Why don't you just look for what must be there to authenticate. I would use logic like this:
char tBuf[64] = "GET /?user=admin&pwd=mypasswd&submit=Log+In HTTP/1.1";
void setup() {
Serial.begin(9600);
if(strstr(tBuf,"user=admin") != NULL && strstr(tBuf,"pwd=mypasswd") !=NULL) {
Serial.println("Logged in");
// do the rest of your stuff here
}
else Serial.println("Login failed");
}
void loop() {
}