function AddUserPawn() {
  const Players = [...document.getElementsByClassName('Players')[0].children];
  Players.length -= 1;
  if (Players.length === 2) {
    document.getElementById('C0').style.display = 'block';
    document.getElementById('C1').style.display = 'block';
    document.getElementById('C2').style.display = 'none';
    document.getElementById('C3').style.display = 'none';
  } else if (Players.length === 3) {
    document.getElementById('C0').style.display = 'block';
    document.getElementById('C1').style.display = 'block';
    document.getElementById('C2').style.display = 'block';
    document.getElementById('C3').style.display = 'none';
  } else if (Players.length === 4) {
    document.getElementById('C0').style.display = 'block';
    document.getElementById('C1').style.display = 'block';
    document.getElementById('C2').style.display = 'block';
    document.getElementById('C3').style.display = 'block';
  } else {
    document.getElementById('C0').style.display = 'block';
    document.getElementById('C1').style.display = 'none';
    document.getElementById('C2').style.display = 'none';
    document.getElementById('C3').style.display = 'none';
  }
}
Authenticating a user, or otherwise establishing a new user session, without invalidating any existing session identifier gives an attacker the opportunity to steal authenticated sessions.