diff --git a/app/api/cron/autosync/route.ts b/app/api/cron/autosync/route.ts
index f8d17a0..f744f03 100644
--- a/app/api/cron/autosync/route.ts
+++ b/app/api/cron/autosync/route.ts
@@ -9,7 +9,7 @@ export async function POST(req: Request) {
 
   try {
     const secret = req.headers.get("x-cron-secret");
-    if (!env.cronSecret || !secret || secret !== env.cronSecret) {
+    if (!env.cronSharedSecret || !secret || secret !== env.cronSharedSecret) {
       return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
     }
 
diff --git a/app/api/cron/health/route.ts b/app/api/cron/health/route.ts
index 56a2c1d..e9c36f4 100644
--- a/app/api/cron/health/route.ts
+++ b/app/api/cron/health/route.ts
@@ -9,7 +9,7 @@ export async function POST(req: Request) {
 
   try {
     const secret = req.headers.get("x-cron-secret");
-    if (!env.cronSecret || !secret || secret !== env.cronSecret) {
+    if (!env.cronSharedSecret || !secret || secret !== env.cronSharedSecret) {
       return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
     }
 
diff --git a/app/auth/callback/route.ts b/app/auth/callback/route.ts
index beebb27..3629a4f 100644
--- a/app/auth/callback/route.ts
+++ b/app/auth/callback/route.ts
@@ -2,12 +2,18 @@ import { createRouteHandlerClient } from "@supabase/auth-helpers-nextjs";
 import { NextResponse } from "next/server";
 import { cookies } from "next/headers";
 
+import { env } from "@/lib/env";
 import type { Database } from "@/lib/types";
 
 export async function GET(request: Request) {
   const requestUrl = new URL(request.url);
   const code = requestUrl.searchParams.get("code");
-  const next = requestUrl.searchParams.get("next") ?? "/dashboard";
+  const nextParam = requestUrl.searchParams.get("next");
+  const next =
+    nextParam && nextParam.startsWith("/") && !nextParam.startsWith("//")
+      ? nextParam
+      : "/dashboard";
+  const redirectOrigin = env.appOrigin || requestUrl.origin;
 
   if (code) {
     const supabase = createRouteHandlerClient<Database>({ cookies });
@@ -15,18 +21,16 @@ export async function GET(request: Request) {
     if (error) {
       console.error(error);
       return NextResponse.redirect(
-        new URL(`/login?authError=1`, requestUrl.origin),
+        new URL(`/login?authError=1`, redirectOrigin),
       );
     }
     const {
       data: { user },
     } = await supabase.auth.getUser();
     if (!user) {
-      return NextResponse.redirect(
-        new URL(`/login?noUser=1`, requestUrl.origin),
-      );
+      return NextResponse.redirect(new URL(`/login?noUser=1`, redirectOrigin));
     }
   }
 
-  return NextResponse.redirect(new URL(next, requestUrl.origin));
+  return NextResponse.redirect(new URL(next, redirectOrigin));
 }
diff --git a/docs/deployment-production.md b/docs/deployment-production.md
index 4cdca7a..181d13d 100644
--- a/docs/deployment-production.md
+++ b/docs/deployment-production.md
@@ -57,6 +57,34 @@ Before deploying, set production-safe values in `.env` (using `.env.example` as
 
 Keep secrets out of source control.
 
+## Reverse proxy assumptions (Nginx Proxy Manager)
+
+This app is expected to run behind a reverse proxy in production.
+
+- TLS is terminated at the proxy.
+- Proxy forwards standard host/protocol headers (`Host`, `X-Forwarded-Proto`, `X-Forwarded-Host`).
+- Public origin is configured explicitly with `NEXT_PUBLIC_APP_URL`.
+
+To keep callback behavior stable behind proxy layers, auth redirect responses use the configured application origin from environment configuration rather than relying only on request host detection.
+
+## Callback URL expectations
+
+- `NEXT_PUBLIC_APP_URL` must be set to the public HTTPS URL used by users.
+- Supabase auth redirect/callback configuration must allow:
+  - `<NEXT_PUBLIC_APP_URL>/auth/callback`
+
+The callback route also validates the `next` parameter as an internal path to prevent unsafe external redirects.
+
+## Cron protection expectations
+
+Operational cron endpoints are protected with a shared secret.
+
+- Required header: `x-cron-secret`
+- Server secret source: `CRON_SHARED_SECRET`
+- Backward compatibility fallback: `CRON_SECRET`
+
+For new production setups, set `CRON_SHARED_SECRET` and use that value for all cron callers.
+
 ## Jenkins pipeline flow
 
 The repository includes a declarative `Jenkinsfile` with the following stages:
diff --git a/lib/env.ts b/lib/env.ts
index b809dbe..1fb90ef 100644
--- a/lib/env.ts
+++ b/lib/env.ts
@@ -3,6 +3,18 @@ const requiredEnvs = [
   "NEXT_PUBLIC_SUPABASE_ANON_KEY",
 ] as const;
 
+const configuredAppUrl = process.env.NEXT_PUBLIC_APP_URL?.trim();
+
+function getAppOrigin() {
+  if (!configuredAppUrl) return "http://localhost:3000";
+
+  try {
+    return new URL(configuredAppUrl).origin;
+  } catch {
+    return "http://localhost:3000";
+  }
+}
+
 for (const key of requiredEnvs) {
   if (!process.env[key]) {
     // Keep as runtime warning for smoother local setup.
@@ -12,9 +24,12 @@ for (const key of requiredEnvs) {
 }
 
 export const env = {
-  appUrl: process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000",
+  appUrl: configuredAppUrl ?? "http://localhost:3000",
+  appOrigin: getAppOrigin(),
   jobSecret: process.env.JOB_SECRET ?? "",
   cronSecret: process.env.CRON_SECRET ?? "",
+  cronSharedSecret:
+    process.env.CRON_SHARED_SECRET ?? process.env.CRON_SECRET ?? "",
   encryptionKey: process.env.ENCRYPTION_KEY ?? "",
   pleskCredEncKey: process.env.PLESK_CRED_ENC_KEY ?? "",
   stripeSecretKey: process.env.STRIPE_SECRET_KEY ?? "",