echolot/server/internal/adminui/auth.go

// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// Package adminui serves the operator's web interface.
//
// Everything here is behind authentication, without exception. The previous arrangement — an
// unauthenticated listener kept safe by binding to loopback — worked exactly until the address
// changed, and then failed silently and publicly. Binding address is a deployment detail; it is
// not an access control, and this package does not treat it as one.
//
// Rendered server-side with html/template and no JavaScript. The pages are lists and forms; a
// framework would add a build step, a dependency tree and an update treadmill to a program that
// currently has none of those.
package adminui

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"strings"
	"time"

	"echo-lot.app/server/internal/adminauth"
	"echo-lot.app/server/internal/oidc"
	"echo-lot.app/server/internal/runs"
	"echo-lot.app/server/internal/store"
)

const (
	sessionCookie = "echolot_admin"
	stateCookie   = "echolot_oidc"
	csrfField     = "csrf"
)

// Server is the admin interface.
type Server struct {
	Store    *store.Store
	Runs     *runs.Store
	OIDC     *oidc.Verifier // admin client; nil when no IdP is configured
	Sessions *adminauth.Sessions
	Throttle *adminauth.Throttle

	// AdminUser is the break-glass username; the password hash lives in the store.
	AdminUser string
	// BaseURL is where this UI is reachable, for building the OIDC redirect. Must match the URI
	// registered at the IdP exactly.
	BaseURL string
	// ClientSecret authenticates the confidential admin client at the token endpoint.
	ClientSecret string
	// Secure marks cookies Secure. Off only for loopback HTTP, where there is no network to
	// intercept and browsers refuse Secure cookies over plaintext anyway.
	Secure bool

	// EnrollLink builds the §2.1 bootstrap link for a token. Injected rather than rebuilt here,
	// so the SPKI pin and public URL stay owned by the control server that actually knows them.
	EnrollLink func(token string) string
	// SelfTest and Version render on the dashboard.
	SelfTest func() any
	Version  string
}

// Handler builds the routes. Only /healthz is reachable without a session.
func (s *Server) Handler() http.Handler {
	mux := http.NewServeMux()

	// Unauthenticated: a health check that required a session would be no use to a monitor, and
	// it discloses nothing beyond "the process is up".
	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		fmt.Fprintf(w, `{"ok":true,"version":%q}`+"\n", s.Version)
	})

	mux.HandleFunc("GET /login", s.loginForm)
	mux.HandleFunc("POST /login", s.loginSubmit)
	mux.HandleFunc("GET /auth/start", s.oidcStart)
	mux.HandleFunc("GET /admin/callback", s.oidcCallback)
	mux.HandleFunc("POST /logout", s.logout)

	mux.HandleFunc("GET /", s.guard(s.dashboard))
	mux.HandleFunc("GET /devices", s.guard(s.devices))
	mux.HandleFunc("POST /devices/{id}/revoke", s.guard(s.revokeDevice))
	mux.HandleFunc("POST /enroll-tokens", s.guard(s.mintToken))
	mux.HandleFunc("GET /runs", s.guard(s.runsList))
	mux.HandleFunc("GET /runs/{device}/{id}", s.guard(s.runView))
	mux.HandleFunc("POST /runs/{device}/{id}/delete", s.guard(s.runDelete))

	return mux
}

// guard requires a valid session, and checks CSRF on anything that changes state.
func (s *Server) guard(h func(http.ResponseWriter, *http.Request, *adminauth.Session)) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		sess := s.session(r)
		if sess == nil {
			http.Redirect(w, r, "/login", http.StatusSeeOther)
			return
		}
		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			// SameSite=Lax already blocks cross-site form posts in current browsers, but this
			// is the control that does not depend on the browser being current.
			if !s.csrfOK(r, sess) {
				http.Error(w, "stale form — reload the page and try again", http.StatusForbidden)
				return
			}
		}
		h(w, r, sess)
	}
}

func (s *Server) session(r *http.Request) *adminauth.Session {
	c, err := r.Cookie(sessionCookie)
	if err != nil {
		return nil
	}
	sess, err := s.Sessions.Parse(c.Value)
	if err != nil {
		return nil
	}
	return sess
}

// csrfToken derives a per-session token. Derived rather than stored so it needs no server-side
// state and cannot drift out of sync with the session it belongs to.
func (s *Server) csrfToken(sess *adminauth.Session) string {
	sum := sha256.Sum256([]byte("csrf|" + sess.Subject + "|" + sess.Expires.String()))
	return base64.RawURLEncoding.EncodeToString(sum[:16])
}

func (s *Server) csrfOK(r *http.Request, sess *adminauth.Session) bool {
	if err := r.ParseForm(); err != nil {
		return false
	}
	return r.PostFormValue(csrfField) == s.csrfToken(sess)
}

func (s *Server) setSession(w http.ResponseWriter, subject, display string) {
	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookie,
		Value:    s.Sessions.Issue(subject, display),
		Path:     "/",
		HttpOnly: true, // the cookie is a bearer credential; script has no business reading it
		Secure:   s.Secure,
		SameSite: http.SameSiteLaxMode,
	})
}

func (s *Server) logout(w http.ResponseWriter, r *http.Request) {
	http.SetCookie(w, &http.Cookie{
		Name: sessionCookie, Value: "", Path: "/", MaxAge: -1,
		HttpOnly: true, Secure: s.Secure, SameSite: http.SameSiteLaxMode,
	})
	http.Redirect(w, r, "/login", http.StatusSeeOther)
}

// ---- local password ---------------------------------------------------------------------

func (s *Server) loginSubmit(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseForm(); err != nil {
		http.Error(w, "bad form", http.StatusBadRequest)
		return
	}
	// The delay is applied before the answer, so a wrong guess costs time whether or not the
	// username exists — the timing carries no information either way.
	if d := s.Throttle.Delay(); d > 0 {
		time.Sleep(d)
	}
	user := r.PostFormValue("username")
	pass := r.PostFormValue("password")

	cred := s.Store.LocalAdmin()
	if cred == nil || !cred.Verify(user, pass) {
		s.Throttle.Failed()
		slog.Info("admin login failed", "user", user, "from", clientIP(r))
		s.render(w, r, "login", map[string]any{
			"Error": "Incorrect username or password.",
			"OIDC":  s.oidcAvailable(),
		})
		return
	}
	s.Throttle.Succeeded()
	slog.Info("admin login", "user", user, "method", "local", "from", clientIP(r))
	s.setSession(w, "local:"+cred.Username, cred.Username)
	http.Redirect(w, r, "/", http.StatusSeeOther)
}

// ---- OIDC -------------------------------------------------------------------------------

func (s *Server) oidcAvailable() bool {
	return s.OIDC != nil && s.OIDC.Config().Enabled() && s.BaseURL != ""
}

// oidcStart redirects to the IdP with state and PKCE.
//
// PKCE even though this is a confidential client: it costs one hash and closes code interception
// independently of the secret, which is worth having when the redirect crosses a browser.
func (s *Server) oidcStart(w http.ResponseWriter, r *http.Request) {
	if !s.oidcAvailable() {
		http.Error(w, "no identity provider is configured on this server", http.StatusNotImplemented)
		return
	}
	d, err := s.OIDC.Discover(r.Context())
	if err != nil {
		http.Error(w, "identity provider unreachable: "+err.Error(), http.StatusBadGateway)
		return
	}
	state, verifier := randomToken(), randomToken()
	challenge := sha256.Sum256([]byte(verifier))

	// state and the PKCE verifier ride in one short-lived cookie: the callback must prove it
	// belongs to the browser that started the flow, or an attacker can feed us their own code.
	http.SetCookie(w, &http.Cookie{
		Name: stateCookie, Value: state + "." + verifier, Path: "/",
		HttpOnly: true, Secure: s.Secure, SameSite: http.SameSiteLaxMode, MaxAge: 600,
	})

	q := url.Values{
		"response_type":         {"code"},
		"client_id":             {s.OIDC.Config().ClientID},
		"redirect_uri":          {s.redirectURI()},
		"scope":                 {"openid profile email"},
		"state":                 {state},
		"code_challenge":        {base64.RawURLEncoding.EncodeToString(challenge[:])},
		"code_challenge_method": {"S256"},
	}
	http.Redirect(w, r, d.AuthorizationEndpoint+"?"+q.Encode(), http.StatusSeeOther)
}

func (s *Server) redirectURI() string {
	return strings.TrimRight(s.BaseURL, "/") + "/admin/callback"
}

func (s *Server) oidcCallback(w http.ResponseWriter, r *http.Request) {
	if !s.oidcAvailable() {
		http.Error(w, "no identity provider configured", http.StatusNotImplemented)
		return
	}
	c, err := r.Cookie(stateCookie)
	if err != nil {
		http.Error(w, "sign-in did not start here — try again from the login page", http.StatusBadRequest)
		return
	}
	http.SetCookie(w, &http.Cookie{Name: stateCookie, Value: "", Path: "/", MaxAge: -1})

	state, verifier, ok := strings.Cut(c.Value, ".")
	if !ok || state == "" || r.URL.Query().Get("state") != state {
		http.Error(w, "sign-in state did not match — start again", http.StatusBadRequest)
		return
	}
	code := r.URL.Query().Get("code")
	if code == "" {
		http.Error(w, "no authorization code returned: "+r.URL.Query().Get("error"), http.StatusBadRequest)
		return
	}

	idToken, err := s.exchange(r.Context(), code, verifier)
	if err != nil {
		slog.Info("admin oidc exchange failed", "err", err, "from", clientIP(r))
		http.Error(w, "could not complete sign-in", http.StatusBadGateway)
		return
	}
	claims, err := s.OIDC.Verify(r.Context(), idToken)
	if err != nil {
		slog.Info("admin oidc token rejected", "err", err, "from", clientIP(r))
		http.Error(w, "the identity token was not accepted", http.StatusForbidden)
		return
	}
	if !s.OIDC.IsAdmin(claims) {
		// Named explicitly: "you signed in but you are not an admin" is a different problem from
		// "your password is wrong", and the group is the thing to go and check.
		slog.Info("admin access denied: not in group", "account", claims.AccountID(),
			"want_group", s.OIDC.Config().AdminGroup, "have", claims.Groups)
		http.Error(w, fmt.Sprintf(
			"Signed in as %s, but that account is not in the %q group, so it cannot administer "+
				"this server.", claims.Display(), s.OIDC.Config().AdminGroup), http.StatusForbidden)
		return
	}
	slog.Info("admin login", "account", claims.AccountID(), "method", "oidc", "from", clientIP(r))
	s.setSession(w, claims.AccountID(), claims.Display())
	http.Redirect(w, r, "/", http.StatusSeeOther)
}

// exchange trades the authorization code for tokens at the IdP.
func (s *Server) exchange(ctx context.Context, code, verifier string) (string, error) {
	d, err := s.OIDC.Discover(ctx)
	if err != nil {
		return "", err
	}
	form := url.Values{
		"grant_type":    {"authorization_code"},
		"code":          {code},
		"redirect_uri":  {s.redirectURI()},
		"client_id":     {s.OIDC.Config().ClientID},
		"code_verifier": {verifier},
	}
	if s.ClientSecret != "" {
		form.Set("client_secret", s.ClientSecret)
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, d.TokenEndpoint,
		strings.NewReader(form.Encode()))
	if err != nil {
		return "", err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	resp, err := (&http.Client{Timeout: 15 * time.Second}).Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	body, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if resp.StatusCode != http.StatusOK {
		return "", fmt.Errorf("token endpoint: %s: %s", resp.Status, strings.TrimSpace(string(body)))
	}
	var tok struct {
		IDToken string `json:"id_token"`
	}
	if err := json.Unmarshal(body, &tok); err != nil {
		return "", err
	}
	if tok.IDToken == "" {
		return "", fmt.Errorf("token endpoint returned no id_token")
	}
	return tok.IDToken, nil
}

func randomToken() string {
	b := make([]byte, 32)
	_, _ = rand.Read(b)
	return base64.RawURLEncoding.EncodeToString(b)
}

// clientIP is for logs only. X-Forwarded-For is deliberately ignored: nothing is meant to sit in
// front of this listener, so a header claiming otherwise is a caller's assertion about itself.
func clientIP(r *http.Request) string {
	if i := strings.LastIndex(r.RemoteAddr, ":"); i > 0 {
		return r.RemoteAddr[:i]
	}
	return r.RemoteAddr
}