This is mostly an awareness article that summarizes previously deployed techniques, and illustrates a situation where those techniques can cause maximum harm.

For background, a HN submission a year ago about these kinds of ads linking to phones, from Ars Technica [1], which some choice comments being one naming such an ad company [2] while a different subthread explores another [3][4].

Prior art in this space includes the work of Boris Smus [5], who then went on to develop the guest pairing mode for Chromecast using this technique. There have been other efforts over the years, some before, but certainly after, and of course the use of sound to transmit digital information is an old trick that makes modems possible, but in those days the lines didn't have the bandwidth to carry ultrasound.

[1] https://news.ycombinator.com/item?id=10562207 [2] https://news.ycombinator.com/item?id=10563384 [3] https://news.ycombinator.com/item?id=10563369 [4] https://news.ycombinator.com/item?id=10563031 [5] https://news.ycombinator.com/item?id=10562787