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WWW2Exec - __malloc_hook & __free_hook

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Malloc Hook

As you can Official GNU site, the variable __malloc_hook is a pointer pointing to the address of a function that will be called whenever malloc() is called stored in the data section of the libc library. Therefore, if this address is overwritten with a One Gadget for example and malloc is called, the One Gadget will be called.

To call malloc it's possible to wait for the program to call it or by calling printf("%10000$c") which allocates too bytes many making libc calling malloc to allocate them in the heap.

More info about One Gadget in:

One Gadget

Note that hooks are disabled for GLIBC >= 2.34. There are other techniques that can be used on modern GLIBC versions. See: https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md.

Free Hook

This was abused in one of the example from the page abusing a fast bin attack after having abused an unsorted bin attack:

Unsorted Bin Attack

It's posisble to find the address of __free_hook if the binary has symbols with the following command:

gef➤  p &__free_hook

In the post you can find a step by step guide on how to locate the address of the free hook without symbols. As summary, in the free function:

In the mentioned break in the previous code in $eax will be located the address of the free hook.

Now a fast bin attack is performed:

  • First of all it's discovered that it's possible to work with fast chunks of size 200 in the __free_hook location:

    • If we manage to get a fast chunk of size 0x200 in this location, it'll be possible to overwrite a function pointer that will be executed

  • For this, a new chunk of size 0xfc is created and the merged function is called with that pointer twice, this way we obtain a pointer to a freed chunk of size 0xfc*2 = 0x1f8 in the fast bin.

  • Then, the edit function is called in this chunk to modify the fd address of this fast bin to point to the previous __free_hook function.

  • Then, a chunk with size 0x1f8 is created to retrieve from the fast bin the previous useless chunk so another chunk of size 0x1f8 is created to get a fast bin chunk in the __free_hook which is overwritten with the address of system function.

  • And finally a chunk containing the string /bin/sh\x00 is freed calling the delete function, triggering the __free_hook function which points to system with /bin/sh\x00 as parameter.

References

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