> A straightforward way to exploit an implementation differential like this is if you have a software system that contains two different implementations of IDNA

> A straightforward way to exploit an implementation differential like this is if you have a software system that contains two different implementations of IDNA 2003 processing user input. Apparently, if this behavior depends on Unicode version, you have a bug over time even if it's a single implementation, *if you process the same input twice*. The correct fix is to process it once and store the result.