diff --git a/src/Http/Controllers/PortalController.php b/src/Http/Controllers/PortalController.php
index aeb7e61..c970038 100644
--- a/src/Http/Controllers/PortalController.php
+++ b/src/Http/Controllers/PortalController.php
@@ -87,6 +87,35 @@ class PortalController extends Controller
 
     // ── Captive portal callback (called by UniFi external portal redirect) ───
 
+    /**
+     * A redirect target that cannot be turned into a phishing or script payload.
+     *
+     * Accepts only absolute http/https URLs and site-relative paths. Rejects
+     * javascript:/data:/vbscript: schemes, and protocol-relative "//host" values which
+     * redirect() would happily send off-site. Anything rejected falls back to the
+     * dashboard rather than erroring — the guest is already online at this point, so a
+     * bad `url` parameter should not look like a failed login.
+     */
+    private function safeRedirectTarget(?string $url): string
+    {
+        $url = trim((string) $url);
+
+        if ($url === '' || str_starts_with($url, '//')) {
+            return '/';
+        }
+
+        // Site-relative is always fine.
+        if (str_starts_with($url, '/')) {
+            return $url;
+        }
+
+        $scheme = strtolower((string) parse_url($url, PHP_URL_SCHEME));
+
+        return in_array($scheme, ['http', 'https'], true) && parse_url($url, PHP_URL_HOST)
+            ? $url
+            : '/';
+    }
+
     public function captiveCallback(Request $request, UnifiApiClient $unifi)
     {
         // UniFi redirects guest to this URL with ?id=<ap_mac>&ap=<ap_mac>&t=<timestamp>&url=<original_url>&ssid=<ssid>
@@ -104,6 +133,23 @@ class PortalController extends Controller
             return response('Missing MAC address', 400);
         }
 
+        // SECURITY: everything identifying the device is request-supplied, and this
+        // endpoint WRITES network state (authorizeGuest puts a MAC on the mapped VLAN).
+        // Before this check any signed-in account — a student, a parent — could call
+        //   GET /portal/wifi/callback?mac=<someone else's MAC>&url=…
+        // and admit a device they do not own, up to max_devices.
+        //
+        // A well-formed MAC is the floor, not a fix: it stops the parameter being used
+        // to smuggle junk into authorizeGuest and into the PortalSession row, but it
+        // cannot prove the caller owns the device. Properly binding it needs the AP
+        // handshake — UniFi can sign the redirect, or we ask the controller whether this
+        // MAC is CURRENTLY associated to $apMac before admitting it. Both need testing
+        // against real hardware, so they are deliberately not guessed at here.
+        $clientMac = str_replace('-', ':', $clientMac);
+        if (! preg_match('/^([0-9a-f]{2}:){5}[0-9a-f]{2}$/', $clientMac)) {
+            return response('Malformed MAC address', 400);
+        }
+
         // Find the VLAN mapping for this user's group/OU
         $mapping = $this->resolveMapping($user);
         if (! $mapping) {
@@ -161,9 +207,16 @@ class PortalController extends Controller
             'expires_at'       => now()->addMinutes($minutes),
         ]);
 
-        // Redirect to the original URL the user was trying to reach
-        $redirectUrl = $request->input('url', '/');
-        return redirect($redirectUrl);
+        // Redirect to the original URL the user was trying to reach.
+        //
+        // SECURITY: this was `redirect($request->input('url', '/'))` — an unvalidated
+        // open redirect on an authenticated route, i.e. a phishing primitive wearing
+        // this school's domain. The whole point of a captive portal is to send someone
+        // to an ARBITRARY external site they were already trying to reach, so an
+        // allowlist is not available; what is available is refusing anything that is
+        // not a plain http(s) URL, which blocks javascript:, data: and protocol-relative
+        // //evil.example payloads.
+        return redirect($this->safeRedirectTarget($request->input('url')));
     }
 
     public function disconnectSession(Request $request, PortalSession $session, UnifiApiClient $unifi)