Don't ever share photos of the barcode on your boarding pass. These could easily be read by anyone with free software. They often contain personally identifiable information like contact details, passport numbers and driver license information, as well as your reservation number and frequent flier number. Cybercriminals can use this information for identity theft, unauthorized credit card accounts and fraudulent purchases.
Travel details can also be used to cancel or change your flights, or access your mileage points and use them for rewards which are nearly impossible to recover. Another common social engineering attack hackers may try is pretending to be an airline representative and requesting for credit card details or additional personal information about the traveler.
Beyond not sharing your boarding pass barcodes, keep in mind other basic cybersecurity rules when traveling. You should never share photos, check-in/location information, and status updates while you're still on vacation. It's best to share memories once you're back home.
Tell your friends and family before their next trip, resist the urge to take that boarding pass selfie!