Just bought Cyberpower OR700 1U UPS (20lbs). Will I need to buy rack mount rails to also support the back or will the cage nuts at the front be enough to support the UPS?

I tend towards always putting heavy cantilever stuff on 4-post rails even if they're generic rails. Lighter 2-post stuff sags enough as it is.

I’ve got a 2U UPS on a rack with just the 4nuts and bolts. So I’d say yes. What’s your rack?

I put my UPSes on generic rails that support it from the sides. I just wasn’t comfortable with the cage nuts and bolts taking the full stress of the UPS in my rack.

I also have that UPS rack mounted, just the cage nut bolts. It doesn't concern me at all. I have it towards the bottom of the rack since its heavy(ish) but its also not too deep so there isn't a lot of flex on it.