diff --git a/.github/workflows/build-and-release.yml b/.github/workflows/build-and-release.yml
index ab99c65..1e81185 100644
--- a/.github/workflows/build-and-release.yml
+++ b/.github/workflows/build-and-release.yml
@@ -76,6 +76,7 @@ jobs:
           flutter build apk --release --flavor prod \
             --build-name="${{ github.ref_name }}" \
             --build-number="${{ github.run_number }}" \
+            --dart-define=BACKEND_BASE_URL="${{ vars.BACKEND_BASE_URL || 'https://api.schuly.dev' }}" \
             --dart-define=SENTRY_DSN="$SENTRY_DSN" \
             --dart-define=SENTRY_RELEASE="${{ github.ref_name }}" \
             --dart-define=SENTRY_ENVIRONMENT="production"
@@ -88,6 +89,7 @@ jobs:
           flutter build appbundle --release --flavor prod \
             --build-name="${{ github.ref_name }}" \
             --build-number="${{ github.run_number }}" \
+            --dart-define=BACKEND_BASE_URL="${{ vars.BACKEND_BASE_URL || 'https://api.schuly.dev' }}" \
             --dart-define=SENTRY_DSN="$SENTRY_DSN" \
             --dart-define=SENTRY_RELEASE="${{ github.ref_name }}" \
             --dart-define=SENTRY_ENVIRONMENT="production"
@@ -134,6 +136,7 @@ jobs:
           flutter build ios --release --no-codesign \
             --build-name="${{ github.ref_name }}" \
             --build-number="${{ github.run_number }}" \
+            --dart-define=BACKEND_BASE_URL="${{ vars.BACKEND_BASE_URL || 'https://api.schuly.dev' }}" \
             --dart-define=SENTRY_DSN="$SENTRY_DSN" \
             --dart-define=SENTRY_RELEASE="${{ github.ref_name }}" \
             --dart-define=SENTRY_ENVIRONMENT="production"
diff --git a/lib/config/backend_config.dart b/lib/config/backend_config.dart
index 57e5ee7..83bdf19 100644
--- a/lib/config/backend_config.dart
+++ b/lib/config/backend_config.dart
@@ -39,6 +39,20 @@ class BackendConfig {
   static String normalise(String? value) =>
       (value ?? '').trim().replaceAll(RegExp(r'/+$'), '');
 
+  /// True if [value] uses plaintext `http://` to a non-loopback host, so
+  /// credentials and tokens would travel in the clear. Used to warn before a
+  /// self-hoster saves an insecure custom backend.
+  static bool isInsecure(String? value) {
+    final uri = Uri.tryParse(normalise(value));
+    if (uri == null || uri.scheme != 'http') return false;
+    final h = uri.host.toLowerCase();
+    return h.isNotEmpty &&
+        h != 'localhost' &&
+        h != '127.0.0.1' &&
+        h != '::1' &&
+        !h.endsWith('.localhost');
+  }
+
   /// Probes [baseUrl] by fetching the anonymous `GET /api/app`. A reachable
   /// Schuly backend returns a JSON object with a `clientId`; on success this
   /// returns its reported `version` (or `'unknown'` if the field is missing).
diff --git a/lib/ui/onboarding/onboarding_screen.dart b/lib/ui/onboarding/onboarding_screen.dart
index 2afe848..1e21f5f 100644
--- a/lib/ui/onboarding/onboarding_screen.dart
+++ b/lib/ui/onboarding/onboarding_screen.dart
@@ -374,6 +374,14 @@ class _ServerPage extends StatelessWidget {
                   hint: 'https://schuly.example.com',
                   autocorrect: false,
                 ),
+                if (BackendConfig.isInsecure(urlController.text)) ...[
+                  const SizedBox(height: 8),
+                  Text(
+                    'Plaintext http:// - your login would be sent unencrypted. Use https:// unless this is a trusted local network.',
+                    textAlign: TextAlign.center,
+                    style: typography.xs.copyWith(color: colors.error),
+                  ),
+                ],
               ],
               if (error != null) ...[
                 const SizedBox(height: 12),
diff --git a/lib/ui/settings/settings_screen.dart b/lib/ui/settings/settings_screen.dart
index 2e0f2cf..e1d7033 100644
--- a/lib/ui/settings/settings_screen.dart
+++ b/lib/ui/settings/settings_screen.dart
@@ -115,6 +115,13 @@ class _ServerDialogState extends State<_ServerDialog> {
   String? _error;
   bool _busy = false;
 
+  @override
+  void initState() {
+    super.initState();
+    // Live-update the insecure-URL warning as the user types.
+    _urlCtrl.addListener(() => setState(() {}));
+  }
+
   @override
   void dispose() {
     _urlCtrl.dispose();
@@ -230,6 +237,13 @@ class _ServerDialogState extends State<_ServerDialog> {
               hint: 'https://schuly.example.com',
               autocorrect: false,
             ),
+            if (BackendConfig.isInsecure(_urlCtrl.text)) ...[
+              const SizedBox(height: 6),
+              Text(
+                'Plaintext http:// - your login would be sent unencrypted. Use https:// unless this is a trusted local network.',
+                style: typography.xs.copyWith(color: colors.error),
+              ),
+            ],
           ],
           if (_error != null) ...[
             const SizedBox(height: 8),
diff --git a/package.json b/package.json
index e41e8e2..eea09ae 100644
--- a/package.json
+++ b/package.json
@@ -4,15 +4,15 @@
   "description": "Task runner for the Schuly Flutter app. Use `bun run <script>`.",
   "scripts": {
     "dev": "flutter run --flavor dev",
-    "prod": "flutter run --flavor prod",
+    "prod": "flutter run --flavor prod --dart-define=BACKEND_BASE_URL=https://api.schuly.dev",
     "analyze": "flutter analyze",
     "test": "flutter test",
     "format": "dart format lib",
     "build:apk:dev": "flutter build apk --flavor dev --release",
-    "build:apk:prod": "flutter build apk --flavor prod --release",
-    "build:ios": "flutter build ios --flavor prod --no-codesign",
+    "build:apk:prod": "flutter build apk --flavor prod --release --dart-define=BACKEND_BASE_URL=https://api.schuly.dev",
+    "build:ios": "flutter build ios --flavor prod --no-codesign --dart-define=BACKEND_BASE_URL=https://api.schuly.dev",
     "install:dev": "flutter build apk --flavor dev --release && flutter install --flavor dev",
-    "install:prod": "flutter build apk --flavor prod --release && flutter install --flavor prod",
+    "install:prod": "flutter build apk --flavor prod --release --dart-define=BACKEND_BASE_URL=https://api.schuly.dev && flutter install --flavor prod",
     "adb:reverse": "adb reverse tcp:5033 tcp:5033",
     "install:dev:usb": "bun run adb:reverse && bun run install:dev",
     "build:apk:dev:url": "flutter build apk --flavor dev --release --dart-define=BACKEND_BASE_URL=$BACKEND_BASE_URL",