diff --git a/server/auth.js b/server/auth.js
index 48c1243..5a63838 100644
--- a/server/auth.js
+++ b/server/auth.js
@@ -1,7 +1,7 @@
 import { createHash, randomUUID, timingSafeEqual, createHmac, randomFillSync } from 'node:crypto'
 import { readFile, writeFile } from 'node:fs/promises'
 import path from 'node:path'
-import { parseCookies } from './helpers.js'
+import { parseCookies, cookieSecureFlag } from './helpers.js'
 import { DATA_DIR } from './paths.js'
 
 const TOTP_SECRET_FILE = path.join(DATA_DIR, 'totp-secret.json')
@@ -18,7 +18,7 @@ const ADMIN_PASSWORD = process.env.ADMIN_PASSWORD
 const adminSessions = new Map()
 
 function cookieFlags() {
-  return process.env.NODE_ENV === 'production' ? '; Secure' : ''
+  return cookieSecureFlag()
 }
 
 export function sha256(value) {
diff --git a/server/helpers.js b/server/helpers.js
index 818a683..50af0e2 100644
--- a/server/helpers.js
+++ b/server/helpers.js
@@ -505,10 +505,18 @@ export function hasVisitorConsent(req) {
   return cookies['vbn_analytics_consent'] === 'yes'
 }
 
+// Secure cookies are required in production unless explicitly disabled with
+// ALLOW_INSECURE_COOKIES=true — needed when the app is reached over plain
+// HTTP (e.g. by LAN/VPN IP during a server migration, before TLS is set up),
+// because browsers silently drop Secure cookies on http:// origins.
+export function cookieSecureFlag() {
+  if (process.env.ALLOW_INSECURE_COOKIES === 'true') return ''
+  return process.env.NODE_ENV === 'production' ? '; Secure' : ''
+}
+
 export function setConsentCookie(res, consent) {
   const value = consent ? 'yes' : 'no'
-  const secureFlag = process.env.NODE_ENV === 'production' ? '; Secure' : ''
-  res.append('Set-Cookie', `vbn_analytics_consent=${value}; Max-Age=31536000; Path=/; SameSite=Lax${secureFlag}`)
+  res.append('Set-Cookie', `vbn_analytics_consent=${value}; Max-Age=31536000; Path=/; SameSite=Lax${cookieSecureFlag()}`)
 }
 
 export function isPrivateOrLocalIp(ip) {
diff --git a/server/study-helpers.js b/server/study-helpers.js
index 6ef2de4..6232bc1 100644
--- a/server/study-helpers.js
+++ b/server/study-helpers.js
@@ -1,6 +1,6 @@
 import { createHash, randomUUID } from 'node:crypto'
 import path from 'node:path'
-import { parseCookies } from './helpers.js'
+import { parseCookies, cookieSecureFlag } from './helpers.js'
 import {
   STUDY_SESSION_COOKIE,
   STUDY_SESSION_TTL_MS,
@@ -131,7 +131,7 @@ export function getStudyAvatarUrl(subject) {
 // ── Session management ─────────────────────────────────────────────────────
 
 export function cookieFlags() {
-  return process.env.NODE_ENV === 'production' ? '; Secure' : ''
+  return cookieSecureFlag()
 }
 
 export function createStudySession(userId) {