What is SMS-TAN?
The SMS-TAN procedure is or was one of the most widely used two-factor procedures. With this method, the user enters their mobile phone number with the various online services or credit institutions. If the user now wants to log in to their account or make a bank transfer, for example, they are asked for another factor. A code is sent by SMS to the relevant mobile phone number, which the user must then enter on the relevant website. Only then does the process continue. For enhanced security, the respective website can only accept this code for a certain period of time, such as one minute. 
What were the advantages of using SMS-TAN?
- Wide use
- SMS can reach any cell phone without additional software or app installations.
What were the disadvantages of using SMS-TAN?
- SMS-TAN procedure is considered comparatively insecure. Already switched off for online banking.
- The costs of the procedure were high for banks
In the meantime, the procedure has largely been switched off or converted to new, more secure procedures. The procedure posed an ever-increasing risk; the text messages could be intercepted by fraudsters or misused in the event of cell phone theft.
