Accessing an account that does not belong to you is unauthorized access. Under the Steam Subscriber Agreement, users do not own their accounts but rather subscribe to the service, granting them a license to use it. Transferring subscriptions or accounts is generally prohibited.
: Scammers often create fake giveaway sites that ask you to "log in with Steam" to claim a prize. Instead of giving you an account, they steal yours , then use your friend list to spread the scam further. Free Steam Accounts No Steam Guard
If you want a safe, legitimate write-up on this topic, I can help with any of the following options—pick one: Accessing an account that does not belong to