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Any chance someone who knows more assembly than me can explain how the symbol names for dlsym() are retrieved?

ie I would have expected to see 'ptrace', 'scanf' and 'printf' in the strings output, but they must be obfuscated in some way (otherwise I guess there's no point using the dlopen/dlsym trick at all.)

I only see one call to dlsym (at 8048506), so it seems to me the program is doing something tricky to build each symbol name string and then calling a routine there to dlsym() it.

That's about where my x86-fu fails me, though, and I remember I should be working on other things. :/




The function names are hidden in the .text section; each character is xored with 0x55. You can see the xoring here:

  80484e0: 83 f2 55              xor    $0x55,%edx
The encoded strings are:

  >>> def ascii_to_xored_hex(s, xorval):
  ...   return ''.join(['%02x' % (ord(c) ^ xorval) for c in s])
  ... 
  >>> ascii_to_xored_hex('ptrace', 0x55)
  '252127343630'
  >>> ascii_to_xored_hex('printf', 0x55)
  '25273c3b2133'
They're hidden in plain sight!

  mrj10@mjlap:~/Downloads$ xxd hackme | grep 2521
  0000680: 008d 7600 2521 2734 3630 0090 2636 343b  ..v.%!'460..&64;
  mrj10@mjlap:~/Downloads$ xxd hackme | grep 2527
  0000690: 3300 6690 2527 3c3b 2133 0090 6afb 4c8d  3.f.%'<;!3..j.L.

  To the disassembler, these strings look like and-xor sequences.  e.g., for 'ptrace':

   8048684: 25 21 27 34 36        and    $0x36342721,%eax
   8048689: 30 00                 xor    %al,(%eax)
As you can see from the hexdump, these did show up when he ran strings (e.g., %!'460 and %'<;!3 ), they just weren't recognizable.


Nice catch! I didn't notice this because I restricted my search to between the printf calls. The strings output makes more sense now!


Neat, thanks. :)

I have a hardcopy of Paul Carter's "PC Assembly Language" book at home that I started reading once but never finished. Some day... :/




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