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hacktricks/reversing-and-exploiting/linux-exploiting-basic-esp/arbitrary-write-2-exec/aws2exec-.dtors-and-.fini_array.md

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AWS2Exec - .dtors & .fini_array

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.dtors

{% hint style="danger" %} Nowadays is very weird to find a binary with a .dtors section. {% endhint %}

The destructors are functions that are executed before program finishes (after the main function returns).
The addresses to these functions are stored inside the .dtors section of the binary and therefore, if you manage to write the address to a shellcode in __DTOR_END__ , that will be executed before the programs ends.

Get the address of this section with:

objdump -s -j .dtors /exec
rabin -s /exec | grep “__DTOR”

Usually you will find the DTOR markers between the values ffffffff and 00000000. So if you just see those values, it means that there isn't any function registered. So overwrite the 00000000 with the address to the shellcode to execute it.

{% hint style="warning" %} Ofc, you first need to find a place to store the shellcode in order to later call it. {% endhint %}

.fini_array

Essentially this is a structure with functions that will be called before the program finishes, like .dtors. This is interesting if you can call your shellcode just jumping to an address, or in cases where you need to go back to main again to exploit the vulnerability a second time.

objdump -s -j .fini_array ./greeting

./greeting:     file format elf32-i386

Contents of section .fini_array:
 8049934 a0850408

#Put your address in 0x8049934

Note that this won't create an eternal loop because when you get back to main the canary will notice, the end of the stack might be corrupted and the function won't be recalled again. So with this you will be able to have 1 more execution of the vuln.

{% hint style="danger" %} Note that with Full Relro, the section .fini_array is made read-only. {% endhint %}

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