diff --git a/modules/images/image-loading-optimization/detection.php b/modules/images/image-loading-optimization/detection.php
index 3dcd945e91..00f5e10c72 100644
--- a/modules/images/image-loading-optimization/detection.php
+++ b/modules/images/image-loading-optimization/detection.php
@@ -40,12 +40,17 @@ function ilo_print_detection_script() {
 	 */
 	$detection_time_window = apply_filters( 'perflab_image_loading_detection_time_window', 5000 );
 
+	$query_vars = ilo_get_normalized_query_vars();
+	$slug       = ilo_get_page_metrics_slug( $query_vars );
+
 	$detect_args = array(
 		'serveTime'           => $serve_time,
 		'detectionTimeWindow' => $detection_time_window,
 		'isDebug'             => WP_DEBUG,
 		'restApiEndpoint'     => rest_url( ILO_REST_API_NAMESPACE . ILO_PAGE_METRICS_ROUTE ),
 		'restApiNonce'        => wp_create_nonce( 'wp_rest' ),
+		'pageMetricsSlug'     => $slug,
+		'pageMetricsHmac'     => ilo_get_slug_hmac( $slug ), // TODO: Or would a nonce make more sense with the $slug being the action?
 	);
 	wp_print_inline_script_tag(
 		sprintf(
diff --git a/modules/images/image-loading-optimization/detection/detect.js b/modules/images/image-loading-optimization/detection/detect.js
index 7498e3d81d..dec3879ba3 100644
--- a/modules/images/image-loading-optimization/detection/detect.js
+++ b/modules/images/image-loading-optimization/detection/detect.js
@@ -104,6 +104,8 @@ function getBreadcrumbs( leafElement ) {
  * @param {boolean} args.isDebug             Whether to show debug messages.
  * @param {string}  args.restApiEndpoint     URL for where to send the detection data.
  * @param {string}  args.restApiNonce        Nonce for writing to the REST API.
+ * @param {string}  args.pageMetricsSlug     Slug for page metrics.
+ * @param {string}  args.pageMetricsHmac     HMAC for the page metric slug.
  */
 export default async function detect( {
 	serveTime,
@@ -111,6 +113,8 @@ export default async function detect( {
 	isDebug,
 	restApiEndpoint,
 	restApiNonce,
+	pageMetricsSlug,
+	pageMetricsHmac,
 } ) {
 	const runTime = new Date().valueOf();
 
@@ -259,7 +263,9 @@ export default async function detect( {
 
 	/** @type {PageMetrics} */
 	const pageMetrics = {
-		url: win.location.href, // TODO: Consider sending canonical URL instead.
+		url: win.location.href,
+		slug: pageMetricsSlug,
+		hmac: pageMetricsHmac,
 		viewport: {
 			width: win.innerWidth,
 			height: win.innerHeight,
diff --git a/modules/images/image-loading-optimization/storage/data.php b/modules/images/image-loading-optimization/storage/data.php
index c2ac6b38e4..63e5ccc3b7 100644
--- a/modules/images/image-loading-optimization/storage/data.php
+++ b/modules/images/image-loading-optimization/storage/data.php
@@ -96,6 +96,30 @@ function ilo_get_normalized_query_vars() {
 	return $normalized_query_vars;
 }
 
+/**
+ * Gets slug for page metrics.
+ *
+ * @see ilo_get_normalized_query_vars()
+ *
+ * @param array $query_vars Normalized query vars.
+ * @return string Slug.
+ */
+function ilo_get_page_metrics_slug( $query_vars ) {
+	return md5( wp_json_encode( $query_vars ) );
+}
+
+/**
+ * Compute HMAC for page metrics slug.
+ *
+ * This is used in the REST API to authenticate the storage of new page metrics from a given URL.
+ *
+ * @param string $slug Page metrics slug.
+ * @return false HMAC.
+ */
+function ilo_get_slug_hmac( $slug ) {
+	return hash_hmac( 'sha1', $slug, wp_salt() );
+}
+
 /**
  * Unshift a new page metric onto an array of page metrics.
  *
diff --git a/modules/images/image-loading-optimization/storage/post-type.php b/modules/images/image-loading-optimization/storage/post-type.php
index 578dcc2db2..d3bfefdc32 100644
--- a/modules/images/image-loading-optimization/storage/post-type.php
+++ b/modules/images/image-loading-optimization/storage/post-type.php
@@ -37,28 +37,18 @@ function ilo_register_page_metrics_post_type() {
 }
 add_action( 'init', 'ilo_register_page_metrics_post_type' );
 
-/**
- * Gets slug for page metrics post.
- *
- * @param string $url URL.
- * @return string Slug for URL.
- */
-function ilo_get_page_metrics_slug( $url ) {
-	return md5( $url );
-}
-
 /**
  * Get page metrics post.
  *
- * @param string $url URL.
+ * @param string $slug Page metrics slug.
  * @return WP_Post|null Post object if exists.
  */
-function ilo_get_page_metrics_post( $url ) {
+function ilo_get_page_metrics_post( $slug ) {
 	$post_query = new WP_Query(
 		array(
 			'post_type'              => ILO_PAGE_METRICS_POST_TYPE,
 			'post_status'            => 'publish',
-			'name'                   => ilo_get_page_metrics_slug( $url ),
+			'name'                   => $slug,
 			'posts_per_page'         => 1,
 			'no_found_rows'          => true,
 			'cache_results'          => true,
@@ -114,7 +104,6 @@ function ilo_parse_stored_page_metrics( WP_Post $post ) {
  * The $validated_page_metric parameter has the following array shape:
  *
  * {
- *      'url': string,
  *      'viewport': array{
  *          'width': int,
  *          'height': int
@@ -122,21 +111,20 @@ function ilo_parse_stored_page_metrics( WP_Post $post ) {
  *      'elements': array
  * }
  *
- * @param array $validated_page_metric Page metric, already validated by REST API.
- *
+ * @param string $url                   URL for the page metrics. This is used purely as metadata.
+ * @param string $slug                  Page metrics slug (computed from query vars).
+ * @param array  $validated_page_metric Page metric, already validated by REST API.
  * @return int|WP_Error Post ID or WP_Error otherwise.
  */
-function ilo_store_page_metric( array $validated_page_metric ) {
-	$url = $validated_page_metric['url'];
-	unset( $validated_page_metric['url'] ); // Not stored in post_content but rather in post_title/post_name.
+function ilo_store_page_metric( $url, $slug, array $validated_page_metric ) {
 	$validated_page_metric['timestamp'] = time();
 
 	// TODO: What about storing a version identifier?
 	$post_data = array(
-		'post_title' => $url,
+		'post_title' => $url, // TODO: Should we keep this? It can help with debugging.
 	);
 
-	$post = ilo_get_page_metrics_post( $url );
+	$post = ilo_get_page_metrics_post( $slug );
 
 	if ( $post instanceof WP_Post ) {
 		$post_data['ID']        = $post->ID;
@@ -150,7 +138,7 @@ function ilo_store_page_metric( array $validated_page_metric ) {
 			$page_metrics = array();
 		}
 	} else {
-		$post_data['post_name'] = ilo_get_page_metrics_slug( $url );
+		$post_data['post_name'] = $slug;
 		$page_metrics           = array();
 	}
 
diff --git a/modules/images/image-loading-optimization/storage/rest-api.php b/modules/images/image-loading-optimization/storage/rest-api.php
index 4844392d3a..fb2c2c9868 100644
--- a/modules/images/image-loading-optimization/storage/rest-api.php
+++ b/modules/images/image-loading-optimization/storage/rest-api.php
@@ -65,6 +65,22 @@ function ilo_register_endpoint() {
 						return true;
 					},
 				),
+				'slug'     => array(
+					'type'     => 'string',
+					'required' => true,
+					'pattern'  => '^[0-9a-f]{32}$',
+				),
+				'hmac'     => array(
+					'type'              => 'string',
+					'required'          => true,
+					'pattern'           => '^[0-9a-f]+$',
+					'validate_callback' => static function ( $hmac, WP_REST_Request $request ) {
+						if ( ! hash_equals( $hmac, ilo_get_slug_hmac( $request->get_param( 'slug' ) ) ) ) {
+							return new WP_Error( 'invalid_hmac', __( 'HMAC comparison failure.', 'performance-lab' ) );
+						}
+						return true;
+					},
+				),
 				'viewport' => array(
 					'description' => __( 'Viewport dimensions', 'performance-lab' ),
 					'type'        => 'object',
@@ -142,7 +158,7 @@ function ilo_handle_rest_request( WP_REST_Request $request ) {
 	ilo_set_page_metric_storage_lock();
 
 	$page_metric = $request->get_json_params();
-	$result      = ilo_store_page_metric( $page_metric );
+	$result      = ilo_store_page_metric( $page_metric['url'], $page_metric['slug'], $request->get_json_params() );
 
 	if ( $result instanceof WP_Error ) {
 		return $result;
@@ -152,7 +168,7 @@ function ilo_handle_rest_request( WP_REST_Request $request ) {
 		array(
 			'success' => true,
 			'post_id' => $result,
-			'data'    => ilo_parse_stored_page_metrics( ilo_get_page_metrics_post( $page_metric['url'] ) ), // TODO: Remove this debug data.
+			'data'    => ilo_parse_stored_page_metrics( ilo_get_page_metrics_post( $page_metric['slug'] ) ), // TODO: Remove this debug data.
 		)
 	);
 }