If you believe that your Steam account has been compromised, take the following steps immediately:
- Scan your computer for viruses, key loggers, spyware, and other malicious code that can steal your account data.
- If you are still able to log in, reset your Steam password by selecting “Change Password” in your Steam client setting/preferences.
- Reset the password for your email account associated with Steam.
- Revoke your Steam Web API key on https://steamcommunity.com/dev/apikey
- Deauthorize all other devices in Steam Guard: go to your Steam Account Details page > Manage Steam Guard > Deauthorize all other devices.
- Reset your Steam Trade URL: go to http://steamcommunity.com/my/tradeoffers/privacy and click “Create New URL” in the Third-Party Sites section.