I have a Posts model class that has a relation with Users model.
When I call API to get Posts with the owner of it (user), it returns with all user info including password.
How to prevent model from returning sensitive information like passwords?
You should create new classes that you return from your actions. Only include the fields/information you want to return. These classes are also known as Data Transfer Objects (DTO).
You can use [JsonIgnore] to avoid serializing the property value:
public class Users
{
public int Id { get; set; }
[System.Text.Json.Serialization.JsonIgnore]
public string Password{ get; set; }
//...
}