diff --git a/template/server/src/index.ts b/template/server/src/index.ts
index eb22fab..4ddbcd6 100644
--- a/template/server/src/index.ts
+++ b/template/server/src/index.ts
@@ -138,6 +138,21 @@ function addrOf(req: IncomingMessage): string {
 	return createHmac('sha256', salt).update(clientOf(req)).digest('hex').slice(0, 16);
 }
 
+/**
+ * Whether the request was sent by this site's own pages. A page elsewhere could have a visitor's browser post here
+ * on its behalf; the browser names that page's origin, and the request is refused. The cookie is not sent across
+ * sites in any case, so what such a page could do is re-key a visitor's own seats under a name, to no end but nuisance.
+ */
+function fromThisSite(req: IncomingMessage): boolean {
+	const origin = req.headers.origin;
+	if (!origin) return true;
+	try {
+		return new URL(origin).host === req.headers.host;
+	} catch {
+		return false;
+	}
+}
+
 /** The seats the browser's cookie banks, as room and token; a cookie this server never wrote reads as none. */
 function bankedSeats(req: IncomingMessage): { roomId: string; token: string }[] {
 	const header = req.headers.cookie ?? '';
@@ -324,6 +339,7 @@ async function handle(req: IncomingMessage, res: ServerResponse): Promise<void>
 		return send(res, 200, rooms.claim(body.phrase));
 	}
 	if (parts[0] === 'api' && parts[1] === 'seats' && req.method === 'POST') {
+		if (!fromThisSite(req) || !String(req.headers['content-type'] ?? '').startsWith('application/json')) throw new RoomError('Not from here.', 403);
 		const body = await readBody(req);
 		if (parts.length === 2) {
 			const seats = provenBank(req, body.seats, body.forget);