Threat model: the network and SSH servers

This continues the threat model for the sync server.

Someone on the network

Every SSH connection, each jump host included, checks the host key. A changed key stops the connection, and only Replace key gets past it. Host certificates count only with a valid signature from a CA you trust and a matching name. Old, weak algorithms are off unless you turn them on for a host. Sync uses HTTPS only (plain HTTP only to the same computer), and carries end-to-end encrypted data anyway. Single sign-on and passkey sign-ins return only to the app that started them.

The limit is the first connection, which trusts the key you accept, as OpenSSH does. For anything important, compare the fingerprint with the server’s over another channel.

A malicious SSH server

What a server prints can’t run code in the app. Clipboard writes (OSC 52) are asked about and capped at 64 KiB, and clipboard reads are ignored. Terminal links open only after you confirm, and only http, https and mailto. Pasting strips control characters and asks before a multi-line paste. SFTP downloads keep file names inside the folder you chose. Agent forwarding is off unless you turn it on, and forwards Sternway’s keys, never your SSH agent.

The limit: a server you log in to can show you anything in the terminal. What you type and run there is yours.

Where to read more

• Threat model: teams and devices
• What leaves your device