Exploit exercises - Protostar - Heap 3

In the heap 3 task 3 portions of memory are allocated on the heap. Then 3 command-line arguments are copied without bounds checking and later the allocated memory is freed. We are supposed to exploit Doug Lea way of allocation and deallocation of memory. There is a nice online page with a description of the way how to exploit it.

Exploitation is based on the fact that while memory is being freed, if there is a free block next to the one that is being freed, free function merges two blocks and write some metadata at the calculated addresses. That means that we can make free function overwrite value at the user provided address after corrupting metadata of the allocated blocks. We should make free think that previous block is free and change metadata to influence address calculation. In the task blocks are allocated in a way that is pictured on the image:

Exploit exercises - Protostar - Format 4

Last level of the format section was more interesting. It supposed you to change code execution sequence. Immediately after printf is called there is a call to exit() function. But in order to reuse code in memory there is a PLT(procedure linkage table) that redirects execution to the place according the address of the specified function in GOT (Global Offset Table). Since .plt section in or binary is read-only and jump is made after address dereference we have to change a value that this address references to our value. This article contains very detailed description of PLT, GOT, PIC(Position Independent Code).