Recently, it became public that the source code of the Source Engine (in which TF2 and CS:GO were written) was leaked. The leaked source code seems to be a snapshot of their repository in which the developers store their code. It is believed to be from 2017-2018 (thus not the most recent up-to-date code).

What does this source code leak mean for me?

First of all, Valve had some previous source code leaks of the Source engine. Other developers might be interested in this code to learn how the engine works, and use this code for their own project to build upon the existing code. The leak in itself does not have any impact on the game itself. The source code does not contain any credentials which can be abused. Leaking the source code does not directly lead to any kind of security problem.

There were various reports of exploits/vulnerabilities, is it still safe to play the game?

Short answer: it is safe to play the game. The source code leak does not directly lead to all kind of exploits an security vulnerabilities in the game (as we tweeted yesterday). Today, the TeamFortress team also confirmed this via their twitter account.

But.. there were various claims that an RCE was found in the game, is it really safe?

What exactly is an RCE? The definition is "Remote Code Execution", and is a common way to describe a security vulnerability which can be abused to execute code on a remote machine (e.g. a hacker can execute anything they want on your computer).

Various reddit posts, images of alerts etc. were floating around after the source code was leaked. These are all false claims of an RCE being present in TF2. Leaking the source code does not directly lead to security problems. Note that no program is 100% secure, and maybe security vulnerabilities can be found in the game. This can be done with the help of the source code, but this isn't a requirement for finding one at all. Also note that security vulnerabilities are usually applicable to very specific situations, thus further decreasing the potential impact of abuse. Also note that Valve has an extensive Bug Bounty program, where anyone can report potential security vulnerabilities, with rewards up to $20.000.

In summary: The source code leak does not directly lead to an unsafe situation, and you can play TF2 as much as you like. The TF team also confirmed this themselves. No game/program will be 100% secure -ever- and the source code leak will not influence this fact in a major way. The reports of an exploitable security vulnerability are fake, and there is no reason why you should avoid playing on TF2 servers (casual or community) at the moment.