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https://npipe.io/

Share files between devices on the same network.

How this works

Devices behind the same router share a public IP address (NAT). The server uses this to enable file sharing between them: devices can download files when they have the same source IP address as the uploader.

I'd consider this abuse of NAT, and not how the internet should be used, but it works in most cases.

Is this private?

If you mean privacy from others:

A little bit. A trusted reverse proxy makes sure that the source IP address is set based on the TCP connection. Attackers can spoof this address, but to my knowledge they won't receive the response.

As far as I can tell, the biggest vulnerability here is CG-NAT. ISPs assign the same public IP address to multiple customers (with different ports). So if two customers share the same IP address happen to visit npipe.io, they will be able to download each other's files.

VPN users share the same exit node will encounter the same problem.

If you mean privacy from me:

Files are never written to disk and are automatically deleted after 5 minutes. However, don't trust me with your incriminating evidence. As far as you know, this is a CIA operation targeting you.

Is this illegal?

Probably