ad fe ff ff          callq  400450 <puts@plt>
  4005a3:   b8 00 00 00 00          mov    $0x0,%eax
  4005a8:   eb 37                   jmp    4005e1 <main+0x64>
  4005aa:   48 8b 45 e0             mov    -0x20(%rbp),%rax
  4005ae:   48 83 c0 08             add    $0x8,%rax
  4005b2:   48 8b 00                mov    (%rax),%rax
  4005b5:   be 9d 06 40 00          mov    $0x40069d,%esi
  4005ba:   48 89 c7                mov    %rax,%rdi
  4005bd:   e8 ae fe ff ff          callq  400470 <strcmp@plt>
  4005c2:   85 c0                   test   %eax,%eax
  4005c4:   75 0c                   jne    4005d2 <main+0x55>
  4005c6:   bf ab 06 40 00          mov    $0x4006ab,%edi
  4005cb:   e8 80 fe ff ff          callq  400450 <puts@plt>
  4005d0:   eb 0a                   jmp    4005dc <main+0x5f>
  4005d2:   bf bc 06 40 00          mov    $0x4006bc,%edi
  4005d7:   e8 74 fe ff ff          callq  400450 <puts@plt>
  4005dc:   b8 00 00 00 00          mov    $0x0,%eax
  4005e1:   c9                      leaveq 
  4005e2:   c3                      retq   
  4005e3:   66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  4005ea:   00 00 00 
  4005ed:   0f 1f 00                nopl   (%rax)

00000000004005f0 <__libc_csu_init>:
  4005f0:   41 57                   push   %r15
  4005f2:   41 89 ff                mov    %edi,%r15d
  4005f5:   41 56                   push   %r14
  4005f7:   49 89 f6                mov    %rsi,%r14
  4005fa:   41 55                   push   %r13
  4005fc:   49 89 d5                mov    %rdx,%r13
  4005ff:   41 54                   push   %r12
  400601:   4c 8d 25 08 08 20 00    lea    0x200808(%rip),%r12        # 600e10 <__frame_dummy_init_array_entry>
  400608:   55                      push   %rbp
  400609:   48 8d 2d 08 08 20 00    lea    0x200808(%rip),%rbp        # 600e18 <__init_array_end>
  400610:   53                      push   %rbx
  400611:   4c 29 e5                sub    %r12,%rbp
  400614:   31 db                   xor    %ebx,%ebx
  400616:   48 c1 fd 03             sar    $0x3,%rbp
  40061a:   48 83 ec 08             sub    $0x8,%rsp
  40061e:   e8 f5 fd ff ff          callq  400418 <_init>
  400623:   48 85 ed                test   %rbp,%rbp
  400626:   74 1e                   je     400646 <__libc_csu_init+0x56>
  400628:   0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
  40062f:   00 
  400630:   4c 89 ea                mov    %r13,%rdx
  400633:   4c 89 f6                mov    %r14,%rsi
  400636:   44 89 ff                mov    %r15d,%edi
  400639:   41 ff 14 dc             callq  *(%r12,%rbx,8)
  40063d:   48 83 c3 01             add    $0x1,%rbx
  400641:   48 39 eb                cmp    %rbp,%rbx
  400644:   75 ea                   jne    400630 <__libc_csu_init+0x40>
  400646:   48 83 c4 08             add    $0x8,%rsp
  40064a:   5b                      pop    %rbx
  40064b:   5d                      pop    %rbp
  40064c:   41 5c                   pop    %r12
  40064e:   41 5d                   pop    %r13
  400650:   41 5e                   pop    %r14
  400652:   41 5f                   pop    %r15
  400654:   c3                      retq   
  400655:   66 66 2e 0f 1f 84 00    data32 nopw %cs:0x0(%rax,%rax,1)
  40065c:   00 00 00 00 

0000000000400660 <__libc_csu_fini>:
  400660:   f3 c3                   repz retq 

Disassembly of section .fini:

0000000000400664 <_fini>:
  400664:   48 83 ec 08             sub    $0x8,%rsp
  400668:   48 83 c4 08             add    $0x8,%rsp
  40066c:   c3                      retq 

Memory address, hexadecimal and assembly. Let’s zoom in on the part the really is important to us:

  4005bd:    e8 ae fe ff ff           callq  400470 <strcmp@plt>   *** THIS LINE have the strcmp()
  4005c2:    85 c0                    test   %eax,%eax             ***
  4005c4:    75 0c                    jne    4005d2 <main+0x55>    *** JNE = Jump on Inequality
  4005c6:    bf ab 06 40 00           mov    $0x4006ab,%edi
  4005cb:    e8 80 fe ff ff           callq  400450 <puts@plt>
  4005d0:    eb 0a                    jmp    4005dc <main+0x5f>
  4005d2:    bf bc 06 40 00           mov    $0x4006bc,%edi
  4005d7:    e8 74 fe ff ff           callq  400450 <puts@plt>

Ok, let’s invert the logic on JNE:

  4005c4:    75 0c                    jne    4005d2 <main+0x55>

Before we do the trick let’s test the program:

./cracking-tutorial-2 I-do-not-know-the-password

Try again 

Now, let’s open the binary using vim:

vim cracking-tutorial-2

And then execute:

ESC + :%!xxd + ENTER You will see VIM change to HEX mode.

Next, search by the hexadecimal codes we want to replace:

ESC + /750c + ENTER

The coursor will stop exactly on the point we need to replace 75 to 74 that means je instead of jne

After changing you need to exit HEX MODE BEFORE SAVING..

ESC + :%!xxd -r + ENTER

ESC + :x to save and exit

Now final test:

./cracking-tutorial-2 rodolfo

Congratulations 

Hope you enjoyed.

Rodolfo

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