diff --git a/frontend/src/routes/documents/[id]/+page.svelte b/frontend/src/routes/documents/[id]/+page.svelte
index 88540618..0baa8985 100644
--- a/frontend/src/routes/documents/[id]/+page.svelte
+++ b/frontend/src/routes/documents/[id]/+page.svelte
@@ -88,11 +88,20 @@ async function loadTranscriptionBlocks() {
 	}
 }
 
+const UUID_RE = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
+
 async function saveBlock(
 	blockId: string,
 	text: string,
 	mentionedPersons: import('$lib/types').PersonMention[]
 ) {
+	// Path-injection defence in depth (Sina #5505): both ids are server-controlled
+	// today, but reject anything that isn't a UUID before interpolating it into
+	// the URL — a future feature accepting user-supplied ids must not silently
+	// bypass this check.
+	if (!UUID_RE.test(doc.id) || !UUID_RE.test(blockId)) {
+		throw new Error(`Invalid id for save: doc=${doc.id} block=${blockId}`);
+	}
 	const res = await fetch(`/api/documents/${doc.id}/transcription-blocks/${blockId}`, {
 		method: 'PUT',
 		headers: { 'Content-Type': 'application/json' },