diff --git a/app/Services/DeploymentService.php b/app/Services/DeploymentService.php
index de90114..d521e59 100644
--- a/app/Services/DeploymentService.php
+++ b/app/Services/DeploymentService.php
@@ -290,9 +290,19 @@ class DeploymentService
             @mkdir($dir, 0775, true);
         }
 
+        // Sign the marker so the host updater (watch.sh) can reject a forged/limited write to the
+        // shared ./run dir before running a root update: line 1 = timestamp, line 2 = its HMAC.
+        // Falls back to the bare timestamp when no key is configured (dev / pre-HMAC installs), which
+        // the host accepts too.
+        $ts = now()->toIso8601String();
+        $key = (string) config('clusev.update_hmac_key');
+        $body = $key !== ''
+            ? $ts.PHP_EOL.hash_hmac('sha256', $ts, $key).PHP_EOL
+            : $ts.PHP_EOL;
+
         // Returns false when the (bind-mounted) sentinel dir is not writable by the app user —
         // the caller surfaces that instead of showing a "running" state that never resolves.
-        $written = @file_put_contents($path, now()->toIso8601String().PHP_EOL) !== false;
+        $written = @file_put_contents($path, $body) !== false;
 
         if ($written) {
             // Best-effort start marker (a failure here must never fail the update): lets the
diff --git a/config/clusev.php b/config/clusev.php
index 142f763..f90d19e 100644
--- a/config/clusev.php
+++ b/config/clusev.php
@@ -49,5 +49,11 @@ return [
     // (terminal disabled). install.sh generates it alongside the other secrets.
     'terminal_secret' => env('TERMINAL_SIDECAR_SECRET', ''),
 
+    // Shared secret used to AUTHENTICATE the update-request marker the app drops for the host updater.
+    // The app signs the marker (HMAC-SHA256) and the host-side watch.sh verifies it before running a
+    // root update, so a stray/limited write to the shared ./run dir cannot drive one. install.sh
+    // generates it alongside the other secrets; the host reads the SAME value from .env.
+    'update_hmac_key' => env('UPDATE_HMAC_KEY', ''),
+
     'license' => 'AGPL-3.0',
 ];
diff --git a/docker/restart-sentinel/watch.sh b/docker/restart-sentinel/watch.sh
index caa58f7..c3130c7 100755
--- a/docker/restart-sentinel/watch.sh
+++ b/docker/restart-sentinel/watch.sh
@@ -35,14 +35,42 @@ COMPOSE_FILE="${CLUSEV_DIR}/docker-compose.prod.yml"
 TAG="clusev-restart"
 log() { printf '%s %s: %s\n' "$(date -Is)" "$TAG" "$*"; }
 
-# Publish an "error" stage to the update-progress feed (run/update-phase.json, bind-mounted into the
-# app container) so the browser shows the failure AT ONCE instead of spinning to its 10-minute
-# timeout. Best-effort — a write failure must never change the update's outcome.
+# run_write_json PAYLOAD — write PAYLOAD to run/update-phase.json SYMLINK-SAFELY (as root, into a
+# directory the app container can write). ./run is container-writable, so a compromised container
+# could pre-plant update-phase.json as a symlink to an arbitrary host path; a plain `> file` would
+# follow it and let this root write escape ./run. We write a temp in ./run and rename(2) over the
+# target: rename replaces the path itself (link included) with our regular file and NEVER follows the
+# link. Best-effort — a write failure must never change the update's outcome.
+run_write_json() {
+  local d="${CLUSEV_DIR}/run" tmp
+  mkdir -p "$d" 2>/dev/null || true
+  tmp="$(mktemp "${d}/.phase.XXXXXX" 2>/dev/null)" || return 0
+  printf '%s\n' "$1" > "$tmp" 2>/dev/null || { rm -f "$tmp" 2>/dev/null; return 0; }
+  chmod 644 "$tmp" 2>/dev/null || true
+  mv -f "$tmp" "${d}/update-phase.json" 2>/dev/null || rm -f "$tmp" 2>/dev/null
+  return 0
+}
+
+# Publish an "error" stage to the update-progress feed so the browser shows the failure AT ONCE
+# instead of spinning to its 10-minute timeout.
 write_update_error() {
-  local f="${CLUSEV_DIR}/run/update-phase.json"
-  mkdir -p "${CLUSEV_DIR}/run" 2>/dev/null || true
-  printf '{"stage":"error","at":%s}\n' "$(date +%s 2>/dev/null || echo 0)" > "$f" 2>/dev/null || true
-  chmod 644 "$f" 2>/dev/null || true
+  run_write_json "$(printf '{"stage":"error","at":%s}' "$(date +%s 2>/dev/null || echo 0)")"
+}
+
+# Authenticate an update request marker. The app (DeploymentService) signs it with UPDATE_HMAC_KEY
+# (line 1 = ISO8601 timestamp, line 2 = HMAC-SHA256(timestamp)); we recompute and compare. Refusing
+# an unsigned/forged marker stops a stray or limited write to ./run from driving a root update.
+# (A full app-container compromise holds the key, so this raises the bar rather than being absolute —
+# the standing guarantee is that a triggered update only re-installs the TRUSTED remote's code.)
+verify_update_request() {
+  local f="$1" key ts mac expect
+  key="$(grep -m1 '^UPDATE_HMAC_KEY=' "${CLUSEV_DIR}/.env" 2>/dev/null | cut -d= -f2-)"
+  [ -n "$key" ] || { log "UPDATE_HMAC_KEY unset — skipping request authentication"; return 0; }
+  ts="$(sed -n '1p' "$f" 2>/dev/null)"
+  mac="$(sed -n '2p' "$f" 2>/dev/null)"
+  [ -n "$ts" ] && [ -n "$mac" ] || { log "update request unsigned/malformed — refusing"; return 1; }
+  expect="$(printf '%s' "$ts" | openssl dgst -sha256 -hmac "$key" 2>/dev/null | sed 's/^.*= *//')"
+  [ -n "$expect" ] && [ "$expect" = "$mac" ] || { log "update request HMAC mismatch — refusing"; return 1; }
   return 0
 }
 
@@ -89,14 +117,24 @@ do_update() {
     write_update_error
     return 1
   fi
+  # Authenticate BEFORE consuming/running: refuse (and clear, so it does not loop the .path unit) a
+  # marker that carries no valid HMAC signature.
+  if ! verify_update_request "$CLUSEV_UPDATE_SIGNAL"; then
+    rm -f "$CLUSEV_UPDATE_SIGNAL"
+    write_update_error
+    return 1
+  fi
   rm -f "$CLUSEV_UPDATE_SIGNAL" && log "update sentinel consumed: $CLUSEV_UPDATE_SIGNAL"
   log "update requested — running update.sh in ${CLUSEV_DIR}"
   # Tee the whole run to a host log the operator can inspect, and cap it with a generous timeout so
   # a truly-stuck step (e.g. a hung build) fails LOUDLY rather than pinning the progress page. On ANY
   # non-zero exit (update.sh or its exec'd install.sh — the exit code propagates through exec) publish
   # the error stage so the browser stops spinning and shows the failure. `timeout` exit 124 = capped.
-  local logf="${CLUSEV_DIR}/run/update.log" rc=0
-  mkdir -p "${CLUSEV_DIR}/run" 2>/dev/null || true
+  # The log lives at the repo ROOT, NOT under ./run: ./run is the container-writable bind mount, so a
+  # log there could be pre-planted as a symlink and this root `tee` would follow it; the repo root is
+  # not mounted into any container, so a container cannot redirect this write.
+  local logf="${CLUSEV_DIR}/update.log" rc=0
+  rm -f "$logf" 2>/dev/null || true
   timeout -k 30 1800 bash "$updater" 2>&1 | tee "$logf" || rc=${PIPESTATUS[0]}
   if [ "$rc" = 0 ]; then
     log "update applied"
diff --git a/install.sh b/install.sh
index e359d13..1ab2c12 100755
--- a/install.sh
+++ b/install.sh
@@ -39,9 +39,14 @@ set_stage() {
 # ── .env mutators ────────────────────────────────────────────────────
 # force_kv: always write (config/derived). set_kv: write only when empty/placeholder.
 force_kv() {
-  local key="$1" val="$2"
+  local key="$1" val esc
+  # Strip stray CR/LF (a newline would split .env into two entries) and neutralise sed replacement
+  # metacharacters (& | \) so an operator-supplied domain/email/port can neither corrupt .env nor
+  # inject a sed command (values reach here unvalidated from CLUSEV_DOMAIN/ADMIN_EMAIL/HTTP_PORT).
+  val="$(printf '%s' "$2" | tr -d '\r\n')"
+  esc="$(printf '%s' "$val" | sed -e 's/[&|\\]/\\&/g')"
   if grep -qE "^${key}=" "$ENV_FILE"; then
-    sed -i "s|^${key}=.*|${key}=${val}|" "$ENV_FILE"
+    sed -i "s|^${key}=.*|${key}=${esc}|" "$ENV_FILE"
   else
     printf '%s=%s\n' "$key" "$val" >> "$ENV_FILE"
   fi
@@ -154,6 +159,13 @@ HOST_IP="$(hostname -I 2>/dev/null | awk '{print $1}')"; [ -n "$HOST_IP" ] || HO
 SERVER_IP="$(curl -fsS --max-time 5 https://api.ipify.org 2>/dev/null || true)"
 [ -n "$SERVER_IP" ] || SERVER_IP="$HOST_IP"
 
+# Reject a non-numeric / out-of-range HTTP port (an operator typo would otherwise reach compose as a
+# bad ${APP_PORT} and fail cryptically). Fall back to 80.
+case "$HTTP_PORT" in
+  ''|*[!0-9]*) warn "HTTP-Port '${HTTP_PORT}' nicht numerisch — nutze 80"; HTTP_PORT=80 ;;
+  *) if [ "$HTTP_PORT" -lt 1 ] || [ "$HTTP_PORT" -gt 65535 ]; then warn "HTTP-Port '${HTTP_PORT}' ausserhalb 1-65535 — nutze 80"; HTTP_PORT=80; fi ;;
+esac
+
 # ── domain DNS check (only when a domain is set) ─────────────────────
 DOMAIN_OK=0
 if [ -n "$DOMAIN" ]; then
@@ -202,6 +214,12 @@ fi
 phase 3/9 "Secrets generieren"
 if [ ! -f "$ENV_FILE" ]; then cp .env.example "$ENV_FILE"; info ".env aus .env.example erstellt";
 else info ".env vorhanden — bestehende Werte bleiben erhalten"; fi
+# .env holds EVERY secret: DB/root/redis passwords, APP_KEY (decrypts the whole fleet's SSH-credential
+# vault + 2FA secrets), the terminal sidecar secret and the update HMAC key. Never leave it
+# world-readable. Own it by the clusev user (the tree owner) and lock it to 0600 BEFORE any secret is
+# written; the subsequent sed -i / >> writes preserve this mode.
+chown clusev:clusev "$ENV_FILE" 2>/dev/null || true
+chmod 600 "$ENV_FILE"
 
 set_kv DB_PASSWORD       "$(rand_hex 24)"
 set_kv DB_ROOT_PASSWORD  "$(rand_hex 24)"
diff --git a/update.sh b/update.sh
index 54906d0..c8fa800 100755
--- a/update.sh
+++ b/update.sh
@@ -32,9 +32,15 @@ export GIT_HTTP_LOW_SPEED_TIME="${GIT_HTTP_LOW_SPEED_TIME:-30}"
 # Written first as "fetch" so the screen leaves the time-guess behind the moment the pull begins,
 # and so a stale stage from a previous update is reset.
 set_stage() {
-  mkdir -p "$REPO_DIR/run" 2>/dev/null || true
-  printf '{"stage":"%s","at":%s}\n' "$1" "$(date +%s 2>/dev/null || echo 0)" > "$REPO_DIR/run/update-phase.json" 2>/dev/null || true
-  chmod 644 "$REPO_DIR/run/update-phase.json" 2>/dev/null || true
+  # Symlink-safe write: ./run is the container-writable bind mount, so update-phase.json could be
+  # pre-planted as a symlink; write a temp in ./run and rename(2) over the target so this root write
+  # replaces the link itself and never follows it (mirror of run_write_json in the restart-sentinel).
+  local d="$REPO_DIR/run" tmp
+  mkdir -p "$d" 2>/dev/null || true
+  tmp="$(mktemp "${d}/.phase.XXXXXX" 2>/dev/null)" || return 0
+  printf '{"stage":"%s","at":%s}\n' "$1" "$(date +%s 2>/dev/null || echo 0)" > "$tmp" 2>/dev/null || { rm -f "$tmp" 2>/dev/null; return 0; }
+  chmod 644 "$tmp" 2>/dev/null || true
+  mv -f "$tmp" "${d}/update-phase.json" 2>/dev/null || rm -f "$tmp" 2>/dev/null
   return 0
 }
 set_stage fetch