[Tedgp908]

TF2, HL:2 and Gmod are very, very similar, especially on the network side. Plug up those firewalls!

From meepen (ex moat dev):

“Hey guys! If you didn't hear, some source code was leaked over last night. This source code includes HL2, CS:GO, CS:S, Portal, Portal 2, DoD:S and some Half-Life games
What does this mean for Garry's Mod? Is it still safe to play on Garry's Mod servers? The source code that was leaked was entirely owned by Valve, not Facepunch. As such only the base of the engine (TF2 and CS:S) were leaked. However just because they were leaked does not mean too much for Garry's Mod, as the source code for CS:S was leaked in 2007, and Garry's Mod has ran on the updated version of that engine for the entire time.
Just because source code was leaked for a game doesn't mean that all kinds of ways to get viruses open up, it just means people will find vulnerabilities easier without as much work or skill. With that said, the chance of someone finding a wormable exploit (like the cough virus long ago) is increased. This does not mean it will be used for nefarious purposes; especially this quickly. To find a wormable exploit means one thing, creating the worm is another.”

This is 90% correct. Just because source code is leaked doesn't inherently mean anything running the Source engine is vulnerable. There is no difference in writing code for a closed source/open source project. Especially with how large Valve is and how common the Source Engine, they will go to extreme lengths to ensure the security of the engine.  Everyone here on a daily basis uses some elements of open source code every day without the threat of it being compromised.

I disagree with this guy's claim that you can now find a vulnerability without much work or skill; to find a vulnerability in something like this you need to understand what is going on in the code. Which cannot be done without significant skill.