fix(auth): build password-reset links from an operator-set public address, never the Host header (M462 #4981)

buildResetURL used r.Host and r.TLS, so a forgot-password request with a
forged Host emailed the victim a real reset token on a link to the
attacker's server. Links now come only from network_settings.public_url
(migration 0062), and no reset email is sent while it is empty; the response
stays the same opaque 200 and the log says why.

The address is set on a new "Public address" card under Admin → Integrations,
which offers the page's own origin and warns while unset. PUT
/api/admin/network-settings takes either field alone, so the proxy card and
this one can't overwrite each other.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/admin_network.go

@@ -23,10 +23,16 @@ type networkSettingsResp struct {
 	// arrived and count them rather than guess.
 	ForwardedChain string `json:"forwarded_chain"`
 	RemoteAddr     string `json:"remote_addr"`
+	// PublicURL is where users reach Minstrel; reset emails link to it and
+	// are not sent while it is empty.
+	PublicURL string `json:"public_url"`
 }
 
+// Both fields are optional so the proxy card and the public-address card can
+// each save their own value without overwriting the other's.
 type updateNetworkSettingsReq struct {
-	TrustedProxyHops int `json:"trusted_proxy_hops"`
+	TrustedProxyHops *int    `json:"trusted_proxy_hops"`
+	PublicURL        *string `json:"public_url"`
 }
 
 func (h *handlers) handleGetNetworkSettings(w http.ResponseWriter, r *http.Request) {
@@ -39,13 +45,37 @@ func (h *handlers) handleUpdateNetworkSettings(w http.ResponseWriter, r *http.Re
 		writeErr(w, apierror.BadRequest("invalid_body", "malformed JSON"))
 		return
 	}
-	if err := h.netSettings.SetHops(r.Context(), req.TrustedProxyHops); err != nil {
-		if errors.Is(err, netsettings.ErrHopsOutOfRange) {
-			writeErr(w, apierror.BadRequest("invalid_hops", err.Error()))
+	if req.TrustedProxyHops == nil && req.PublicURL == nil {
+		writeErr(w, apierror.BadRequest("invalid_body", "nothing to update"))
+		return
+	}
+	// Validate everything before writing anything, so a bad public URL can't
+	// leave the hops half-saved.
+	if req.TrustedProxyHops != nil && (*req.TrustedProxyHops < 0 || *req.TrustedProxyHops > netsettings.MaxTrustedProxyHops) {
+		writeErr(w, apierror.BadRequest("invalid_hops", netsettings.ErrHopsOutOfRange.Error()))
+		return
+	}
+	if req.PublicURL != nil {
+		if _, err := netsettings.NormalizePublicURL(*req.PublicURL); err != nil {
+			writeErr(w, apierror.BadRequest("invalid_public_url", err.Error()))
+			return
+		}
+	}
+	if req.TrustedProxyHops != nil {
+		if err := h.netSettings.SetHops(r.Context(), *req.TrustedProxyHops); err != nil {
+			if errors.Is(err, netsettings.ErrHopsOutOfRange) {
+				writeErr(w, apierror.BadRequest("invalid_hops", err.Error()))
+				return
+			}
+			writeErrWithLog(w, h.logger, "admin network: update failed", apierror.Internal(err))
+			return
+		}
+	}
+	if req.PublicURL != nil {
+		if err := h.netSettings.SetPublicURL(r.Context(), *req.PublicURL); err != nil {
+			writeErrWithLog(w, h.logger, "admin network: public URL update failed", apierror.Internal(err))
 			return
 		}
-		writeErrWithLog(w, h.logger, "admin network: update failed", apierror.Internal(err))
-		return
 	}
 	// Echo the payload recomputed under the NEW value, so the card can show
 	// immediately what the change did to this request's own address rather
@@ -61,5 +91,6 @@ func (h *handlers) networkSettingsPayload(r *http.Request) networkSettingsResp {
 		DetectedClientIP: auth.ClientIP(r, hops),
 		ForwardedChain:   r.Header.Get("X-Forwarded-For"),
 		RemoteAddr:       r.RemoteAddr,
+		PublicURL:        h.netSettings.PublicURL(),
 	}
 }

internal/api/auth_forgot.go

@@ -69,7 +69,7 @@ func (h *handlers) handleForgotPassword(w http.ResponseWriter, r *http.Request)
 		if err == nil && user.Email != nil && *user.Email != "" {
 			matched = true
 			auditTarget = user.ID
-			if sendErr := h.sendResetEmail(r.Context(), r, user); sendErr != nil {
+			if sendErr := h.sendResetEmail(r.Context(), user); sendErr != nil {
 				h.logger.Warn("forgot-password: send failed",
 					"email", email, "err", sendErr)
 				// fall through; response is still 200
@@ -89,7 +89,7 @@ func (h *handlers) handleForgotPassword(w http.ResponseWriter, r *http.Request)
 // sendResetEmail generates a token, persists it, renders + sends the
 // email. Returns the underlying error (caller logs it but doesn't
 // surface to the HTTP response).
-func (h *handlers) sendResetEmail(ctx context.Context, r *http.Request, user dbq.User) error {
+func (h *handlers) sendResetEmail(ctx context.Context, user dbq.User) error {
 	if h.mailer == nil {
 		return errors.New("forgot-password: no mailer configured")
 	}
@@ -109,7 +109,10 @@ func (h *handlers) sendResetEmail(ctx context.Context, r *http.Request, user dbq
 		return err
 	}
 
-	resetURL := buildResetURL(r, token)
+	resetURL, err := buildResetURL(h.netSettings.PublicURL(), token)
+	if err != nil {
+		return err
+	}
 	textBody, htmlBody, err := mailer.RenderResetEmail(mailer.ResetEmailVars{
 		Username:    user.Username,
 		ResetURL:    resetURL,
@@ -127,14 +130,18 @@ func (h *handlers) sendResetEmail(ctx context.Context, r *http.Request, user dbq
 	return h.mailer.Send(ctx, *user.Email, mailer.ResetEmailSubject, textBody, htmlBody)
 }
 
-func buildResetURL(r *http.Request, token string) string {
-	scheme := "http"
-	if r.TLS != nil {
-		scheme = "https"
+// errNoPublicURL stops a reset email from going out before the operator has
+// said where Minstrel lives. It surfaces in the log, not the response, which
+// stays the same opaque 200 either way.
+var errNoPublicURL = errors.New("forgot-password: no public URL set (Admin → Integrations → Public address); reset email not sent")
+
+// buildResetURL builds the emailed link from the operator-set public URL.
+// It deliberately ignores the request: the Host header is whatever the
+// requester sent, and building from it let a forged Host plant a real reset
+// token on a link to someone else's server.
+func buildResetURL(publicURL, token string) (string, error) {
+	if publicURL == "" {
+		return "", errNoPublicURL
 	}
-	host := r.Host
-	if host == "" {
-		host = "localhost"
-	}
-	return scheme + "://" + host + "/reset-password/" + token
+	return publicURL + "/reset-password/" + token, nil
 }

internal/api/auth_forgot_test.go

@@ -7,11 +7,59 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"testing"
 
+	"github.com/jackc/pgx/v5/pgxpool"
+
 	"git.fabledsword.com/bvandeusen/minstrel/internal/mailer"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/netsettings"
 )
 
+// withPublicURL gives h a network-settings service holding url, restoring an
+// empty value afterwards so other tests see the default.
+func withPublicURL(t *testing.T, h *handlers, pool *pgxpool.Pool, url string) {
+	t.Helper()
+	ns, err := netsettings.New(context.Background(), pool, nil)
+	if err != nil {
+		t.Fatalf("netsettings: %v", err)
+	}
+	if err := ns.SetPublicURL(context.Background(), url); err != nil {
+		t.Fatalf("set public url: %v", err)
+	}
+	t.Cleanup(func() { _ = ns.SetPublicURL(context.Background(), "") })
+	h.netSettings = ns
+}
+
+// With no public URL set, a reset email is not sent at all, and the response
+// is still the same opaque 200.
+func TestForgotPassword_NoPublicURL_SendsNothing(t *testing.T) {
+	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
+		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
+	}
+	h, pool := testHandlers(t)
+	fake := &mailer.FakeSender{}
+	h.mailer = fake
+	withPublicURL(t, h, pool, "")
+
+	user := seedUser(t, pool, "nourl", "pw", false)
+	if _, err := pool.Exec(context.Background(),
+		"UPDATE users SET email = 'nourl@example.com' WHERE id = $1", user.ID); err != nil {
+		t.Fatalf("seed email: %v", err)
+	}
+	req := httptest.NewRequest(http.MethodPost, "/api/auth/forgot-password",
+		bytes.NewReader([]byte(`{"email":"nourl@example.com"}`)))
+	rec := httptest.NewRecorder()
+	h.handleForgotPassword(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want 200", rec.Code)
+	}
+	if len(fake.Sent) != 0 {
+		t.Errorf("sent %d emails with no public URL set, want 0", len(fake.Sent))
+	}
+}
+
 func TestForgotPassword_KnownEmail_SendsMail(t *testing.T) {
 	if os.Getenv("MINSTREL_TEST_DATABASE_URL") == "" {
 		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
@@ -19,6 +67,7 @@ func TestForgotPassword_KnownEmail_SendsMail(t *testing.T) {
 	h, pool := testHandlers(t)
 	fake := &mailer.FakeSender{}
 	h.mailer = fake
+	withPublicURL(t, h, pool, "https://music.example.com")
 
 	user := seedUser(t, pool, "forgotuser", "pw", false)
 	if _, err := pool.Exec(context.Background(),
@@ -29,7 +78,8 @@ func TestForgotPassword_KnownEmail_SendsMail(t *testing.T) {
 	body := `{"email":"forgot@example.com"}`
 	req := httptest.NewRequest(http.MethodPost, "/api/auth/forgot-password",
 		bytes.NewReader([]byte(body)))
-	req.Host = "minstrel.example.com"
+	// A forged Host must not reach the link: it comes from the public URL.
+	req.Host = "attacker.example.net"
 	rec := httptest.NewRecorder()
 	h.handleForgotPassword(rec, req)
 
@@ -43,6 +93,12 @@ func TestForgotPassword_KnownEmail_SendsMail(t *testing.T) {
 	if got.To != "forgot@example.com" {
 		t.Errorf("To = %q, want forgot@example.com", got.To)
 	}
+	if !strings.Contains(got.TextBody, "https://music.example.com/reset-password/") {
+		t.Errorf("reset link not built from the public URL:\n%s", got.TextBody)
+	}
+	if strings.Contains(got.TextBody+got.HTMLBody, "attacker.example.net") {
+		t.Error("the request's Host header reached the emailed link")
+	}
 	// Token row was inserted.
 	var tokenCount int
 	if err := pool.QueryRow(context.Background(),

internal/db/dbq/models.go

@@ -430,6 +430,7 @@ type MissingReacquisition struct {
 type NetworkSetting struct {
 	ID               bool
 	TrustedProxyHops int32
+	PublicUrl        string
 }
 
 type PasswordReset struct {

internal/db/dbq/network_settings.sql.go

@@ -10,23 +10,34 @@ import (
 )
 
 const getNetworkSettings = `-- name: GetNetworkSettings :one
-SELECT id, trusted_proxy_hops FROM network_settings WHERE id = true
+SELECT id, trusted_proxy_hops, public_url FROM network_settings WHERE id = true
 `
 
 func (q *Queries) GetNetworkSettings(ctx context.Context) (NetworkSetting, error) {
 	row := q.db.QueryRow(ctx, getNetworkSettings)
 	var i NetworkSetting
-	err := row.Scan(&i.ID, &i.TrustedProxyHops)
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
+	return i, err
+}
+
+const updatePublicURL = `-- name: UpdatePublicURL :one
+UPDATE network_settings SET public_url = $1 WHERE id = true RETURNING id, trusted_proxy_hops, public_url
+`
+
+func (q *Queries) UpdatePublicURL(ctx context.Context, publicUrl string) (NetworkSetting, error) {
+	row := q.db.QueryRow(ctx, updatePublicURL, publicUrl)
+	var i NetworkSetting
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
 	return i, err
 }
 
 const updateTrustedProxyHops = `-- name: UpdateTrustedProxyHops :one
-UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING id, trusted_proxy_hops
+UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING id, trusted_proxy_hops, public_url
 `
 
 func (q *Queries) UpdateTrustedProxyHops(ctx context.Context, trustedProxyHops int32) (NetworkSetting, error) {
 	row := q.db.QueryRow(ctx, updateTrustedProxyHops, trustedProxyHops)
 	var i NetworkSetting
-	err := row.Scan(&i.ID, &i.TrustedProxyHops)
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
 	return i, err
 }

internal/db/migrations/0062_network_public_url.down.sql

@@ -0,0 +1 @@
+ALTER TABLE network_settings DROP COLUMN IF EXISTS public_url;

internal/db/migrations/0062_network_public_url.up.sql

@@ -0,0 +1,8 @@
+-- The address users reach Minstrel at, e.g. https://music.example.com.
+--
+-- Password-reset links used to be built from the request's Host header,
+-- which the requester controls: a forgot-password call with a forged Host
+-- would email the victim a real reset token on a link to the attacker's
+-- server. Links are now built only from this operator-set value, and none
+-- are sent while it is empty (M462 #4981).
+ALTER TABLE network_settings ADD COLUMN public_url text NOT NULL DEFAULT '';

internal/db/queries/network_settings.sql

@@ -3,3 +3,6 @@ SELECT * FROM network_settings WHERE id = true;
 
 -- name: UpdateTrustedProxyHops :one
 UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING *;
+
+-- name: UpdatePublicURL :one
+UPDATE network_settings SET public_url = $1 WHERE id = true RETURNING *;

internal/netsettings/publicurl_test.go

@@ -0,0 +1,34 @@
+package netsettings
+
+import "testing"
+
+func TestNormalizePublicURL(t *testing.T) {
+	ok := map[string]string{
+		"":                             "",
+		"  ":                           "",
+		"https://music.example.com":    "https://music.example.com",
+		"https://music.example.com/":   "https://music.example.com",
+		"http://192.168.1.10:4533":     "http://192.168.1.10:4533",
+		" https://Music.Example.com/ ": "https://Music.Example.com",
+	}
+	for in, want := range ok {
+		got, err := NormalizePublicURL(in)
+		if err != nil || got != want {
+			t.Errorf("NormalizePublicURL(%q) = %q, %v; want %q", in, got, err, want)
+		}
+	}
+	for _, in := range []string{
+		"music.example.com",               // no scheme
+		"ftp://music.example.com",         // wrong scheme
+		"https://",                        // no host
+		"https://music.example.com/app",   // path
+		"https://music.example.com/?x=1",  // query
+		"https://music.example.com/#frag", // fragment
+		"https://user:pw@music.example.com",
+		"javascript:alert(1)",
+	} {
+		if _, err := NormalizePublicURL(in); err == nil {
+			t.Errorf("NormalizePublicURL(%q) accepted, want ErrInvalidPublicURL", in)
+		}
+	}
+}

internal/netsettings/service.go

@@ -12,6 +12,8 @@ import (
 	"context"
 	"errors"
 	"log/slog"
+	"net/url"
+	"strings"
 	"sync"
 
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -32,13 +34,18 @@ const (
 // so the API layer can answer 400 instead of surfacing a constraint violation.
 var ErrHopsOutOfRange = errors.New("trusted proxy hops must be between 0 and 10")
 
+// ErrInvalidPublicURL is returned by SetPublicURL for anything that isn't a
+// bare http(s) origin, so the API layer can answer 400.
+var ErrInvalidPublicURL = errors.New("public URL must be an http:// or https:// address with a host and no path, query or fragment")
+
 // Service caches the network settings and owns their persistence.
 type Service struct {
 	pool   *pgxpool.Pool
 	logger *slog.Logger
 
-	mu   sync.RWMutex
-	hops int
+	mu        sync.RWMutex
+	hops      int
+	publicURL string
 }
 
 // New loads the settings once and caches them.
@@ -58,6 +65,7 @@ func New(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger) (*Service
 		return s, err
 	}
 	s.hops = int(row.TrustedProxyHops)
+	s.publicURL = row.PublicUrl
 	return s, nil
 }
 
@@ -106,3 +114,53 @@ func (s *Service) SetHops(ctx context.Context, hops int) error {
 	}
 	return nil
 }
+
+// PublicURL returns the operator-set address users reach Minstrel at, with no
+// trailing slash, or "" when it hasn't been set. Links that leave the app (a
+// password-reset email) are built from this and never from the request's
+// Host header, which the requester controls. Nil-safe like Hops.
+func (s *Service) PublicURL() string {
+	if s == nil {
+		return ""
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.publicURL
+}
+
+// NormalizePublicURL validates raw as a bare http(s) origin and returns it
+// without a trailing slash. "" is valid and means unset.
+func NormalizePublicURL(raw string) (string, error) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return "", nil
+	}
+	u, err := url.Parse(raw)
+	if err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" ||
+		u.User != nil || (u.Path != "" && u.Path != "/") || u.RawQuery != "" || u.Fragment != "" {
+		return "", ErrInvalidPublicURL
+	}
+	return u.Scheme + "://" + u.Host, nil
+}
+
+// SetPublicURL validates, persists and caches the public URL. "" clears it.
+func (s *Service) SetPublicURL(ctx context.Context, raw string) error {
+	normalized, err := NormalizePublicURL(raw)
+	if err != nil {
+		return err
+	}
+	if s == nil || s.pool == nil {
+		return errors.New("network settings unavailable")
+	}
+	row, err := dbq.New(s.pool).UpdatePublicURL(ctx, normalized)
+	if err != nil {
+		return err
+	}
+	s.mu.Lock()
+	s.publicURL = row.PublicUrl
+	s.mu.Unlock()
+	if s.logger != nil {
+		s.logger.Info("netsettings: public URL updated", "public_url", normalized)
+	}
+	return nil
+}