Unlink exploit

For this method to work u need to have

• A global pointer
• Heap overflow

Method to exploit :

1) Creation of fake chunk :

• create a fake chunk inside a chunk pointed by global pointer with prev_size , size , fd , bk .

• create fd and bk of fake chunk such that

  •   *(fd+12) = *(bk+8) = *(global ptr) = chunk_0
  •   With this we bypass the check P->fd->bk != P
    

• Set size of fake chunk as :

  •   chunk_0->size = *fd // prev_size
  •   this bypass the check made by libc chunksize(P) != fd->prev_size

2) Modifying chunk_1

• set chunk1->prev_size = size_requested

  • such that it points at start of fake chunk
  • this makes our chunk_0 to shrink
    

• set chunk1->size prev inuse bit 0

3) Unlink

• Now free(chunk_1)

• our fake chunk gets unlinked and causes to overwrite the global pointer

• We can now have arbitaray write