As in, would they be able to access your server?
From what I understand tailscale is basically wire guard but made convenient. And how they do that is by managing you wire guard keys for you. So I would have assumed they could use the keys to access your network. HOWever while trying to look into this just now I found out tailnet lock exist and it says “When tailnet lock is enabled, even if Tailscale infrastructure is malicious or hacked, attackers can’t send or receive traffic on your tailnet.”
If you do not trust Tailscale as a company, here is an open source re-implementation of the server called headscale. Some/all clients are open source as well. So, you can review all components yourself or pay for a professional third-party review. Otherwise, if you take a binary blob from any origin, including Tailscale, and have it run with privileges on your server, there are few limits on what this blob can do. Yes, backdoors are technically possible, but probably bad for Tailscale’s business if that ever came to light.
I’ve never heard of professional third-party review of open source code. That’s a service people offer?
The biggest downside, as I understand it, is that it’s difficult to convince others to use your tailnet
You can enable Funnel, which doesn’t require others to have the TS client.
Had no idea that existed, I wonder what the security is like
