This change is going live today. Direct UDP heartbeats from Half-Life 1 servers will no longer be supported. You must update your server to get the latest Steam binaries, which support the new protocol.
We have released an update to Half-Life 1 dedicated servers. This update fixes a potential vulnerability in the challenge/response protocol uses for out of band queries (in particular A2S_RULES and A2S_PLAYERS responses). The update also fixes sprays not functioning correctly when new users join a server.
Also as a reminder, we will be disabling the older heartbeating protocol later this week so you need to update to be listed on the master server. This change only effect the server side, any clients (i.e server list generators or management tools) will be unaffected by this change.