There are reports that, in the wake of the Steam Summer Sale, there has been the biggest ever spike in VAC bans seen in the history of Steam. On 6th July Valve put in place 40,445 user bans according to the Steam Database.
As a reminder, Valve Anti-Cheat System (VAC) bans occur when a user has cheats detected on their system. It is an automated system designed to detect cheats which are often manifested as modifications to a game's core executable files and dynamic link libraries. Not all games support the VAC system but many popular competitive ones do.
The Steam Summer Sale ended on 5th July and it looks like a whole host of cheaters took advantage of the low pricing to try and set up new accounts and indulge in their cheating ways. Perhaps one or two reported success of re/gaining access to a certain game with their cheats enabled and many others bought in. Eurogamer theorises that would-be cheaters created new accounts with some keenly priced games to test the viability of cheats with their main accounts. Polygon thinks that a lot of the cheat probing was undertaken by CS:GO players. However, Steam's 'banhammer' somehow detected thousands of cheaters and neutralised over 40,000 of them on the following day.
Previously, the biggest day for bans was 15,227, during October last year. Of the 40,445 bans last Thursday just over 5,000 of them were in-game bans based upon complaint reports from fellow players.