A vulnerability in the current server-masterserver protocol was found that can cause servers to not show up in the server list anymore. Use one of the following methods to protect your server(s):

1) Apply this patch to your source code.
2) Use this script to patch your binaries.
3) Use this iptables rules by m!nus on your server (NOTE: Adjust the port if you're not running on 8303!):

Code:

iptables -I INPUT -p udp --dport 8303 -m u32 --u32 '0>>22&0x3C@18=0x66776572' -j DROP

NOTE: If you're using method 2 or 3, you won't be able to detect NAT issues anymore, they'll show up as "WARNING: Master server is not responding, switching master".