Next time when you are throwing away a used boarding pass with a barcode on it, please consider tossing it into a shredder instead. Why?
Two-dimensional barcodes and QR codes can hold a great deal of information, and the codes printed on airline boarding passes may allow someone to discover more about you, your future travel plans, and your frequent flyer account.
Thanks to KrebsOnSecurity for this valuable info.
The standards for the boarding pass barcodes are widely available and have been for years. Check out this document (PDF) from the International Air Transport Association (IATA) for more on how the barcode standards work and have been implemented in various forms.
For the same reason, It’s also not a good idea posting your boarding pass on social networks like Facebook without blurring out the critical information such as barcodes. In some worst cases, it can get your account stolen.
Interested in learning what’s in your boarding pass barcode? Take a picture of the barcode with your phone, and upload it to this site. This blog on the same topic from several years back includes some helpful hints on how to decode the various information fields that get dumped by the barcode reader.