Less than a day ago, there was a leak of the source code for previous versions of TF2 and CSGO, and there was a lot of reports that the TF2 source code contained a Remote Code Execution exploit, which theoretically would allow a hacker to take control of your PC by executing arbitrary code through TF2 whilst on the same server as you.

I'd like everybody to know that Valve has officially reviewed the leaked code and have commented that there is nothing to worry about. The source code that was leaked was related to code depots released to close Valve partners in late 2017, and it was actually originally leaked all the way back in 2018, meaning that the current build of TF2 is absolutely fine to play on.

There were also posts showcasing the RCE exploit affecting TF2 clients[pbs.twimg.com], but there's a high chance these were fake too, and created by people attempting to scaremonger.

tl;dr? TF2 is safe, guys. You're all fine, and you can all go and play on NGS again; there's absolutely no need to worry. If anybody has any questions or concerns, I'd be happy to answer them and hopefully dispel your fears.

More info for those interested:
Megathread created to discuss the source code leaks