As long as you are playing at an online casino that is legal and of good reputation, you can be assured that any details you provide are always secure. The casino will be licensed by a respectable jurisdiction, have an eCOGRA seal of approval, and accept only the safest payment methods.
As part of the verification process, online casinos sometimes ask players to scan or fax a copy of both sides of your credit card. Legit online casinos ask you to cover or blot out the three-digit CVC code that verifies a credit card. Give out your card number, but not the CVC code.