OK, so why is that any different than sharing your phone number, credit card, address, birth date, email address, answers to "security" questions, etc. With the websites, apps, or companies that I mentioned in my original list? Frankly why should I care if anybody knows my KTN? I would be a lot more concerned about my credit card data than my KTN. KTN only helps somebody that is booking travel as me, with the associated other data and biometrics. (Not likely.) I am genuinely curious if you know of some risk here that I am not aware of, or if this is just a generic concern because it feels like it is the equivalent of your passport and therefore special. And if it is the latter I would suggest the feeling is unfounded in genuine risk.