[MC-111859] Movement Validation Wrong

The bug

The MC server contains code to the effect of:

  private static boolean b(PacketPlayInFlying paramPacketPlayInFlying) {
    if ((Doubles.isFinite(paramPacketPlayInFlying.a(0.0D))) && (Doubles.isFinite(paramPacketPlayInFlying.b(0.0D))) && (Doubles.isFinite(paramPacketPlayInFlying.c(0.0D))) && (Floats.isFinite(paramPacketPlayInFlying.b(0.0F))) && (Floats.isFinite(paramPacketPlayInFlying.a(0.0F)))) {
      return false;
    }
    if ((Math.abs(paramPacketPlayInFlying.a(0.0D)) > 30000000.0D) || (Math.abs(paramPacketPlayInFlying.c(0.0D)) > 30000000.0D)) {
      return false;
    }
    return true;
  }

Aside from not checking the magnitude of the Y coord, the first part of the NaN checks can be bypassed as long as magnitude < 3E7.

Sending NaN allows the server to be crashed.

Examples

Sending this packet triggers the watchdog:

new CPacketPlayer.Position(99_999_999, Double.NaN, 50, false)

Here are some samples of the watchdog crashes:

Sending this packet does not trigger the watchdog, although it does cause severe lag:

new CPacketPlayer.Position(Double.NaN, 64, 99_999_999, false)

It also generated 2 region files:

r.-1.58593.mca
r.0.58593.mca

Movement Validation Wrong

The bug

Examples

Sending this packet triggers the watchdog:

Sending this packet does *not* trigger the watchdog, although it does cause severe lag:

Sending this packet does not trigger the watchdog, although it does cause severe lag: