# Diary - Balsn 2020

tl;dr

• Overflow from stdin stucture till main_arena.
• Create fake fastbin chunks to get overlapping chunk and leak.
• Overwrite __malloc_hook using fastbin attack.

We were able to solve only one pwn challenge this time. According to the author the method explained in this post is an unintentded solution.

Challenge Points: 455
Solves: 18
Solved By: 3agl3

## Analysis

Binary was a standard menu driven heap program with usual mitigations.

## Reversing

First the program asks for a name of size 32 bytes which is stored on bss.
After this we are given 5 options:

• Show name: Prints the name.
• Write Diary: Creates a chunk using calloc (max size is 0x80). Size is stored as the first 4 bytes and the pointer is stored in a global array (Max 14 allocations).
• Read Diary: Prints the content of a chunk.
• Edit Diary: Edit the content of a chunk. Can be only done once.
• Tear out page: Frees a chunk and set a bit to 1. This bit will be used to prevent UAF.

Since calloc is used, any chunk that goes to tcache cannot be reused.

## Bugs

• Since the name is non null terminated and since it is stored just above the pointer table, we can get heap leak by calling Show name after creating atleast 1 chunk.
• In Edit Diary, the index can be negative.

## Exploitation

Since heap leak is easy, let’s do that.

Next we need a libc leak. The issue here is that the program uses calloc which nulls out the content of a chunk before giving it to the user. This does not happen if the mmap bit of the chunk is set. Let’s find a way to do that. For this, we will be using the second bug. We had several targets on bss at negative offset. Two of them were stdin and stdout pointers. We can’t use stdout since program uses puts and printf which will use vtable. So we are going to use stdin.
If we check using a debugger, we can see that main_arena lies after stdin meaning we can create fake chunks. Remember that we only have 1 edit. So we will have to set everything together.
We create a chunk with data that looks like a header of a chunk.

Now in the edit operation, we put the pointer to this fake chunk inside fastbin. Next time when we ask for a chunk of size 0x41, calloc returns our fake chunk. Using this, we can overwrite the size of next chunk while also setting the mmap bit. Now we just need to add another chunk and view it to get leak.

## Getting shell

To overwrite __malloc_hook, we need a fastbin which is poiting to this. Since we don’t have libc leak at the time of doing the edit, we have to create a fastbin that points to __malloc_hook. Actually no. Remember that our tcache of size 0x70 is currently empty. Meaning if we have a fastbin pointing to __malloc_hook and then allocates a chunk, the remaining chunks will be moved to tcache. Let’s try to fill tcache of size 0x70. Consider this case.

Here we have a chain of fake chunks which ultimately points to __malloc_hook. If we put chunk 1 in the fastbin and allocates a chunk, calloc sees that tcache is empty and start moving the chunks. When it reaches chunk 8, tcache is full and the remaining is kept in fastbin and chunk 1 is returned to the user. Next time when we allocates, we will get __malloc_hook and we can overwrite it with one_gadget. Next call to malloc will give us a shell.

## Conclusion

Even though this was not the intended solution, it was a really good heap challenge. Thanks to Balsn for such a ctf.
You can find the full exploit here