Valve has begun issuing cease and desist letters to sites that utilise Steam services and its OpenID API to enable users to gamble against each other and on competitive games using in-game skins and items with real world value from games such as Counter Strike: GO and Dota 2.

23 websites – including CS:GO Lotto, CS:GO Big and Dota 2 Lounge – were among those sent letters. Last week Valve promised to stamp out gambling websites that allow users to verify their Steam accounts through an OpenID API and which create bot accounts to access the Steam marketplace and trade items which have been won or lost between players.

"Using the OpenID API and making the same web calls as Steam users to run a gambling business is not allowed by our API nor our user agreements," Valve's Erik Johnson said in a statement.

"We are going to start sending notices to these sites requesting they cease operations through Steam, and further pursue the matter as necessary."

Website CS:GO Big announced on Twitter that it would be "shutting down temporarily", also uploading a copy of the cease and desist letter it received so users would be aware which other sites have been sent notices and will be affected.

The validity of the letter was verified by Kotaku.

Another site affected, CS:GO 500, released a statement saying: "Our main goal here at CS:GO 500 is to provide an enjoyable service for our users that is both legitimate and interactive. This will continue to be our goal, as we plan on adjusting to the upcoming limitations set by Valve. We assure our users that an alternative is already being worked on, one that will comply with Steam's Terms of Service."

Players of Valve games like CS:GO and Dota 2 are able to purchase keys through official means to access crates that drop random cosmetic items. The rarity and desirability of these items is determined on the official marketplace of Valve's digital sales platform Steam, through which players can buy items with real-money and sell them for Steam store credit. These gambling sites enable users to bet these items against each other with winners determined by what amounts to a virtual coin-flip.

Valve's actions come during an ongoing lawsuit against them regarding the morally and legally dubious websites which it doesn't officially back but which it has allowed to exist up until this point. A recent scandal involving prominent YouTubers shined a spotlight on such websites, prompting Valve to act.

YouTube personalities ProSyndicate and TmarTN were discovered to be part-owners of CS:GO Lotto, a site they had been promoting in videos without disclosing their relationship to the website. Other YouTubers and Twitch streamers then revealed the extent of their relationship with the site and how videos promoting other sites had been rigged.

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