According to The Investor, a Korea-based publication, the Samsung Galaxy S8 will sport a rear-facing fingerprint scanner because Samsung and Synaptics ran out of time. The Galaxy S8 was supposed to feature an in-display fingerprint scanner, but the two companies simply did not have enough time to implement it there, so Samsung opted to include it on the back of the device, which might explain a rather odd placement of said fingerprint scanner.
We've seen a ton of Galaxy S8 leaks thus far, and in pretty much every leak of the phone's back side that leaked in the last couple of weeks, you've been able to see that the fingerprint scanner is placed next to the phone's rear-facing camera. That is a rather odd placement of the fingerprint scanner, mainly because the Galaxy S8 comes with a rather large 5.8-inch display, and despite the fact it offers really thin bezels, it's not exactly a tiny smartphone, and reaching that fingerprint scanner might be somewhat difficult. That problem becomes even bigger when it comes to the Galaxy S8 Plus which is expected to sport a 6.2-inch panel, and the placement of the fingerprint scanner remains the same. Now, regarding an in-display fingerprint scanner, Synaptics showed off that tech last year, but it seems they were unable to implement it in the Galaxy S8 as fast as it was expected. It's worth mentioning that the upcoming iPhone is also rumored to sport such technology, but we'll see what happens.
That being said, the Galaxy S8 is rumored to ship with both an iris scanner, and a facial recognition technology, so in addition to the rear-facing fingerprint scanner, we'll have plenty of options when it comes to security. It remains to be seen how well will the facial recognition technology function, but it seems like Samsung is quite confident in such tech, at least according to the latest rumors, as the company still did not confirm anything of the sort. Samsung did, however, confirm that the Galaxy S8 will land during their Unpacked event on March 29, and chances are that the 'Plus' variant will be announced alongside the Galaxy S8, but we'll see.