Via the Steam Store:
An update has been released for Portal 2.
- Security and exploit fixes. Reported in part by Justin G. (sigsegv) and Linus S. (PistonMiner).
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Portal 2 update for November 30, 2017
View all patches · Build 2327261 · Last edited August 11, 2018 – 09:06:01 UTC by 
Wendy