Updates to Left 4 Dead 2 have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:
New mutation: Four Swordsmen of the Apocalypse
By default in single player and co-op gamemodes custom content will eclipse game content.
Addon campaigns custom content now loads first to avoid conflicts.
In competitive modes addon loading is a server option.
Fixed an exploit that allowed people to modify materials on pure servers.
Default consistency checks now include pak01_dir.vpk. Addon authors should not hack the pak01_dir.vpk directly but instead include content in a VPK.
The "whitelist.cfg" file located in the "steamapps\common\left 4 dead 2\left4dead2" directory is now functional. Instructions on how to use this file are located in the file itself. Basically this allows the server operators to decide what files need consistency checks.