From 2256f8359b795261b4492f0630c2ee9c0a7908f3 Mon Sep 17 00:00:00 2001
From: Gerhard Scheikl <hatl@hatl.at>
Date: Sun, 31 May 2026 23:02:40 +0200
Subject: [PATCH] feat(security): server-side guard preventing removal of last
 MFA factor

---
 app/api/security/unenroll/route.ts | 66 ++++++++++++++++++++++++++++++
 app/security/security-client.tsx   | 16 +++++++-
 2 files changed, 80 insertions(+), 2 deletions(-)
 create mode 100644 app/api/security/unenroll/route.ts

diff --git a/app/api/security/unenroll/route.ts b/app/api/security/unenroll/route.ts
new file mode 100644
index 0000000..8be610e
--- /dev/null
+++ b/app/api/security/unenroll/route.ts
@@ -0,0 +1,66 @@
+import { NextResponse, type NextRequest } from 'next/server';
+import { createSupabaseServerClient } from '@/lib/supabase/server';
+import { jsonNoStore } from '@/lib/admin/response';
+
+export const runtime = 'nodejs';
+export const dynamic = 'force-dynamic';
+
+/**
+ * Remove one of the current user's MFA factors. Requires an aal2 session.
+ *
+ * Mandatory MFA means every user must keep at least one VERIFIED factor. The
+ * client disables the "Remove" button on the last factor, but that is only a
+ * first line of defense — this route is the real server-side guard: if deleting
+ * the requested factor would leave the user with zero verified factors we
+ * refuse with 409 and never call GoTrue. The factor must belong to the caller.
+ */
+export async function POST(request: NextRequest): Promise<NextResponse> {
+  const supabase = createSupabaseServerClient();
+  const {
+    data: { user },
+  } = await supabase.auth.getUser();
+  if (!user) return jsonNoStore({ error: 'unauthorized' }, { status: 401 });
+
+  const { data: aal } = await supabase.auth.mfa.getAuthenticatorAssuranceLevel();
+  if (aal?.currentLevel !== 'aal2') {
+    return jsonNoStore({ error: 'aal2_required' }, { status: 403 });
+  }
+
+  let body: { factorId?: string };
+  try {
+    body = (await request.json()) as { factorId?: string };
+  } catch {
+    return jsonNoStore({ error: 'bad_request' }, { status: 400 });
+  }
+  const factorId = (body.factorId ?? '').trim();
+  if (!factorId) {
+    return jsonNoStore({ error: 'invalid_factor' }, { status: 400 });
+  }
+
+  const { data: list, error: listErr } = await supabase.auth.mfa.listFactors();
+  if (listErr) return jsonNoStore({ error: 'failed' }, { status: 500 });
+
+  const all = list?.all ?? [];
+  const target = all.find((f) => f.id === factorId);
+  if (!target) {
+    return jsonNoStore({ error: 'invalid_factor' }, { status: 404 });
+  }
+
+  // Only VERIFIED factors count as protection. If the user would be left with
+  // no verified factors after this removal, refuse.
+  const verified = all.filter((f) => f.status === 'verified');
+  const remainingVerified = verified.filter((f) => f.id !== factorId).length;
+  if (target.status === 'verified' && remainingVerified === 0) {
+    return jsonNoStore(
+      { error: 'cannot_remove_last_factor' },
+      { status: 409 },
+    );
+  }
+
+  const { error: unenrollErr } = await supabase.auth.mfa.unenroll({ factorId });
+  if (unenrollErr) {
+    return jsonNoStore({ error: unenrollErr.message }, { status: 400 });
+  }
+
+  return jsonNoStore({ ok: true });
+}
diff --git a/app/security/security-client.tsx b/app/security/security-client.tsx
index 5d115d7..15bf71f 100644
--- a/app/security/security-client.tsx
+++ b/app/security/security-client.tsx
@@ -50,8 +50,20 @@ export function SecurityClient({
     setNotice(null);
     setBusyId(id);
     try {
-      const { error } = await supabase.auth.mfa.unenroll({ factorId: id });
-      if (error) throw error;
+      const res = await fetch('/api/security/unenroll', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ factorId: id }),
+      });
+      if (!res.ok) {
+        const json = (await res.json().catch(() => null)) as {
+          error?: string;
+        } | null;
+        if (json?.error === 'cannot_remove_last_factor') {
+          throw new Error('You must keep at least one two-factor method enabled.');
+        }
+        throw new Error(json?.error || 'Could not remove this method.');
+      }
       router.refresh();
     } catch (e) {
       setError((e as Error).message);