QR codes are repeatedly pasted over at charging stations or vending machines, for example. However, they are sometimes also sent with letters. This is how people are tricked into entering personal information on fake websites or installing malicious apps. So check the target carefully. Be skeptical of stick-on codes. QR codes should be designed in such a way that they are forgery-proof. For example, they should be placed behind a pane of glass to prevent them from being replaced.