Depending on your situation, you might also want to tell them your name, seat assignment, cabin class, or any other personal information which they legitimately need to know. In general, there is no good reason to provide anyone with your booking code or confirmation number (or whatever your airline calls it).

You should be extremely cautious and generally avoid sharing your flight confirmation number (also known as a PNR or Record Locator) publicly or with people you do not trust. This 6-digit alphanumeric code is the "master key" to your entire travel reservation. Anyone with this number and your last name can log into the airline's "Manage Booking" portal. Once inside, they can see your full legal name, date of birth, passport details, contact information, and payment methods. More dangerously, they could potentially change your seat, cancel your flight, or even re-route your journey without your permission. In the age of social media, many travelers accidentally expose this information by posting photos of their boarding passes or computer screens; hackers can easily harvest this data to steal frequent flyer miles or commit identity theft. If you need to share your flight details with friends or family for coordination, it is much safer to send a screenshot of just the flight times and flight numbers while blacking out the confirmation code and barcode. Treat your PNR with the same level of security as you would your bank account password.