Sharif CTF Quals 2013 - Exploiting 100 - mellow

Task:

nc ctf.sharif.edu 61451

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We have a simple service that accepts our input and if it doesn't allow us to get a flag it shutdowns the connection.

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 - Heap 2

In this task we are able to control the sequence of malloc and free operations. Also the binary seems to be different from the source because binary allocates not enough memory for the auth struct. If you write auth 1, only 4 bytes are allocated for a whole 36 bytes auth structure. So if we are able to create another structure that has nonzero byte at 32-byte offset we will get a flag. Let's allocate service struct that has unlimited size and therefore can overwrite auth at offset 32 from auth pointer.

Exploit exercises - Protostar - Heap 1

In this task there is a bit more calls to malloc function. As we can see first strcpy is able to overwrite all other allocated data except first allocation. Data allocated in a way that is shown on the picture.

Next strcpy gets address from the allocated memory and write there second argument. So we can overwrite an arbitrary place in memory.
Since main function uses puts function to print something after second strcpy was finished and the fact that winner function doesn't use puts function, we can overwrite address in GOT for puts to move execution flow to the winner.

Exploit exercises - Protostar - Heap 0

Heap 0 level teaches us that variables that are placed in a heap are referenced by different addresses in memory far from stack. Two sequential allocations have a high probability to be one after the other in memory. We can exploit this by overflowing buffer.

Exploit exercises - Protostar - Format 0

This task was quite educating for me. It contained very simple buffer overflow vulnerability that should be done via format string in less than 10 bytes of input.

Exploit exercises - Protostar - Stack 5

After a lot of effort spend on trying to exploit this simple program (http://www.exploit-exercises.com/protostar/stack5) I've finally executed root shell.

Here are some difficulties that I have encountered: