diff --git a/src/serverfs/usr/sbin/linbo-torrent b/src/serverfs/usr/sbin/linbo-torrent
index e388923..925a0b9 100755
--- a/src/serverfs/usr/sbin/linbo-torrent
+++ b/src/serverfs/usr/sbin/linbo-torrent
@@ -52,11 +52,18 @@ usage(){
 }
 
 # update image permissions
+# *.macct files hold samba machine account password hashes. The [linbo]
+# rsync module serves LINBODIR as nobody without login, so they must never
+# belong to nobody, at any depth (backups, .incoming, ...). They are left out
+# of the nobody chown rather than taken back afterwards.
+# images/.incoming is the staging dir of the central image sync
+# (linuxmuster-tools), which keeps it root 700 while an upload waits there,
+# so it is left out of the nobody chown too.
 update_permissions(){
-  chown nobody:nogroup "$LINBOIMGDIR" -R
+  find "$LINBOIMGDIR" -type f -name "*.macct" -exec chown root:root {} + -exec chmod 600 {} +
+  find "$LINBOIMGDIR" -path "$LINBOIMGDIR/.incoming" -prune \
+    -o ! \( -type f -name "*.macct" \) -exec chown -h nobody:nogroup {} +
   chown "$OTRUSER":"$OTRUSER" "$OTRDIR" -R
-  chown root:root "$LINBOIMGDIR"/*/*.macct
-  chmod 600 "$LINBOIMGDIR"/*/*.macct
 }
 
 # save torrent hash
diff --git a/src/serverfs/usr/share/linuxmuster/linbo/rsync-post-upload.sh b/src/serverfs/usr/share/linuxmuster/linbo/rsync-post-upload.sh
index 736c36f..64dc6e5 100755
--- a/src/serverfs/usr/share/linuxmuster/linbo/rsync-post-upload.sh
+++ b/src/serverfs/usr/share/linuxmuster/linbo/rsync-post-upload.sh
@@ -118,8 +118,12 @@ case "$EXT" in
       if [ -n "$unicodepwd" ]; then
         echo "Saving samba password hash and kvno for $image."
         template="$LINBOTPLDIR/machineacct"
-        sed -e "s|@@unicodepwd@@|$unicodepwd|" \
-          -e "s|@@suppcredentials@@|$suppcredentials|" "$template" > "$imagemacct"
+        # create it anew with mode 600, the [linbo] rsync module can read it
+        # as nobody while it is still 644, and writing into an existing file
+        # keeps its mode until the chmod
+        rm -f "$imagemacct"
+        (umask 077; sed -e "s|@@unicodepwd@@|$unicodepwd|" \
+          -e "s|@@suppcredentials@@|$suppcredentials|" "$template" > "$imagemacct")
         chmod 600 "$imagemacct"
         # remove obsolete macct file if present
         rm -f "$IMGDIR/${BASE}.macct"
diff --git a/tests/shell/test_linbo-torrent.sh b/tests/shell/test_linbo-torrent.sh
new file mode 100644
index 0000000..5b0f869
--- /dev/null
+++ b/tests/shell/test_linbo-torrent.sh
@@ -0,0 +1,158 @@
+#!/bin/sh
+#
+# test_linbo-torrent.sh
+# tom.lehmann@netzint.de
+# 20260929
+#
+# update_permissions: machine account files (*.macct) anywhere below the
+# images dir end up root:root 600 and are never handed to nobody on the way,
+# everything else keeps going to nobody:nogroup (opentracker dir to its
+# user) except the image sync's staging dir images/.incoming, symlinks are
+# not followed, and no errors without any *.macct.
+#
+# linbo-torrent is a bash script on the server, so the function runs under
+# bash here, whichever shell runs the tests. chown is replaced by a stub on
+# PATH that logs "owner path" per file instead of changing anything, since
+# the tests don't run as root (busybox ash would ignore the stub and run its
+# own chown applet). chmod is the real one.
+
+SCRIPT="$(dirname "$0")/../../src/serverfs/usr/sbin/linbo-torrent"
+
+oneTimeSetUp() {
+  . "$(dirname "$0")/lib/extract_function.sh"
+  extract_function "$SCRIPT" update_permissions > "$SHUNIT_TMPDIR/linbo_torrent_update_permissions.sh"
+  mkdir -p "$SHUNIT_TMPDIR/bin"
+  cat > "$SHUNIT_TMPDIR/bin/chown" <<'EOF'
+#!/bin/sh
+# chown stub: logs "owner path" for every file chown would change. -R logs
+# the whole tree without following symlinks; a symlink given without -h
+# logs its target, as chown would change that one.
+recursive=""
+nodereference=""
+for arg; do
+  case "$arg" in
+    -R) recursive=1 ;;
+    -h) nodereference=1 ;;
+  esac
+done
+owner=""
+rc=0
+for arg; do
+  case "$arg" in -*) continue ;; esac
+  if [ -z "$owner" ]; then
+    owner="$arg"
+  elif [ ! -e "$arg" ] && [ ! -L "$arg" ]; then
+    echo "chown: cannot access '$arg': No such file or directory" >&2
+    rc=1
+  elif [ -n "$recursive" ]; then
+    find "$arg" | sed "s|^|$owner |" >> "$CHOWN_LOG"
+  elif [ -L "$arg" ] && [ -z "$nodereference" ]; then
+    echo "$owner $(readlink -f "$arg")" >> "$CHOWN_LOG"
+  else
+    echo "$owner $arg" >> "$CHOWN_LOG"
+  fi
+done
+exit "$rc"
+EOF
+  chmod 755 "$SHUNIT_TMPDIR/bin/chown"
+}
+
+setUp() {
+  images="$SHUNIT_TMPDIR/images"
+  outside="$SHUNIT_TMPDIR/outside"
+  CHOWN_LOG="$SHUNIT_TMPDIR/chown.log"
+  STDERR_LOG="$SHUNIT_TMPDIR/stderr.log"
+  rm -rf "$images" "$outside" "$CHOWN_LOG" "$STDERR_LOG"
+  mkdir -p "$images/win10/backups/202609011200" "$images/tmp" \
+    "$images/.incoming/win11" "$images/opentracker" "$outside"
+  for f in win10/win10.qcow2 win10/win10.qcow2.info win10/win10.qcow2.macct \
+    win10/backups/202609011200/win10.qcow2 \
+    win10/backups/202609011200/win10.qcow2.macct \
+    tmp/win10.qcow2.macct \
+    .incoming/win11/win11.qcow2 .incoming/win11/win11.qcow2.macct \
+    opentracker/torrent.whitelist; do
+    echo "$f" > "$images/$f"
+    chmod 644 "$images/$f"
+  done
+  echo secret > "$outside/secret"
+  chmod 644 "$outside/secret"
+}
+
+run_update_permissions() {
+  CHOWN_LOG="$CHOWN_LOG" PATH="$SHUNIT_TMPDIR/bin:$PATH" \
+    LINBOIMGDIR="$images" OTRDIR="$images/opentracker" OTRUSER="_opentracker" \
+    bash -c '. "$1" && update_permissions' sh \
+    "$SHUNIT_TMPDIR/linbo_torrent_update_permissions.sh" 2> "$STDERR_LOG"
+}
+
+# owner a file got last
+owner_of() {
+  awk -v f="$1" '$2 == f { owner = $1 } END { print owner }' "$CHOWN_LOG"
+}
+
+macct_files() {
+  echo "$images/win10/win10.qcow2.macct"
+  echo "$images/win10/backups/202609011200/win10.qcow2.macct"
+  echo "$images/tmp/win10.qcow2.macct"
+  echo "$images/.incoming/win11/win11.qcow2.macct"
+}
+
+test_macct_files_end_up_root_600_at_any_depth() {
+  run_update_permissions
+  for f in $(macct_files); do
+    assertEquals "owner of $f" "root:root" "$(owner_of "$f")"
+    assertEquals "mode of $f" "600" "$(stat -c %a "$f")"
+  done
+}
+
+test_macct_files_are_never_given_to_nobody() {
+  run_update_permissions
+  for f in $(macct_files); do
+    assertFalse "$f given to nobody" "grep -qxF 'nobody:nogroup $f' '$CHOWN_LOG'"
+  done
+}
+
+test_other_files_go_to_nobody_unchanged_mode() {
+  run_update_permissions
+  for f in "$images" "$images/win10" "$images/win10/win10.qcow2" \
+    "$images/win10/win10.qcow2.info" "$images/win10/backups" \
+    "$images/win10/backups/202609011200" \
+    "$images/win10/backups/202609011200/win10.qcow2" "$images/tmp"; do
+    assertEquals "owner of $f" "nobody:nogroup" "$(owner_of "$f")"
+  done
+  assertEquals "644" "$(stat -c %a "$images/win10/win10.qcow2")"
+}
+
+test_incoming_is_left_to_the_image_sync() {
+  run_update_permissions
+  assertFalse ".incoming given to nobody" \
+    "grep -qF 'nobody:nogroup $images/.incoming' '$CHOWN_LOG'"
+  assertEquals "root:root" \
+    "$(owner_of "$images/.incoming/win11/win11.qcow2.macct")"
+}
+
+test_opentracker_dir_goes_to_opentracker_user() {
+  run_update_permissions
+  assertEquals "_opentracker:_opentracker" "$(owner_of "$images/opentracker")"
+  assertEquals "_opentracker:_opentracker" \
+    "$(owner_of "$images/opentracker/torrent.whitelist")"
+}
+
+test_no_errors_without_macct_files() {
+  for f in $(macct_files); do
+    rm -f "$f"
+  done
+  run_update_permissions
+  assertEquals 0 $?
+  assertEquals "" "$(cat "$STDERR_LOG")"
+}
+
+test_symlinks_are_not_followed() {
+  ln -s "$outside/secret" "$images/win10/link.qcow2.macct"
+  ln -s "$outside/secret" "$images/win10/link.qcow2"
+  run_update_permissions
+  assertFalse "symlink target chowned" "grep -qF '$outside' '$CHOWN_LOG'"
+  assertEquals "644" "$(stat -c %a "$outside/secret")"
+}
+
+. "$(dirname "$0")/shunit2"