In a statement posted on the CS:GO and Team Fortress 2 Twitter accounts, Valve said the source code in question is older, dating to 2017 — and that it was already part of an existing leak from 2018.

We have reviewed the leaked code and believe it to be a reposting of a limited CS:GO engine code depot released to partners in late 2017, and originally leaked in 2018.

Today's source code leak opens the possibility of similar attacks against CS:GO and TF2; however, exploits may take some time to develop and Valve may easily counteract them with game updates ...