diff --git a/bin/box b/bin/box
index f076c98..0e840ec 100755
--- a/bin/box
+++ b/bin/box
@@ -630,7 +630,14 @@ cmd_new() {
     load_template "$t"
     m="$(pick_mode)"
     # shellcheck disable=SC2054 # "root,size=..." is a single incus argument
-    if [ "$m" = vm ]; then extra+=(--vm --device "root,size=$T_DISK"); else extra+=(--config security.nesting=true); fi
+    # security.secureboot=false: Incus defaults VMs to secureboot ON, and a
+    # Debian cloud image whose shim is signed with a key the host's OVMF does
+    # not trust dies with "bad shim signature / prohibited by secure boot
+    # policy" and drops to the GRUB menu forever — the kernel never loads. It
+    # is not part of a throwaway box's threat model (the VM boundary is), and
+    # turning it off boots reliably across image rebuilds. Container mode has
+    # no firmware, so it does not apply there.
+    if [ "$m" = vm ]; then extra+=(--vm --device "root,size=$T_DISK" --config security.secureboot=false); else extra+=(--config security.nesting=true); fi
     # The template's identity is stamped ONTO the instance: which template,
     # which user. 'incus copy' preserves user.* keys (audit B2), so a clone
     # knows what it is without ever consulting the template again.
diff --git a/drill/drill.sh b/drill/drill.sh
index a82093c..d8edd7a 100755
--- a/drill/drill.sh
+++ b/drill/drill.sh
@@ -779,7 +779,8 @@ else
   # old stack, wearing the old tag. This is what migrate has to move.
   printf '\n  minting a faithful legacy box on the old stack…\n'
   if mint_legacy=$(incus launch images:debian/13/cloud legacybox --profile claude-dev \
-       --config user.claudebox=1 --vm --device root,size=20GiB 2>&1); then
+       --config user.claudebox=1 --vm --device root,size=20GiB \
+       --config security.secureboot=false 2>&1); then
     wait_box legacybox && ok "legacy box up on the old stack (claudenet, user.claudebox=1)" \
                        || no "legacy box never came up — cannot drill migration"
     box list 2>/dev/null | grep -q '^legacybox' \