Computer security Question:

Download Questions PDF

You have been asked to review the source code for a compiled script that is being used to validate logon credentials for a web application. The file is called "logon_validate" and a typical logon request looks like this –

"GET /cgi-bin/logon_validate?login=test&password=test"

The source code is shown below –

void show_error(void) {

// AUTHENTICATION ERROR

exit(-1);

}

int main(int argc, char **argv) {
char error_on_auth='1';
char user[128];
char pass[128];
char *ch_ptr_begin;
char *ch_ptr_end;

/**********************************/
/* Get Username from Query String */
/**********************************/
ch_ptr_begin=(char *)strstr
(****QUERY_STRING****,"login=");
if (ch_ptr_begin==NULL)
show_error();
ch_ptr_begin+=6;
ch_ptr_end=(char *)strstr(ch_ptr_begin,"&");
if (ch_ptr_end==NULL)
show_error();
*(ch_ptr_end++)='';
strcpy(user,ch_ptr_begin);


/**********************************/
/* Get Password from Query String */
/**********************************/
ch_ptr_begin=(char *)strstr(ch_ptr_end,"password=");
if (ch_ptr_begin==NULL)
show_error();
ch_ptr_begin+=9;
ch_ptr_end=(char *)strstr(ch_ptr_begin,"&");
if (ch_ptr_end!=NULL) *(ch_ptr_end++)='';
strcpy(pass,ch_ptr_begin);


if ((strcmp(user,GOOD_USER)==0) &&
(strcmp(pass,GOOD_PASS)==0))
error_on_auth='0';

if (error_on_auth=='0') {

// AUTHENTICATION OK!!


} else {

// AUTHENTICATION ERROR
show_error();


}

// return(0); hehe could be evil ;PPPPP
exit(0);

}
This pseudo-code is taken from the NGSec Web Auth Games
http://quiz.ngsec.biz:8080/game1/level6/replicant.php

Do you see any problems with this script?
How could an attacker exploit this script to bypass
the authentication mechanisms in this script?
What are some mitigation options?




Note: Goal of question – This is most likely the most complex question being asked during the interview due to the fact that the applicant will need to apply multiple layers of analysis, including both the attacker and defender perspectives.