Apple stopped signing iOS 8 for the iPhone 5 within months of iOS 9’s release in 2015. Today, nearly a decade later, the only versions of iOS that Apple continues to sign for the iPhone 5 are the final builds of iOS 10—specifically 10.3.3 and 10.3.4. Without that signature, your iPhone 5, your computer, and any cable or software tool you own are powerless.
: A full downgrade will wipe all data on the device. Ensure you have a manual backup, as iOS 10 backups cannot be restored to iOS 8. downgrade iphone 5 to ios 8
: Using older iOS versions leaves the device vulnerable to unpatched security exploits. Apple stopped signing iOS 8 for the iPhone
The iPhone 5 has a 32-bit processor (A6 chip). It is susceptible to the "DeerTick" exploit used by Odysseus, but it is complex. : A full downgrade will wipe all data on the device
Furthermore, the iPhone 5’s hardware is perfectly capable of running its final OS, iOS 10. While slower than iOS 8, iOS 10.3.4 is at least patched against the GPS rollover bug (which affects iOS 9 and earlier) and offers a modicum of security.