fix(rsvp): defend the edit flow against forwarded invitation links

When a guest submitted via their invitation and then forwarded the link
(or someone copied the URL), the recipient was shown the original guest's
response and a "Change my response" button. Two real problems:
  - Privacy leak: the original guest's reply was visible
  - Integrity: the recipient could silently overwrite the response

The fix is two layered defences plus a recovery path, matching the
industry pattern used by Eventbrite / Partiful / Lu.ma:

Backend
  - GET /access/{token} now compares the device fingerprint of the
    current request to the fingerprint stored on the existing RSVP.
    When they don't match, the rsvp field is omitted from the
    response and a new rsvp_submitted_elsewhere flag is set instead.
    The original guest's reply stays private.
  - PATCH /rsvp/{token} runs the same gate before scoring. A foreign
    device gets 403 with a hint to request an edit link.
  - The fingerprint check is intentionally narrow (user_agent only),
    so a guest jumping between Wi-Fi and mobile data on the same
    phone still sails through.

Recovery path
  - New POST /access/{token}/request-edit-link mints a short-lived
    edit nonce (Redis, 30-min TTL, SHA-256-hashed), then emails it
    to the guest's address on file via the existing EmailSender.
    Rate-limited to 3 per token per hour.
  - GET /access/{token}?edit=<nonce> and PATCH /rsvp with edit_nonce
    in the body both accept the nonce as a bypass for the
    same-device check. Lets the real guest edit from a new phone
    when their original device is gone.
  - New SendRSVPEditLink method on auth.EmailSender, implemented by
    every concrete sender (log stub / Resend / SMTP / SES), with a
    branded HTML+text template that explains the "we sent this
    because we didn't recognise the device" framing.

Frontend
  - rsvp/[token].vue learns the new "responded elsewhere" state.
    Renders "This invitation has already been used" + a
    "Send me an edit link" CTA when the access response says we
    have somewhere to deliver it. Empty-state copy reads "If you
    forwarded the link, please ask the original guest to reach
    out to the host".
  - When the URL carries ?edit=<nonce>, the page passes it on the
    /access call (so the backend unhides the RSVP), opens the edit
    form pre-populated, and forwards the nonce on PATCH.
  - Removed two leftover leaks from earlier — the page no longer
    shows internal "Risk score N · band" to confirmed or blocked
    guests; the blocked-attempt copy now reads "Something about
    this attempt looked off" rather than "suspicious access
    attempt".

Defensive nil-guard
  - The access handler's NATS publish goroutine now skips when
    deps.AccessPublisher is nil (matches the rsvp publisher's
    existing guard); without it the handler nil-panicked in tests
    that don't wire NATS.

Tests
  - TestFingerprintsSimilar (unit) covers the same-UA / different-UA
    / missing-UA matrix.
  - TestForwardedInvitationLinkDefence (integration) walks the full
    flow: submit from UA-A, hide on UA-B, request link, follow nonce
    from UA-B and edit, then verify a UA-C with a forged nonce is
    still refused.
  - Full integration suite passes (183.5s).

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

internal/api/rsvp_edit.go

@@ -0,0 +1,111 @@
+package api
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/redis/go-redis/v9"
+)
+
+// RSVP-edit nonce store. When a guest revisits their invitation from a
+// device that doesn't match the one they originally responded from, we
+// hide their RSVP details from the access response so a forwarded link
+// can't leak (or alter) their reply. They can request a one-time edit
+// link sent to their email/SMS; that link carries a short-lived nonce
+// stored here.
+//
+// We store sha256(raw) so a database snapshot of Redis doesn't give an
+// attacker working nonces; the raw value only ever lives in the email
+// link the guest receives.
+
+const (
+	editNonceTTL       = 30 * time.Minute
+	editNonceKeyPrefix = "gg:rsvp_edit:"
+)
+
+type editNonceStore struct {
+	rdb *redis.Client
+}
+
+// newEditNonceStore returns nil when Redis isn't configured — the calling
+// handler treats nil as "edit-link flow disabled" and falls back to the
+// strict same-device rule.
+func newEditNonceStore(rdb *redis.Client) *editNonceStore {
+	if rdb == nil {
+		return nil
+	}
+	return &editNonceStore{rdb: rdb}
+}
+
+// Mint creates a fresh nonce bound to guestID, stores its hash with a
+// 30-minute TTL, and returns the raw value for embedding in the email
+// link. Hex-encoded so it survives URL transit without further escaping.
+func (s *editNonceStore) Mint(ctx context.Context, guestID uuid.UUID) (raw string, err error) {
+	if s == nil {
+		return "", errors.New("edit nonce store not configured")
+	}
+	buf := make([]byte, 24)
+	if _, err := rand.Read(buf); err != nil {
+		return "", err
+	}
+	raw = hex.EncodeToString(buf)
+	sum := sha256.Sum256([]byte(raw))
+	key := editNonceKeyPrefix + hex.EncodeToString(sum[:])
+	if err := s.rdb.Set(ctx, key, guestID.String(), editNonceTTL).Err(); err != nil {
+		return "", err
+	}
+	return raw, nil
+}
+
+// Verify reports whether raw is a still-valid nonce for guestID.
+// Doesn't consume the nonce; expiry happens naturally via TTL so the
+// guest can refresh the edit page or briefly close + reopen the link
+// during the half-hour window without losing their session.
+func (s *editNonceStore) Verify(ctx context.Context, raw string, guestID uuid.UUID) (bool, error) {
+	if s == nil || raw == "" {
+		return false, nil
+	}
+	sum := sha256.Sum256([]byte(raw))
+	key := editNonceKeyPrefix + hex.EncodeToString(sum[:])
+	val, err := s.rdb.Get(ctx, key).Result()
+	if err != nil {
+		if errors.Is(err, redis.Nil) {
+			return false, nil
+		}
+		return false, err
+	}
+	return val == guestID.String(), nil
+}
+
+// fingerprintsSimilar reports whether two device fingerprints look like
+// the same browser. The check is intentionally narrow: just `user_agent`,
+// because it's the strongest signal of "same browser" and the one least
+// affected by normal day-to-day variation (changing networks, switching
+// rooms, etc).
+//
+// Conservative-by-default: when either side lacks a user_agent string we
+// return false, which causes the access handler to hide the RSVP. The
+// legitimate guest in that edge case can still recover via the
+// request-edit-link flow.
+//
+// Two fingerprints with the same user_agent but different IPs are
+// considered the same device. This is on purpose: a guest jumping
+// between Wi-Fi and mobile data is the same person on the same phone.
+// The Gate's full scoring (run on submit/edit, not on access) catches
+// the more sophisticated mismatches.
+func fingerprintsSimilar(stored, current map[string]any) bool {
+	if stored == nil || current == nil {
+		return false
+	}
+	s, _ := stored["user_agent"].(string)
+	c, _ := current["user_agent"].(string)
+	if s == "" || c == "" {
+		return false
+	}
+	return s == c
+}

internal/api/rsvp_edit_test.go

@@ -0,0 +1,52 @@
+package api
+
+import "testing"
+
+func TestFingerprintsSimilar(t *testing.T) {
+	const ua = "Mozilla/5.0 (Macintosh; Intel Mac OS X 14_2) AppleWebKit/605"
+	other := "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537"
+
+	tests := []struct {
+		name           string
+		stored, current map[string]any
+		want           bool
+	}{
+		{
+			name:    "identical UA, different IPs counts as same device",
+			stored:  map[string]any{"user_agent": ua, "accept_language": "en"},
+			current: map[string]any{"user_agent": ua, "accept_language": "fr"},
+			want:    true,
+		},
+		{
+			name:    "different UA never matches",
+			stored:  map[string]any{"user_agent": ua},
+			current: map[string]any{"user_agent": other},
+			want:    false,
+		},
+		{
+			name:    "missing UA on current side is treated as mismatch",
+			stored:  map[string]any{"user_agent": ua},
+			current: map[string]any{},
+			want:    false,
+		},
+		{
+			name:    "missing UA on stored side is treated as mismatch",
+			stored:  map[string]any{},
+			current: map[string]any{"user_agent": ua},
+			want:    false,
+		},
+		{
+			name:    "both nil is mismatch (conservative default)",
+			stored:  nil,
+			current: nil,
+			want:    false,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := fingerprintsSimilar(tc.stored, tc.current); got != tc.want {
+				t.Errorf("fingerprintsSimilar = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}

internal/api/rsvps.go

@@ -34,6 +34,7 @@ type rsvpHandler struct {
 	rsvps      *storage.RSVPRepo
 	accessLogs *storage.AccessLogRepo
 	allowlist  *storage.AllowlistRepo
+	editNonces *editNonceStore
 	scorer     fraudScorer
 	pub        rsvpPublisher
 }
@@ -43,6 +44,12 @@ type submitRSVPRequest struct {
 	PlusOnes     int            `json:"plus_ones"`
 	DietaryNotes *string        `json:"dietary_notes"`
 	Fingerprint  map[string]any `json:"fingerprint"`
+	// EditNonce is the short-lived value carried back from the magic
+	// edit link the guest received by email. It bypasses the
+	// same-device check on PATCH so a guest who jumped to a new phone
+	// can still edit their RSVP. Empty on POST and on edits from the
+	// original device.
+	EditNonce string `json:"edit_nonce,omitempty"`
 }
 
 type submitRSVPResponse struct {
@@ -117,6 +124,13 @@ func (h *rsvpHandler) submit(w http.ResponseWriter, r *http.Request) {
 // PATCH /rsvp/{token} — revise an existing RSVP. Same fraud check as POST.
 // The prior state is snapshotted into rsvp_revisions inside Update; edits
 // past MaxRSVPEdits return 429.
+//
+// As of the forwarded-link defence: a PATCH is only accepted when either
+// (a) the current device fingerprint matches the one stored on the
+// existing RSVP, or (b) the caller presented a valid edit nonce in the
+// request body. Otherwise we refuse with 403 + a hint to request an edit
+// link — this stops anyone the original guest forwarded the link to from
+// quietly altering the response.
 func (h *rsvpHandler) edit(w http.ResponseWriter, r *http.Request) {
 	tk, ok := h.loadValidToken(w, r)
 	if !ok {
@@ -136,6 +150,28 @@ func (h *rsvpHandler) edit(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Forwarded-link gate. Load the existing RSVP (if any) so we can
+	// compare device fingerprints; absent RSVP means this is a quirky
+	// PATCH-before-POST flow that Update will 404 below anyway.
+	if existing, err := h.rsvps.GetByGuest(r.Context(), guest.ID); err == nil && existing != nil {
+		current := mergeFingerprint(req.Fingerprint, collectFingerprint(r))
+		if !fingerprintsSimilar(existing.DeviceFingerprint, current) {
+			// Bypass via edit nonce delivered to the guest's email.
+			bypassed := false
+			if h.editNonces != nil && req.EditNonce != "" {
+				if ok, _ := h.editNonces.Verify(r.Context(), req.EditNonce, guest.ID); ok {
+					bypassed = true
+				}
+			}
+			if !bypassed {
+				writeError(w, http.StatusForbidden,
+					"this invitation looks like it's being opened from a different device; "+
+						"please request an edit link from the previous screen")
+				return
+			}
+		}
+	}
+
 	decision, fingerprint, ip, ok := h.scoreAccess(w, r, event, guest, tk, req.Fingerprint)
 	if !ok {
 		return
@@ -238,6 +274,7 @@ type decodedRSVPRequest struct {
 	PlusOnes     int
 	DietaryNotes *string
 	Fingerprint  map[string]any
+	EditNonce    string
 }
 
 func decodeSubmitRSVP(w http.ResponseWriter, r *http.Request) (decodedRSVPRequest, bool) {
@@ -260,6 +297,7 @@ func decodeSubmitRSVP(w http.ResponseWriter, r *http.Request) (decodedRSVPReques
 		PlusOnes:     req.PlusOnes,
 		DietaryNotes: req.DietaryNotes,
 		Fingerprint:  req.Fingerprint,
+		EditNonce:    req.EditNonce,
 	}, true
 }
 

internal/api/server.go

@@ -102,6 +102,7 @@ func NewServer(deps ServerDeps) (*Server, error) {
 	analyticsRepo := storage.NewAnalyticsRepo(deps.DB)
 	brandingRepo := storage.NewBrandingRepo(deps.DB)
 	allowlistRepo := storage.NewAllowlistRepo(deps.DB)
+	editNonces := newEditNonceStore(deps.Redis)
 	feedbackRepo := storage.NewFeedbackRepo(deps.DB)
 
 	// Branding image store. Empty UploadsDir leaves it nil and the upload
@@ -194,6 +195,8 @@ func NewServer(deps ServerDeps) (*Server, error) {
 			rsvps:         rsvpRepo,
 			collabs:       collabRepo,
 			branding:      brandingRepo,
+			editNonces:    editNonces,
+			emails:        emails,
 			gen:           auth.NewGenerator(),
 			ttl:           deps.TokenTTL,
 			pub:           deps.AccessPublisher,
@@ -208,6 +211,7 @@ func NewServer(deps ServerDeps) (*Server, error) {
 			rsvps:      rsvpRepo,
 			accessLogs: accessRepo,
 			allowlist:  allowlistRepo,
+			editNonces: editNonces,
 			scorer:     deps.FraudScorer,
 			pub:        deps.RSVPPublisher,
 		},
@@ -443,6 +447,13 @@ func (s *Server) Handler() http.Handler {
 	// Block B: .ics download. Same rate-limit class as /access since the
 	// payload is similarly cheap and the abuse profile is identical (one
 	// token per attacker).
+	// Forwarded-link defence: when a guest opens their invitation from a
+	// device the original RSVP wasn't submitted from, /access hides the
+	// reply. This endpoint mints a short-lived edit nonce and emails it
+	// so the real guest can recover from a new phone / new laptop.
+	mux.Handle("POST /access/{token}/request-edit-link",
+		rl("rsvp_edit_request", 3, time.Hour, pathKey("token"), http.HandlerFunc(s.tokens.requestEditLink)))
+
 	mux.Handle("GET /access/{token}/calendar.ics",
 		rl("calendar_ics", 60, time.Hour, pathKey("token"), http.HandlerFunc(s.tokens.calendar)))
 	mux.Handle("POST /rsvp/{token}",

internal/api/tokens.go

@@ -36,6 +36,8 @@ type tokenHandler struct {
 	rsvps         *storage.RSVPRepo
 	collabs       *storage.CollaboratorRepo
 	branding      *storage.BrandingRepo
+	editNonces    *editNonceStore
+	emails        auth.EmailSender
 	gen           *auth.Generator
 	ttl           time.Duration
 	pub           accessPublisher
@@ -412,7 +414,22 @@ type accessResponse struct {
 	// RSVP is the guest's current submission, if any. Populated so the RSVP
 	// page can show an edit form instead of a fresh submit form when the
 	// guest revisits their invitation link (Tier 2 Block A).
+	//
+	// As of the forwarded-link defence (Tier 2 Block G follow-up): this is
+	// only populated when the current device looks like the device that
+	// originally submitted, OR when the caller presented a valid edit nonce.
+	// A forwarded-link recipient sees a nil RSVP + RSVPSubmittedElsewhere=true
+	// instead, so the original guest's response stays private and unmodifiable.
 	RSVP *domain.RSVP `json:"rsvp,omitempty"`
+	// RSVPSubmittedElsewhere signals "there's an RSVP on file but we're
+	// hiding it because this looks like a different device". The frontend
+	// renders a "this invitation has already been responded to" view +
+	// (when CanRequestEditLink) a "send me an edit link" CTA.
+	RSVPSubmittedElsewhere bool `json:"rsvp_submitted_elsewhere,omitempty"`
+	// CanRequestEditLink reports whether we have a way to deliver an edit
+	// link to the guest (email or phone on file). When false the only
+	// path is for the guest to contact the host directly.
+	CanRequestEditLink bool `json:"can_request_edit_link,omitempty"`
 	// Calendar holds the add-to-calendar deep-links and the .ics download
 	// path so the frontend renders four ready-to-click buttons after a
 	// successful RSVP. (Tier 2 Block B.)
@@ -469,23 +486,25 @@ func (h *tokenHandler) access(w http.ResponseWriter, r *http.Request) {
 		h.logger.Error("create access log", "err", err)
 	}
 
-	go func(evt natspub.AccessAttempted) {
-		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-		defer cancel()
-		if err := h.pub.PublishAccessAttempted(ctx, evt); err != nil {
-			h.logger.Error("publish access.attempted", "err", err, "guest_id", evt.GuestID)
-		}
-	}(natspub.AccessAttempted{
-		EventID:     event.ID,
-		GuestID:     guest.ID,
-		TokenID:     tk.ID,
-		AccessLogID: accessLogID,
-		Fingerprint: fingerprint,
-		IPAddress:   ip,
-		UserAgent:   r.UserAgent(),
-		Referrer:    r.Referer(),
-		OccurredAt:  time.Now().UTC(),
-	})
+	if h.pub != nil {
+		go func(evt natspub.AccessAttempted) {
+			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+			if err := h.pub.PublishAccessAttempted(ctx, evt); err != nil {
+				h.logger.Error("publish access.attempted", "err", err, "guest_id", evt.GuestID)
+			}
+		}(natspub.AccessAttempted{
+			EventID:     event.ID,
+			GuestID:     guest.ID,
+			TokenID:     tk.ID,
+			AccessLogID: accessLogID,
+			Fingerprint: fingerprint,
+			IPAddress:   ip,
+			UserAgent:   r.UserAgent(),
+			Referrer:    r.Referer(),
+			OccurredAt:  time.Now().UTC(),
+		})
+	}
 
 	var brandingPayload *domain.Branding
 	if h.branding != nil {
@@ -518,17 +537,140 @@ func (h *tokenHandler) access(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Forwarded-link defence (Tier 2 Block G follow-up). If a previous
+	// submission exists, only surface its details when the current
+	// device looks like the one that submitted, OR when the caller
+	// presented a valid edit nonce in ?edit=<nonce>. Anything else gets
+	// the "responded elsewhere" view — no leak, no edit.
+	rsvpPayload := existingRSVP
+	var rsvpSubmittedElsewhere, canRequestEditLink bool
+	if existingRSVP != nil && !fingerprintsSimilar(existingRSVP.DeviceFingerprint, fingerprint) {
+		bypassed := false
+		if nonce := r.URL.Query().Get("edit"); nonce != "" && h.editNonces != nil {
+			if ok, _ := h.editNonces.Verify(r.Context(), nonce, guest.ID); ok {
+				bypassed = true
+			}
+		}
+		if !bypassed {
+			rsvpPayload = nil
+			rsvpSubmittedElsewhere = true
+			canRequestEditLink =
+				h.editNonces != nil &&
+					((guest.Email != nil && *guest.Email != "") ||
+						(guest.Phone != nil && *guest.Phone != ""))
+		}
+	}
+
 	writeJSON(w, http.StatusOK, accessResponse{
-		Guest:     guest,
-		Event:     event,
-		Token:     tk,
-		AccessLog: accessLogID,
-		RSVP:      existingRSVP,
-		Calendar:  h.calendarLinks(event, raw),
-		Branding:  brandingPayload,
+		Guest:                  guest,
+		Event:                  event,
+		Token:                  tk,
+		AccessLog:              accessLogID,
+		RSVP:                   rsvpPayload,
+		RSVPSubmittedElsewhere: rsvpSubmittedElsewhere,
+		CanRequestEditLink:     canRequestEditLink,
+		Calendar:               h.calendarLinks(event, raw),
+		Branding:               brandingPayload,
 	})
 }
 
+// requestEditLinkResponse is the wire shape of POST /access/{token}/request-edit-link.
+type requestEditLinkResponse struct {
+	// Channel hints at where the link went so the frontend can render
+	// "Sent to your email" vs. "Sent by SMS" feedback. Empty when the
+	// store/sender wasn't configured (dev environments without email
+	// wired up — the frontend should still treat that as success).
+	Channel string `json:"channel,omitempty"`
+}
+
+// POST /access/{token}/request-edit-link — public, token-scoped. When a
+// guest opens their invitation from an unfamiliar device the regular
+// access response hides their RSVP. This endpoint lets them prove email
+// or phone ownership instead: we mint a short-lived edit nonce and
+// deliver it to the address on file.
+//
+// Rate limit lives on the route registration (3 per hour per token).
+// The endpoint itself stays generous about the response — we never
+// reveal whether a token has an RSVP attached, just whether the request
+// itself was acceptable.
+func (h *tokenHandler) requestEditLink(w http.ResponseWriter, r *http.Request) {
+	raw := r.PathValue("token")
+	if err := auth.ValidateFormat(raw); err != nil {
+		writeError(w, http.StatusBadRequest, "malformed token")
+		return
+	}
+	tk, err := h.tokens.GetByHash(r.Context(), auth.HashToken(raw))
+	if err != nil {
+		if errors.Is(err, domain.ErrTokenNotFound) {
+			writeError(w, http.StatusNotFound, "token not found")
+			return
+		}
+		writeError(w, http.StatusInternalServerError, "failed to load token")
+		return
+	}
+	if err := tk.IsValid(time.Now().UTC()); err != nil {
+		writeError(w, http.StatusGone, err.Error())
+		return
+	}
+	guest, err := h.guests.Get(r.Context(), tk.GuestID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to load guest")
+		return
+	}
+
+	if h.editNonces == nil {
+		// Redis isn't wired up; the feature is disabled. Tell the caller
+		// honestly rather than pretending we sent something.
+		writeError(w, http.StatusServiceUnavailable, "edit-link delivery isn't configured for this environment")
+		return
+	}
+
+	// Only attempt delivery when there's somewhere to deliver to. Without
+	// email or phone on file the request is a 404 — we don't have a
+	// secure channel for the nonce.
+	hasEmail := guest.Email != nil && *guest.Email != ""
+	if !hasEmail {
+		// Phone-only delivery is a future enhancement (Twilio path is
+		// wired for the broader notifier; not for synchronous edit links
+		// yet). For now treat as no-channel.
+		writeError(w, http.StatusNotFound, "no email on file for this guest")
+		return
+	}
+
+	nonce, err := h.editNonces.Mint(r.Context(), guest.ID)
+	if err != nil {
+		h.logger.Error("mint edit nonce", "err", err, "guest_id", guest.ID)
+		writeError(w, http.StatusInternalServerError, "failed to issue edit link")
+		return
+	}
+
+	// Resolve the event name for the email. Best-effort: if the lookup
+	// fails we still send a link, just with a generic fallback.
+	eventName := "your event"
+	if event, err := h.events.Get(r.Context(), guest.EventID); err == nil {
+		eventName = event.Name
+	}
+
+	link := h.editLink(raw, nonce)
+	if h.emails != nil {
+		if err := h.emails.SendRSVPEditLink(r.Context(), *guest.Email, guest.Name, eventName, link); err != nil {
+			h.logger.Warn("send rsvp edit link", "err", err, "guest_id", guest.ID)
+			// Don't 500 — the nonce already exists in Redis, and we've
+			// logged the link in the dev stub. A 202-ish behaviour:
+			// "accepted; delivery might be best-effort".
+		}
+	}
+	writeJSON(w, http.StatusAccepted, requestEditLinkResponse{Channel: "email"})
+}
+
+func (h *tokenHandler) editLink(rawToken, nonce string) string {
+	base := h.publicBaseURL
+	if base == "" {
+		base = "http://localhost:3000"
+	}
+	return base + "/rsvp/" + rawToken + "?edit=" + nonce
+}
+
 // calendarLinks renders the three provider URLs + .ics path for the event.
 // The raw access token is embedded in the .ics path so the download endpoint
 // stays public (no auth) while still scoped to a single invitation.