passcode (incomplete)


#include <stdio.h>
#include <stdlib.h>

void login(){
	int passcode1;
	int passcode2;

	printf("enter passcode1 : ");
	scanf("%d", passcode1);

	// ha! mommy told me that 32bit is vulnerable to bruteforcing :)
	printf("enter passcode2 : ");
        scanf("%d", passcode2);

	if(passcode1==338150 && passcode2==13371337){
                printf("Login OK!\n");
                system("/bin/cat flag");
                printf("Login Failed!\n");

void welcome(){
	char name[100];
	printf("enter you name : ");
	scanf("%100s", name);
	printf("Welcome %s!\n", name);

int main(){
	printf("Toddler's Secure Login System 1.0 beta.\n");


	// something after login...
	printf("Now I can safely trust you that you have credential :)\n");
	return 0;	

We can see the passwords in plain text in the code: 338150 and 13371337. I tried entering those and ended up getting a segmentation fault:

I ran checksec on the program just to see what was there:

There was nothing too out of the ordinary for me here. I went back at the prompt for the question and noticed they mentioned something about a compiler warning. I compiled the code again to focus on the compiler warnings:

I learned online that scanf takes in a pointer to an input instead of the input itself. I think we might able to manipulate this by writing our passcodes in the name string. We can then point scanf to look for the pointer where that passcode is located. I fired up gdb with the pwndebug add-on to it. I made the first break in main. This was just to get the program going to see where the updated memory locations were of the functions and variables. I then disassembled the welcome function because that is where our input goes to:

I will break after scanf just to see what register is holding our data in it:

There seems to be 3 registers that are currently holding our information: rax, r9, and rsp. I tried multiple things after this but was unable to get anywhere. I then found this write-up of the same challenge which made me understand this a bit more: The first thing I had learned from this write-up was that the lea instruction and registers were a way to figure out what the variable is in a given context. In our case, we have the following:

After the data gets scanned, we can make a good guess that the value that just got scanned in will be moved to another register.

Now we have verified that this is the location of our variable's data. However, it is only temporary, since it gets moved due to the code still being run:

This write-up is incomplete.

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