About 12TB of Steam content from 2003-2013 is circulating as a torrent of old Steam2 depots: retail builds plus betas, prototypes, trailers, demos, and soundtracks. Valve has not commented.

Dataminers Gabe Follower and Scolcer say this was not a hack. Steam2's content endpoint was public until Valve switched to SteamPipe in 2013.

No hacking involved, just verified that everything in Steam2 Teraleak was obtained via a publicly accessible [API] endpoint. It's Valve's fault...

Gabe Follower, on X.

Reported Valve material so far: a playable Portal 2 build from 2009 (TF2 stand-in assets, cut GLaDOS and Cave Johnson lines), Left 4 Dead and CS:GO prototypes, assets from F-Stop (the canceled Portal prequel), and a weapon also seen in old Half-Life 2: Episode 3 footage. There is no confirmed Episode 3 game in the dump.

The archive is not Valve-only. Hidden Palace has listed obtainable third-party prototypes including Spec Ops: The Line, Civilization V, Mafia 2, and GTA 3. A depot name list is hundreds of titles wide; that is not the same as extracted playable builds. Tyler McVicker called the third-party slice more legally dangerous than Valve-only files.

Source: Ars Technica