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House of Einherjar

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Basic Information

Code

Goal

  • The goal is to allocate memory in almost any specific address.

Requirements

  • Create a fake chunk when we want to allocate a chunk:

    • Set pointers to point to itself to bypass sanity checks

  • One-byte overflow with a null byte from one chunk to the next one to modify the PREV_INUSE flag.

  • Indicate in the prev_size of the off-by-null abused chunk the difference between itself and the fake chunk

    • The fake chunk size must also have been set the same size to bypass sanity checks

  • For constructing these chunks, you will need a heap leak.

Attack

  • A fake chunk is created inside a chunk controlled by the attacker pointing with fd and bk to the original chunk to bypass protections

  • 2 other chunks (B and C) are allocated

  • Abusing the off by one in the B one the prev in use bit is cleaned and the prev_size data is overwritten with the difference between the place where the C chunk is allocated, to the fake A chunk generated before

    • This prev_size and the size in the fake chunk A must be the same to bypass checks.

  • Then, the tcache is filled

  • Then, C is freed so it consolidates with the fake chunk A

  • Then, a new chunk D is created which will be starting in the fake A chunk and covering B chunk

    • The house of Einherjar finishes here

  • This can be continued with a fast bin attack or Tcache poisoning:

    • Free B to add it to the fast bin / Tcache

    • B's fd is overwritten making it point to the target address abusing the D chunk (as it contains B inside)

    • Then, 2 mallocs are done and the second one is going to be allocating the target address

References and other examples

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