This time we got both binary and the source code. Unfortunately PIE is enabled, so we don't know where our binary will be in the memory.
First interesting function is authenticate(). In fact it doesn't perform any authentication, it just sets the username.It's also not vulnerable to path traversals as they check if our username contains only [A-Za-z]. open_todos() opens the file in `todos/[username]` and we know that wintermuted uses this application to store his password, so maybe we could just guess his username and read his todos?
In menu we can select 6 options: `print all`, `print`, `add`, `remove`, `admin` and `quit`. This time admin function does nothing, so it's not our goal to get there.On `quit` our todos are written to file. But store_todo() and print_todo() seem vulnerable. They both contains the following code.```cint idx = read_int();if (idx > TODO_COUNT) {	puts(OUT_OF_BOUNDS_MESSAGE);	return;}```  In C int is signed, but they only check if it is greater than TODO_COUNT and don't check if it's less than 0. We can enter any negative number and read/write the memory before our todos.Our todos are in .bss section and before them there is username variable and the file descriptor. Unfortunately the file is opened only once at the beginning of the program and we can't use it for path traversal..data section does not seem to contain anything useful eiter, but before .data we have .got.plt and it can be useful.TODO_LENGTH is 0x30, so we can specify address we want to read/write only with this precision. With python we can print the closest addresses we can get.```>>> for i in range(10):...  print(hex(0x203140-0x30*i))0x2031400x2031100x2030e00x2030b00x203080 (open)0x203050 (strncat)0x203020 (write)0x202ff00x202fc00x202f90```According to [this website](https://systemoverlord.com/2017/03/19/got-and-plt-for-pwning.html) we'll have either the function address or the address in .plt if it hasn't been resolved yet.
So if the function was called we get some address in libc and in other case we get address of .plt section in our binary.We don't have the libc used on server, so it may cause some problems to find something there. open() and strncat() are both called before we can input anything, but write function is called on exit.
With the address of write() we can calculate of .plt entry for system. We can use it address to overwrite atoi address. (atoi is the best, because it is used every time when we enter our choice in menu and it gets the string with our input).
[exploit](https://github.com/BOAKGP/CTF-Writeups/blob/master/Google%20CTF%202018%20Quals%20Beginners%20Quest/Fridge%20todo%20list/exploit.py)
When we get shell on the server we can list all files in /todos directory. The wintermuted nickname is `CountZero`. We can just log in as him to get all his notes. 
Flag: `CTF{goo.gl/cjHknW}`