diff --git a/frontend/src/components/AuthGate.tsx b/frontend/src/components/AuthGate.tsx
index 12d386a..3ab5e3f 100644
--- a/frontend/src/components/AuthGate.tsx
+++ b/frontend/src/components/AuthGate.tsx
@@ -1,6 +1,13 @@
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import type { User } from '../types'
 
+const SHARED_TIMEOUT_MS = 10 * 60 * 1000
+
+function isSharedDevice() {
+  return document.cookie.split(';').some(c => c.trim() === 'hnf_shared_device=1')
+}
+
+
 interface Props {
   children: (user: User) => React.ReactNode
 }
@@ -23,6 +30,7 @@ export function AuthGate({ children }: Props) {
   const [password, setPassword] = useState('')
   const [error, setError] = useState('')
   const [loading, setLoading] = useState(false)
+  const timerRef = useRef<ReturnType<typeof setTimeout> | null>(null)
 
   useEffect(() => {
     fetch('/api/auth/verify?app=hk-planner', { credentials: 'include' })
@@ -33,6 +41,30 @@ export function AuthGate({ children }: Props) {
       .catch(() => setState('login'))
   }, [])
 
+  useEffect(() => {
+    if (state !== 'authed' || !isSharedDevice()) return
+
+    async function forceLogout() {
+      await fetch('/api/auth/logout', { method: 'POST', credentials: 'include' }).catch(() => {})
+      setUser(null)
+      setState('login')
+    }
+
+    function reset() {
+      if (timerRef.current) clearTimeout(timerRef.current)
+      timerRef.current = setTimeout(forceLogout, SHARED_TIMEOUT_MS)
+    }
+
+    const events = ['mousemove', 'keydown', 'click', 'touchstart'] as const
+    events.forEach(e => window.addEventListener(e, reset, { passive: true }))
+    reset()
+
+    return () => {
+      if (timerRef.current) clearTimeout(timerRef.current)
+      events.forEach(e => window.removeEventListener(e, reset))
+    }
+  }, [state])
+
   async function login(e: React.FormEvent) {
     e.preventDefault()
     setLoading(true)