fix(security): SSRF protection for alarm webhooks (FLAW-05)

Discord and custom webhook URLs are checked when saved and before every
call: https only, Discord URLs must point to discord.com. A guarded HTTP
client resolves hosts itself and connects only to allowed IPs; localhost,
link-local and cloud metadata are always blocked, LAN ranges (incl.
Tailscale CGNAT) and plain http only with the new admin switch.

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

Doku/flaws.md

@@ -14,7 +14,7 @@ Schweregrade: **Hoch** (Datenabfluss oder Rechteumgehung möglich), **Mittel** (
 | FLAW-02 | Realtime | ~~Räume ohne Berechtigungsprüfung~~ | **Behoben** |
 | FLAW-03 | Aufgaben | ~~`ReorderTasks` ändert beliebige Aufgaben per ID (IDOR)~~ | **Behoben** |
 | FLAW-04 | Auth | ~~Fest eincodiertes Fallback-`JWT_SECRET`~~ | **Behoben** |
-| FLAW-05 | Benachrichtigungen | Discord-/Webhook-URLs ungeprüft (SSRF) | Mittel |
+| FLAW-05 | Benachrichtigungen | ~~Discord-/Webhook-URLs ungeprüft (SSRF)~~ | **Behoben** |
 | FLAW-06 | Aufgaben | ~~Jeder Editor einer Aufgabenliste kann sie per `teamId` an weitere Gruppen freigeben~~ | **Behoben** |
 | FLAW-07 | Auth | API-Key-`scopes` werden nie ausgewertet; ein Key ist ein Vollzugriff | Mittel |
 | FLAW-08 | Auth | Webhook-Ingest mit JWT prüft weder Sperrliste noch `token_version` noch `is_active` | Mittel |
@@ -66,6 +66,7 @@ Schweregrade: **Hoch** (Datenabfluss oder Rechteumgehung möglich), **Mittel** (
 - **Maßnahme:** Start verweigern oder Secret beim ersten Start erzeugen und in `/data` ablegen.
 
 ### FLAW-05: SSRF über Alarm-URLs
+- **Status: Behoben.** [netguard.go](../internal/api/netguard.go): URLs werden beim Speichern und vor jedem Versand geprüft (https Pflicht, Discord nur `discord.com/api/webhooks/`). Der HTTP-Client löst den Host selbst auf und verbindet nur zu erlaubten IPs: localhost, Link-Local/Cloud-Metadaten sind immer gesperrt, Heimnetz-Bereiche (RFC 1918, ULA, CGNAT/Tailscale) und http nur mit dem Admin-Schalter „Alarm-Webhooks ins lokale Netz erlauben“. Keine Umgebungs-Proxys, höchstens 3 Weiterleitungen.
 - **Fundstellen:** `handleUpdateUserSettings` (keine Validierung), `sendDiscordNotification`, `sendCustomWebhookNotification`.
 - **Problem:** Jeder Nutzer kann beliebige URLs hinterlegen. Der Server sendet POST-Anfragen dorthin, auch an interne Adressen (`http://169.254.169.254`, Docker-Netz, `localhost`). Über `handleTestUserAlarm` lässt sich das sofort auslösen.
 - **Maßnahme:** Nur `https`, Discord-Host prüfen, private/Loopback-Adressen beim Verbinden sperren.

frontend/src/features/settings/AdminSettingsView.tsx

@@ -132,6 +132,7 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
           smtpFrom: sys.smtpFrom || '',
           smtpTls: sys.smtpTls ?? true,
           smtpSkipVerify: Boolean(sys.smtpSkipVerify),
+          allowPrivateWebhooks: Boolean(sys.allowPrivateWebhooks),
           smtpConfiguredViaEnv: Boolean(sys.smtpConfiguredViaEnv),
         });
       }
@@ -1081,6 +1082,22 @@ export const AdminSettingsView: React.FC<AdminSettingsViewProps> = ({
                   )}
                 </div>
 
+                {/* Alarm webhooks into the local network (SSRF protection) */}
+                <div className="p-4 bg-canvas border border-border rounded-card flex items-center justify-between gap-4">
+                  <div className="pr-4">
+                    <p className="font-bold text-main">Alarm-Webhooks ins lokale Netz erlauben</p>
+                    <p className="text-[11px] text-muted mt-0.5">
+                      Standard: Discord- und Webhook-Alarme gehen nur an öffentliche https-Adressen. Eingeschaltet sind auch Ziele
+                      im Heimnetz erlaubt (z. B. Home Assistant unter 192.168.x.x oder über Tailscale), dort auch per http.
+                      Der Server selbst (localhost) und Cloud-Metadaten bleiben immer gesperrt.
+                    </p>
+                  </div>
+                  <ToggleSwitch
+                    checked={Boolean(settings.allowPrivateWebhooks)}
+                    onChange={(allowPrivateWebhooks) => handleSaveSystemSettings({ allowPrivateWebhooks })}
+                  />
+                </div>
+
                 {/* Public Registration Toggle Switch */}
                 <div className="p-4 bg-canvas border border-border rounded-card flex items-center justify-between gap-4">
                   <div className="pr-4">

frontend/src/lib/types.ts

@@ -178,6 +178,7 @@ export interface SystemSettings {
   smtpFrom?: string;
   smtpTls?: boolean;
   smtpSkipVerify?: boolean;
+  allowPrivateWebhooks?: boolean; // alarm webhooks may reach the LAN (and plain http)
   smtpConfiguredViaEnv?: boolean;
 }
 

internal/api/netguard.go

@@ -0,0 +1,134 @@
+package api
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+// Outgoing alarm requests (Discord, custom webhooks) go to user-supplied URLs. Without checks a user
+// could make the server call internal services (SSRF): localhost, the Docker network or cloud metadata.
+
+var errBlockedTarget = errors.New("Ziel nicht erlaubt: interne oder reservierte Adresse")
+
+func mustCIDRs(list ...string) []*net.IPNet {
+	out := make([]*net.IPNet, 0, len(list))
+	for _, c := range list {
+		_, n, err := net.ParseCIDR(c)
+		if err != nil {
+			panic(err)
+		}
+		out = append(out, n)
+	}
+	return out
+}
+
+var (
+	// Never reachable: this machine, link-local incl. cloud metadata (169.254.169.254), "this network"
+	alwaysBlocked = mustCIDRs("127.0.0.0/8", "::1/128", "169.254.0.0/16", "fe80::/10", "0.0.0.0/8", "::/128")
+	// Home/LAN ranges (incl. CGNAT as used by Tailscale), only with the admin switch
+	// "Ziele im lokalen Netz erlauben"
+	privateRanges = mustCIDRs("10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "fc00::/7", "100.64.0.0/10")
+)
+
+func targetIPAllowed(ip net.IP, allowPrivate bool) bool {
+	if ip == nil || ip.IsMulticast() || ip.IsUnspecified() {
+		return false
+	}
+	if v4 := ip.To4(); v4 != nil {
+		ip = v4
+	}
+	for _, n := range alwaysBlocked {
+		if n.Contains(ip) {
+			return false
+		}
+	}
+	if !allowPrivate {
+		for _, n := range privateRanges {
+			if n.Contains(ip) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// validateWebhookURL checks a URL before it is stored or called. HTTPS is required unless the admin
+// allows local targets (LAN services often speak plain HTTP). Discord URLs must point to Discord.
+func validateWebhookURL(raw string, discord, allowPrivate bool) error {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return nil
+	}
+	u, err := url.Parse(raw)
+	if err != nil || u.Host == "" {
+		return errors.New("Ungültige URL")
+	}
+	switch u.Scheme {
+	case "https":
+	case "http":
+		if !allowPrivate {
+			return errors.New("Nur https-URLs sind erlaubt (http nur mit freigegebenen Zielen im lokalen Netz)")
+		}
+	default:
+		return errors.New("Nur https-URLs sind erlaubt")
+	}
+	if discord {
+		host := strings.ToLower(u.Hostname())
+		if u.Scheme != "https" || (host != "discord.com" && host != "discordapp.com") || !strings.HasPrefix(u.Path, "/api/webhooks/") {
+			return errors.New("Discord-Webhooks müssen mit https://discord.com/api/webhooks/ beginnen")
+		}
+	}
+	if ip := net.ParseIP(u.Hostname()); ip != nil && !targetIPAllowed(ip, allowPrivate) {
+		return errBlockedTarget
+	}
+	return nil
+}
+
+// guardedClient resolves the host itself and only connects to allowed addresses. Checking at dial time
+// (not just the URL) also covers DNS tricks and redirects; proxies from the environment are not used.
+func guardedClient(allowPrivate bool) *http.Client {
+	dialer := &net.Dialer{Timeout: 5 * time.Second}
+	transport := &http.Transport{
+		Proxy: nil,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			host, port, err := net.SplitHostPort(addr)
+			if err != nil {
+				return nil, err
+			}
+			ips, err := net.DefaultResolver.LookupIPAddr(ctx, host)
+			if err != nil {
+				return nil, err
+			}
+			for _, ip := range ips {
+				if targetIPAllowed(ip.IP, allowPrivate) {
+					return dialer.DialContext(ctx, network, net.JoinHostPort(ip.IP.String(), port))
+				}
+			}
+			return nil, fmt.Errorf("%w (%s)", errBlockedTarget, host)
+		},
+		TLSHandshakeTimeout:   5 * time.Second,
+		ResponseHeaderTimeout: 10 * time.Second,
+		MaxIdleConns:          10,
+	}
+	return &http.Client{
+		Timeout:   10 * time.Second,
+		Transport: transport,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			if len(via) >= 3 {
+				return errors.New("zu viele Weiterleitungen")
+			}
+			return nil
+		},
+	}
+}
+
+func (s *Server) allowPrivateWebhooks() bool {
+	sys, _ := s.store.GetSystemSettings()
+	return sys != nil && sys.AllowPrivateWebhooks
+}

internal/api/netguard_test.go

@@ -0,0 +1,79 @@
+package api
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestTargetIPAllowed(t *testing.T) {
+	cases := []struct {
+		ip           string
+		allowPrivate bool
+		want         bool
+	}{
+		{"127.0.0.1", true, false},
+		{"::1", true, false},
+		{"169.254.169.254", true, false}, // cloud metadata
+		{"0.0.0.0", true, false},
+		{"192.168.1.20", false, false},
+		{"192.168.1.20", true, true},
+		{"10.0.0.5", false, false},
+		{"100.101.102.103", true, true}, // Tailscale
+		{"fd00::1", false, false},
+		{"93.184.216.34", false, true},
+		{"::ffff:127.0.0.1", true, false}, // IPv4-mapped loopback
+	}
+	for _, c := range cases {
+		if got := targetIPAllowed(net.ParseIP(c.ip), c.allowPrivate); got != c.want {
+			t.Errorf("targetIPAllowed(%s, private=%v) = %v, want %v", c.ip, c.allowPrivate, got, c.want)
+		}
+	}
+}
+
+func TestValidateWebhookURL(t *testing.T) {
+	ok := []struct {
+		url              string
+		discord, private bool
+	}{
+		{"https://discord.com/api/webhooks/1/abc", true, false},
+		{"https://hooks.example.org/x", false, false},
+		{"http://192.168.1.20:8123/api/webhook/x", false, true},
+		{"", false, false},
+	}
+	for _, c := range ok {
+		if err := validateWebhookURL(c.url, c.discord, c.private); err != nil {
+			t.Errorf("%q should be allowed: %v", c.url, err)
+		}
+	}
+	bad := []struct {
+		url              string
+		discord, private bool
+	}{
+		{"http://hooks.example.org/x", false, false},
+		{"https://evil.example.org/api/webhooks/1", true, false},
+		{"https://127.0.0.1/x", false, true},
+		{"https://169.254.169.254/latest/meta-data", false, true},
+		{"https://192.168.1.20/x", false, false},
+		{"file:///etc/passwd", false, true},
+		{"gopher://x", false, false},
+	}
+	for _, c := range bad {
+		if err := validateWebhookURL(c.url, c.discord, c.private); err == nil {
+			t.Errorf("%q must be rejected", c.url)
+		}
+	}
+}
+
+// A hostname that resolves to localhost (like a test server or a DNS rebinding trick) is refused at
+// connection time even though the URL itself looks harmless.
+func TestGuardedClientRefusesLoopbackAtDial(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(204) }))
+	defer ts.Close()
+	url := strings.Replace(ts.URL, "127.0.0.1", "localhost", 1)
+	if _, err := guardedClient(true).Get(url); err == nil || !strings.Contains(err.Error(), "nicht erlaubt") {
+		t.Fatalf("loopback must be refused at dial time, got %v", err)
+	}
+}

internal/api/server.go

@@ -2325,6 +2325,20 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 		writeError(w, http.StatusBadRequest, "Ungültiges JSON")
 		return
 	}
+	// Alarm URLs are checked before they are stored (SSRF protection, see netguard.go)
+	allowPrivate := s.allowPrivateWebhooks()
+	if req.DiscordWebhookURL != nil {
+		if err := validateWebhookURL(*req.DiscordWebhookURL, true, allowPrivate); err != nil {
+			writeError(w, http.StatusBadRequest, "Discord-Webhook: "+err.Error())
+			return
+		}
+	}
+	if req.CustomWebhookURL != nil {
+		if err := validateWebhookURL(*req.CustomWebhookURL, false, allowPrivate); err != nil {
+			writeError(w, http.StatusBadRequest, "Webhook-URL: "+err.Error())
+			return
+		}
+	}
 	if req.Password != nil && strings.TrimSpace(*req.Password) != "" {
 		hash, err := auth.HashPassword(*req.Password)
 		if err != nil {
@@ -2933,6 +2947,12 @@ func (s *Server) handleAdminUpdateUser(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if req.DiscordWebhookURL != nil {
+		if err := validateWebhookURL(*req.DiscordWebhookURL, true, s.allowPrivateWebhooks()); err != nil {
+			writeError(w, http.StatusBadRequest, "Discord-Webhook: "+err.Error())
+			return
+		}
+	}
 	u, err := s.store.UpdateUser(targetUserID, req.DisplayName, req.Email, req.Role, req.IsActive, req.DiscordWebhookURL, nil)
 	if err != nil {
 		writeError(w, http.StatusBadRequest, err.Error())
@@ -3112,7 +3132,11 @@ func (s *Server) sendEmailNotification(toEmail, subject, bodyText string) error
 }
 
 func (s *Server) sendDiscordNotification(webhookURL, title, description, dueDate, priority string) error {
-	httpClient := &http.Client{Timeout: 10 * time.Second}
+	allowPrivate := s.allowPrivateWebhooks()
+	if err := validateWebhookURL(webhookURL, true, allowPrivate); err != nil {
+		return err
+	}
+	httpClient := guardedClient(allowPrivate)
 	payload := map[string]interface{}{
 		"content": "⏰ **MindSift Aufgaben-Alarm**",
 		"embeds": []map[string]interface{}{
@@ -3142,7 +3166,11 @@ func (s *Server) sendDiscordNotification(webhookURL, title, description, dueDate
 }
 
 func (s *Server) sendCustomWebhookNotification(webhookURL, event, title, notes, dueDate, priority string) error {
-	httpClient := &http.Client{Timeout: 10 * time.Second}
+	allowPrivate := s.allowPrivateWebhooks()
+	if err := validateWebhookURL(webhookURL, false, allowPrivate); err != nil {
+		return err
+	}
+	httpClient := guardedClient(allowPrivate)
 	payload := map[string]interface{}{
 		"event":     event,
 		"title":     title,

internal/domain/models.go

@@ -278,17 +278,20 @@ type TeamMember struct {
 }
 
 type SystemSettings struct {
-	SharingMode          string `json:"sharingMode"` // private | organizations | open
-	AllowRegistration    bool   `json:"allowRegistration"`
-	InstanceName         string `json:"instanceName"`
-	SMTPHost             string `json:"smtpHost"`
-	SMTPPort             string `json:"smtpPort"`
-	SMTPUser             string `json:"smtpUser"`
-	SMTPPass             string `json:"smtpPass"`
-	SMTPFrom             string `json:"smtpFrom"`
-	SMTPTLS              bool   `json:"smtpTls"`
-	SMTPSkipVerify       bool   `json:"smtpSkipVerify"` // accept self-signed certificates
-	SMTPConfiguredViaEnv bool   `json:"smtpConfiguredViaEnv,omitempty"`
+	SharingMode       string `json:"sharingMode"` // private | organizations | open
+	AllowRegistration bool   `json:"allowRegistration"`
+	InstanceName      string `json:"instanceName"`
+	SMTPHost          string `json:"smtpHost"`
+	SMTPPort          string `json:"smtpPort"`
+	SMTPUser          string `json:"smtpUser"`
+	SMTPPass          string `json:"smtpPass"`
+	SMTPFrom          string `json:"smtpFrom"`
+	SMTPTLS           bool   `json:"smtpTls"`
+	SMTPSkipVerify    bool   `json:"smtpSkipVerify"` // accept self-signed certificates
+	// AllowPrivateWebhooks lets alarm webhooks reach LAN addresses (and plain http); localhost and cloud
+	// metadata stay blocked
+	AllowPrivateWebhooks bool `json:"allowPrivateWebhooks"`
+	SMTPConfiguredViaEnv bool `json:"smtpConfiguredViaEnv,omitempty"`
 }
 
 // UnmarshalJSON accepts smtpPort as text or number: older clients sent a number, which made the whole

internal/storage/sqlite.go

@@ -490,7 +490,8 @@ func (s *Store) migrate() error {
 		('smtp_pass', ''),
 		('smtp_from', ''),
 		('smtp_tls', 'true'),
-		('smtp_skip_verify', 'false')`)
+		('smtp_skip_verify', 'false'),
+		('allow_private_webhooks', 'false')`)
 
 	// Clean up legacy roles in team_members and users
 	_, _ = s.db.Exec(`UPDATE team_members SET role = 'admin' WHERE role IN ('team_admin', 'owner')`)
@@ -5090,6 +5091,8 @@ func (s *Store) GetSystemSettings() (*domain.SystemSettings, error) {
 				settings.SMTPTLS = (v != "false" && v != "0")
 			case "smtp_skip_verify":
 				settings.SMTPSkipVerify = v == "true"
+			case "allow_private_webhooks":
+				settings.AllowPrivateWebhooks = v == "true"
 			}
 		}
 	}
@@ -5109,6 +5112,10 @@ func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
 	if settings.SMTPSkipVerify {
 		skipVerifyStr = "true"
 	}
+	privateHooksStr := "false"
+	if settings.AllowPrivateWebhooks {
+		privateHooksStr = "true"
+	}
 	if settings.SMTPPort == "" {
 		settings.SMTPPort = "587"
 	}
@@ -5126,7 +5133,8 @@ func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
 		('smtp_pass', ?),
 		('smtp_from', ?),
 		('smtp_tls', ?),
-		('smtp_skip_verify', ?)
+		('smtp_skip_verify', ?),
+		('allow_private_webhooks', ?)
 		ON CONFLICT(key) DO UPDATE SET value = excluded.value`,
 		settings.SharingMode,
 		regStr,
@@ -5138,6 +5146,7 @@ func (s *Store) UpdateSystemSettings(settings *domain.SystemSettings) error {
 		strings.TrimSpace(settings.SMTPFrom),
 		tlsStr,
 		skipVerifyStr,
+		privateHooksStr,
 	)
 	return err
 }