Websites that will extract information from a barcode or QR code are readily available for free on the Internet. Don't post barcodes or QR codes to social media or elsewhere on the Internet, as they can contain sensitive personal information.
The barcode of boarding passes contain lots of information - and in the case of some airlines, more information than what's printed on the boarding pass, including enough information to gain complete access to the frequent flyer account. With access to the account, one could make reservations, change the email address to retain control of the account, and view any passport numbers or addresses listed in the account.
I know it's popular to post pictures of your boarding passes to social media as you're traveling, but be careful about which part of the boarding pass you reveal: don't reveal the barcode or record locator (which can be used to cancel or modify your flight, even without access to the frequent flyer account).
Other Barcodes or QR Codes
Don't post other barcodes or QR codes either, such as those on concert tickets. It's only safe to post them if you extract the data from the barcode beforehand, but if you use an online barcode reader to do so, then you're giving that site the data in the barcode. Hence, it's safer to either adjust your photo so that the barcode is not in the photo, or edit it out.