whosthere
A ssh server that knows who you are.
Try it (it's harmless)
ssh whoami.filippo.io
How it works
When it tries to authenticate via public key, ssh sends the server all your public keys, one by one, until the server accepts one. One can take advantage of this to enumerate all the client's installed public keys.
On the other hand, GitHub allows everyone to download users' public keys (which is very handy at times). Ben Cox took advantage of that and built a dataset of all GitHub public keys.
This is a pretty vanilla golang.org/x/crypto/ssh Go server that will advertise (publickey,keyboard-interactive) authentication. It won't accept any public key, but it will take a note of them. Once the client is done with public keys, it will try keyboard-interactive, which the server will accept without sending any challenge, so that no user interaction is required.
Then it just lets you open a shell+PTY, uses the public keys and Ben's database to find your username, asks the GitHub API your real name, prints all that and close the terminal.
All the interesting bits are in server.go.
How do I stop it?
If this behavior is problematic for you, you can tell ssh not to present your public keys to the server by default.
Add this line to your ~/.ssh/config
IdentitiesOnly yes
And then specify what keys should be used for each host
Host example.com
IdentityFile ~/.ssh/id_rsa
If you want you can use different keys so that they can't be linked together
Host github.com
IdentityFile ~/.ssh/github_id_rsa