t3code: harden dispatch — dedicated user + validated t3-mint + scoped sudoers

Run t3-dispatch as an unprivileged dedicated user instead of wizard (who has
full sudo). Privileged minting goes through /usr/local/bin/t3-mint, which
validates the target against /etc/ttyd-user-map before minting as that user;
sudoers permits t3-dispatch to run only that wrapper. Compromise of the
network-facing service can mint pairing tokens for mapped users at most.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/sudoers-t3-autopair

@@ -0,0 +1,6 @@
+# The t3-dispatch service (unprivileged user t3-dispatch) may run ONLY the
+# t3-mint wrapper, as root. t3-mint validates the target user against
+# /etc/ttyd-user-map and mints a one-time t3 pairing token as that user.
+# A compromise of the network-facing dispatch service can therefore mint
+# pairing tokens for already-mapped users at most — never arbitrary root.
+t3-dispatch ALL=(root) NOPASSWD: /usr/local/bin/t3-mint

scripts/t3-dispatch.service

@@ -4,7 +4,9 @@ After=network.target
 
 [Service]
 Type=simple
-User=wizard
+# Unprivileged dedicated user; the only privileged action is `sudo t3-mint`
+# (scoped in /etc/sudoers.d/t3-autopair). Compromise => mint tokens at most.
+User=t3-dispatch
 ExecStart=/usr/local/bin/t3-dispatch
 Restart=on-failure
 RestartSec=5

scripts/t3-dispatch/main.go

@@ -62,9 +62,10 @@ func lookup(ak string) (entry, bool) {
 // user, via the scoped sudoers entry) and exchanges it at the instance's
 // /api/auth/bootstrap, relaying the returned t3_session Set-Cookie to the browser.
 func autoPair(e entry, w http.ResponseWriter, r *http.Request) {
-	out, err := exec.Command("sudo", "-n", "-u", e.OsUser,
-		"t3", "auth", "pairing", "create",
-		"--base-dir", "/home/"+e.OsUser+"/.t3", "--ttl", "5m", "--json").Output()
+	// t3-mint (root, via scoped sudoers) validates the OS user is in
+	// /etc/ttyd-user-map, then mints as that user. The dispatch service itself
+	// runs unprivileged and can invoke nothing else.
+	out, err := exec.Command("sudo", "-n", "/usr/local/bin/t3-mint", e.OsUser).Output()
 	if err != nil {
 		log.Printf("mint for %s failed: %v", e.OsUser, err)
 		http.Error(w, "pairing mint failed", http.StatusInternalServerError)

scripts/t3-mint

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Mint a one-time t3 pairing token for a mapped OS user.
+# Runs as root via the scoped sudoers entry for the t3-dispatch service user.
+# Validates the requested user is an actual t3 OS user (a value on the RHS of
+# /etc/ttyd-user-map) before minting as that user. Prints the t3 CLI JSON.
+set -euo pipefail
+os_user="${1:-}"
+[[ "$os_user" =~ ^[a-z_][a-z0-9_-]{0,31}$ ]] || { echo "invalid user" >&2; exit 2; }
+# Must be a mapped t3 OS user (RHS of a non-comment "authentik=os" line).
+awk -F= '!/^[[:space:]]*#/ && NF==2 { gsub(/[[:space:]]/, "", $2); print $2 }' /etc/ttyd-user-map \
+  | grep -qxF "$os_user" || { echo "user not mapped" >&2; exit 3; }
+exec runuser -u "$os_user" -- /usr/bin/t3 auth pairing create \
+  --base-dir "/home/${os_user}/.t3" --ttl 5m --json