Derive cookie Secure and CSRF strictness from the request transport (closes #269)

internal/session/session.go

@@ -17,6 +17,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/logger"
+	"sneak.berlin/go/webhooker/internal/reqtls"
 )
 
 const (
@@ -84,10 +85,9 @@ type Params struct {
 
 // Session manages encrypted session storage.
 type Session struct {
-	store  *sessions.CookieStore
-	key    []byte // raw 32-byte auth key, also used for CSRF cookie signing
-	log    *slog.Logger
-	config *config.Config
+	store *sessions.CookieStore
+	key   []byte // raw 32-byte auth key, also used for CSRF cookie signing
+	log   *slog.Logger
 
 	// idleTimeout is the sliding inactivity window. A session that
 	// sees no authenticated request within this window expires,
@@ -104,6 +104,10 @@ type Session struct {
 // cookie. MaxAge is deliberately left at its zero value: for a store
 // it is set through CookieStore.MaxAge (see newStore), and for a
 // single session it is copied from the store's options.
+//
+// Secure is a parameter rather than a constant because it is the one
+// attribute here that is not a policy -- it is a fact about the
+// connection carrying this particular response. See applyTransport.
 func cookieOptions(secure bool) *sessions.Options {
 	return &sessions.Options{
 		Path:     "/",
@@ -121,14 +125,52 @@ func cookieOptions(secure bool) *sessions.Options {
 // Options never touches Codecs -- so a store configured that way still
 // decodes a 30-day-old cookie, leaving the cookie attribute and the
 // codec disagreeing about the same policy. store.MaxAge sets both.
-func newStore(key []byte, secure bool) *sessions.CookieStore {
+//
+// The store's Secure is fixed at true, and is only a template: every
+// write path overwrites it for the request in hand (applyTransport).
+// It is true rather than false so that a write path added later which
+// forgets to call applyTransport fails loudly -- the browser drops the
+// cookie over plaintext HTTP and the developer sees it immediately --
+// instead of silently shipping the authentication credential without
+// Secure, which is the exact failure this store already had once.
+func newStore(key []byte) *sessions.CookieStore {
 	store := sessions.NewCookieStore(key)
-	store.Options = cookieOptions(secure)
+	store.Options = cookieOptions(true)
 	store.MaxAge(secondsPerDay * sessionMaxAgeDays)
 
 	return store
 }
 
+// applyTransport sets the session cookie's Secure attribute from the
+// transport of the request being answered.
+//
+// This is decided per-request, not once at startup. Deciding it at
+// startup from the configured environment is what this replaces, and
+// it got the DEFAULT posture wrong: "dev" is the environment when
+// WEBHOOKER_ENVIRONMENT is unset, so a deployment terminating TLS at a
+// proxy without also setting the environment emitted the
+// authentication cookie with no Secure attribute -- silently, and on
+// the same response as a CSRF cookie that did have one.
+//
+// gorilla/sessions makes this cheap and local: CookieStore.New gives
+// every session its own copy of the store's Options, and
+// CookieStore.Save renders the cookie from that copy rather than from
+// the store. So the flag is set on the one session being saved,
+// without a second store and without reaching across concurrent
+// requests.
+//
+// The flag tracks the transport in BOTH directions rather than being
+// latched on once seen. Secure on a plaintext response is worse than
+// useless: the browser discards such a cookie without any error, so a
+// latched flag would make a plain-HTTP local run impossible to log
+// into. It is also why every write path must call this, including the
+// deletion cookies in Destroy and Regenerate -- a Secure deletion
+// cookie sent over plaintext is dropped too, leaving the session the
+// caller believed it had just revoked.
+func applyTransport(r *http.Request, sess *sessions.Session) {
+	sess.Options.Secure = reqtls.IsTLS(r)
+}
+
 // New creates a new session manager. The cookie store is
 // initialized during the fx OnStart phase after the database is
 // connected, using a session key that is auto-generated and stored
@@ -139,7 +181,6 @@ func New(
 ) (*Session, error) {
 	s := &Session{
 		log:         params.Logger.Get(),
-		config:      params.Config,
 		idleTimeout: params.Config.SessionIdleTimeout,
 		now:         time.Now,
 	}
@@ -172,7 +213,7 @@ func New(
 			}
 
 			s.key = keyBytes
-			s.store = newStore(keyBytes, !params.Config.IsDev())
+			s.store = newStore(keyBytes)
 			s.log.Info("session manager initialized")
 
 			return nil
@@ -196,12 +237,16 @@ func (s *Session) GetKey() []byte {
 	return s.key
 }
 
-// Save saves the session.
+// Save saves the session. Every session-cookie write in the
+// application goes through here or through Regenerate, which is what
+// makes applyTransport a complete answer rather than a best effort.
 func (s *Session) Save(
 	r *http.Request,
 	w http.ResponseWriter,
 	sess *sessions.Session,
 ) error {
+	applyTransport(r, sess)
+
 	return sess.Save(r, w)
 }
 
@@ -340,6 +385,7 @@ func (s *Session) Regenerate(
 	// Destroy the old session
 	oldSess.Options.MaxAge = -1
 	s.ClearUser(oldSess)
+	applyTransport(r, oldSess)
 
 	err := oldSess.Save(r, w)
 	if err != nil {
@@ -368,7 +414,7 @@ func (s *Session) Regenerate(
 	// Apply the standard session options (the destroyed old
 	// session had MaxAge = -1, which store.New might inherit
 	// from the cookie).
-	newSess.Options = cookieOptions(!s.config.IsDev())
+	newSess.Options = cookieOptions(reqtls.IsTLS(r))
 	newSess.Options.MaxAge = secondsPerDay * sessionMaxAgeDays
 
 	return newSess, nil