https://www.bleepingcomputer.com/news/security/fake-steam-skin-giveaway-site-steals-your-login-credentials/

A phishing scam has been found on the Steam website which advertises a ‘free giveaway’ if users follow a link and log-in to a fraudulent website with their Steam credentials. The link takes people to a landing page with a fake running chat down the left side, which hackers have created to look like site users having a message conversation, making it appear more legitimate.  To take part in the ‘giveaway’, users are asked to sign-in via an authentic-looking Steam login form, which will then steal their credentials and can be used to hijack their account.

To help avoid this kind of scam, you should always be cautious and check where you are entering your login details.  Most simply this can be done by checking the address of the site, for example this particular bogus site was hosted on giveavvay.com instead of the steampowered.com domain.  If your account has been compromised, then take steps to secure it such as changing the password and notifying your contacts.

Further advice can be found here.