UPDATE: Valve issues a statement that CS:GO hasn't been jeopardized and it's safe to play online. The source code was from an outdated build and will not interfere with the current version of CS:GO.
Valve makes a ton from CS:GO. The game has multi-million dollar eSports tournaments every year and pulls in serious revenue via skins and microtransactions. It's a heavily-guarded moneymaker that Valve regulates with strict anti-cheat software. That could be jeopardized with today's leak.
Source code for CS:GO and Team Fortress 2 have leaked out to the public via 4Chan, which could have big ramifications for Valve's money-making franchises. The code is actually outdated, though: the CS:GO code leak dates back to 2017, and the TF2 code is from 2018. The worry is the code could be used to create hacks and mods that circumvent the anti-cheat software embedded into both games. The code itself is licensed out by Valve and isn't available to the public for this reason. Many services and mod sites have gone offline in response to the leak as to not provide hosting for any game-breaking content.
So how did this happen? It stems from a falling out between Valve News Network's Tyler McVicker and a disgruntled and "toxic" employee. The leaker previously worked at McVicker's Lever Softworks studio but was recently fired. According to Jaycie Eyrsdren, a game dev and Valve enthusiast, the leaker released the files to get back at McVicker. The spurned employee also released chat logs with McVicker to try and frame the YouTuber.
McVicker is in direct contact with Valve's legal team, and the leaker has been identified. This spat could have significant legal consequences, and Valve has yet to issue a public comment on the matter.