The bug

net.minecraft.server.rcon.thread.QueryThreadGs4 (Mojang name) can crash because it can incorrectly try to use a cached rules response even though there hasn't been one created yet.

The class is intended to cache rules responses and only generate new ones every 5 seconds. The problem is that the field used for this (lastRulesResponse) is initialized to 0 and the check whether the cached response should be used looks like this:

private byte[] buildRuleResponse(DatagramPacket datagramPacket) throws IOException {
    String[] arrstring;
    long l = Util.getMillis();
    if (l < this.lastRulesResponse + 5000L) {
        byte[] arrby = this.rulesResponse.toByteArray();
        byte[] arrby2 = this.getIdentBytes(datagramPacket.getSocketAddress());
        arrby[1] = arrby2[0];
        arrby[2] = arrby2[1];
        arrby[3] = arrby2[2];
        arrby[4] = arrby2[3];
        return arrby;
    }
    this.lastRulesResponse = l;
    this.rulesResponse.reset();
    ...
}

Util.getMillis() calls in the end System.nanoTime() which can return negative values so the method above would use the empty byte array of rulesResponse and an ArrayIndexOutOfBoundsException would be thrown.