http://revuln.com/files/ReVuln_Exploiti ... obbies.pdf
It's a paper about some vulnerabilities that affected the Steam lobbies used by many games like Counter Strike Global Offensive, Left for Dead 1 and 2, Borderlands 2, all the Codemasters games, Alien Swarm, Day Z and a lot of other games.
The effect is zero servers online, no multiplayer.
The attack is very easy to perform and quite silent, basically one single attacker can take down the multiplayer of all the affected games without particular bandwidth/network requirements.
All the bugs are now fixed but they were there for many years, the scary part is that they were extremely easy to find and exploit... just basic Steamworks programming.
Hope you will appreciate the reading.