diff --git a/api.wordpress.org/public_html/themes/info/1.0/index.php b/api.wordpress.org/public_html/themes/info/1.0/index.php
index 0007b8492c..90bd9d9939 100644
--- a/api.wordpress.org/public_html/themes/info/1.0/index.php
+++ b/api.wordpress.org/public_html/themes/info/1.0/index.php
@@ -1,4 +1,16 @@
 <?php
+/**
+ * Themes API 1.0 endpoint: theme information and queries for WordPress clients.
+ *
+ * This is a stateless public API endpoint, so there is no session or nonce infrastructure.
+ * The request is also parsed before `load_wordpress()` runs below, which means request data
+ * has not been slashed at that point.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Themes
+ */
+
 namespace WordPressdotorg\API\Themes\Info;
 use function WordPressdotorg\API\load_wordpress;
 
@@ -17,19 +29,34 @@
 function send_error( $error, $code = 404 ) {
 	global $format;
 
-	header( ( $_SERVER['SERVER_PROTOCOL'] ?? 'HTTP/1.0' ) . ' ' . $code, true, $code );
+	$protocol = filter_var(
+		$_SERVER['SERVER_PROTOCOL'] ?? '',
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '#^HTTP/[0-9.]+\z#',
+				'default' => 'HTTP/1.0',
+			),
+		)
+	);
+
+	header( $protocol . ' ' . $code, true, $code );
 
 	$response = (object) [
 		'error' => $error	
 	];
 
+	// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- Only used for a substring comparison, never output or stored.
+	$user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
+
 	// Browsers get a nicer action not implemented error.
 	if (
-		'GET' === $_SERVER['REQUEST_METHOD'] &&
-		false === strpos( $_SERVER['HTTP_USER_AGENT'] ?? '', 'WordPress/' ) &&
+		isset( $_SERVER['REQUEST_METHOD'] ) && 'GET' === $_SERVER['REQUEST_METHOD'] &&
+		false === strpos( $user_agent, 'WordPress/' ) &&
 		false !== strpos( $error, 'Action not implemented.' )
 	) {
 		header( 'Content-Type: text/html; charset=utf-8' );
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- Every caller passes a hard-coded message, one of which intentionally contains a link; esc_html() is not loaded here.
 		die( "<p>{$error}</p>" );
 	}
 
@@ -42,9 +69,10 @@ function send_error( $error, $code = 404 ) {
 	}
 
 	if ( 'php' === $format ) {
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- serialized payload, not HTML.
 		echo serialize( $response );
 	} else {
-		// JSON format
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON payload; no wp_json_encode() here.
 		echo json_encode( $response );
 	}
 
@@ -57,9 +85,11 @@ function send_error( $error, $code = 404 ) {
 
 // Set up action and request information.
 if ( defined( 'JSON_RESPONSE' ) && JSON_RESPONSE ) {
+	// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- Fields are type-checked and sanitized by Themes_API; DB access uses prepared WP_Query calls.
 	$request = isset( $_REQUEST['request'] ) ? (object) $_REQUEST['request'] : '';
 	$format = 'json';
 } else {
+	// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- Screened for object injection below; unserialized with `allowed_classes` limited to stdClass.
 	$post_request = isset( $_POST['request'] ) && is_string( $_POST['request'] ) ? $_POST['request'] : '';
 	if ( $post_request ) {
 		// PHP Needs to get a non-urldecoded request, to avoid multibyte character malforming the request,
@@ -79,7 +109,17 @@ function send_error( $error, $code = 404 ) {
 	$format = 'php';
 }
 
-$action = $_REQUEST['action'] ?? '';
+// phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited -- pre-existing; drives this endpoint's switch.
+$action = filter_var(
+	$_REQUEST['action'] ?? '',
+	FILTER_VALIDATE_REGEXP,
+	array(
+		'options' => array(
+			'regexp'  => '/^[a-z_]{1,32}\z/',
+			'default' => '',
+		),
+	)
+);
 
 // Validate the request.
 switch ( $action ) {
@@ -94,7 +134,7 @@ function send_error( $error, $code = 404 ) {
 			}
 
 			foreach ( $slugs as $slug ) {
-				if ( ! $slug || ! is_string( $slug ) || ! preg_match( '/^[a-z0-9-_]+$/', $slug ) ) {
+				if ( ! $slug || ! is_string( $slug ) || ! preg_match( '/^[a-z0-9-_]+\z/', $slug ) ) {
 					send_error( 'Invalid slugs provided' );
 				}
 
@@ -107,7 +147,7 @@ function send_error( $error, $code = 404 ) {
 			if ( ! $slug ) {
 				send_error( 'Slug not provided' );
 			}
-			if ( ! is_string( $slug ) || ! preg_match( '/^[a-z0-9-_]+$/', $slug ) ) {
+			if ( ! is_string( $slug ) || ! preg_match( '/^[a-z0-9-_]+\z/', $slug ) ) {
 				send_error( 'Invalid slug provided' );
 			}
 
@@ -140,6 +180,7 @@ function send_error( $error, $code = 404 ) {
 
 $api->set_status_header();
 
+// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON or serialized payload, not HTML.
 echo $api->get_result( $format );
 
 // Cache when a theme doesn't exist. See the validation handler above.
diff --git a/api.wordpress.org/public_html/themes/info/1.1/index.php b/api.wordpress.org/public_html/themes/info/1.1/index.php
index 7884e76418..2f067a681d 100644
--- a/api.wordpress.org/public_html/themes/info/1.1/index.php
+++ b/api.wordpress.org/public_html/themes/info/1.1/index.php
@@ -1,9 +1,30 @@
 <?php
+/**
+ * Themes API 1.1 endpoint: JSON wrapper around the 1.0 API.
+ *
+ * This is a stateless public API endpoint, so there is no session or nonce infrastructure.
+ * The request is also read before the 1.0 endpoint loads WordPress, which means request
+ * data has not been slashed at that point.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Themes
+ */
 
 header( 'Access-Control-Allow-Origin: *' );
 
 if ( isset( $_GET['callback'] ) && is_string( $_GET['callback'] ) ) {
-	$callback = preg_replace( '/[^a-z0-9_]/i', '', $_GET['callback'] );
+	// A callback that is not a valid JavaScript identifier falls back to plain JSON.
+	$callback = filter_var(
+		$_GET['callback'],
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '/^[a-z_][a-z0-9_]*\z/i',
+				'default' => false,
+			),
+		)
+	);
 } else {
 	$callback = false;
 }
@@ -20,8 +41,10 @@
 
 if ( $callback ) {
 	header( 'Content-Type: text/javascript; charset=UTF-8' );
+	// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- validated callback, JSON payload.
 	echo "$callback($response);";
 } else {
 	header( 'Content-Type: application/json; charset=UTF-8' );
+	// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON payload served as application/json.
 	echo $response;
 }
diff --git a/api.wordpress.org/public_html/themes/info/1.2/index.php b/api.wordpress.org/public_html/themes/info/1.2/index.php
index bcba2ec784..89b8eb60db 100644
--- a/api.wordpress.org/public_html/themes/info/1.2/index.php
+++ b/api.wordpress.org/public_html/themes/info/1.2/index.php
@@ -1,9 +1,31 @@
 <?php
+/**
+ * Themes API 1.2 endpoint: GET-only wrapper with flat request support.
+ *
+ * This is a stateless public API endpoint, so there is no session or nonce infrastructure.
+ * The request is also read before the 1.0 endpoint loads WordPress, which means request
+ * data has not been slashed at that point.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Themes
+ */
 
 // Version 1.2+ only accepts GET requests
-if ( isset( $_SERVER['REQUEST_METHOD'] ) && $_SERVER['REQUEST_METHOD'] === 'POST' ) {
-	header( $_SERVER['SERVER_PROTOCOL'] . ' 405 Method not allowed' );
-	header( 'Allow: GET' );
+if ( isset( $_SERVER['REQUEST_METHOD'] ) && ! in_array( $_SERVER['REQUEST_METHOD'], array( 'GET', 'HEAD', 'OPTIONS' ), true ) ) {
+	$protocol = filter_var(
+		$_SERVER['SERVER_PROTOCOL'] ?? '',
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '#^HTTP/[0-9.]+\z#',
+				'default' => 'HTTP/1.0',
+			),
+		)
+	);
+
+	header( $protocol . ' 405 Method not allowed' );
+	header( 'Allow: GET, HEAD, OPTIONS' );
 	header( 'Content-Type: text/plain' );
 
 	die( 'This API only serves GET requests.' );
@@ -15,8 +37,20 @@
 
 // Support "flat" requests, ie. no '?request[slug]=..` needed, just '?slug=...'
 if ( ! isset( $_GET['request'] ) ) {
+	// 1.2 only supports GET requests.
+	$requested_action = filter_var(
+		$_GET['action'] ?? '',
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '/^[a-z_]{1,32}\z/',
+				'default' => '',
+			),
+		)
+	);
+
 	$_GET = $_REQUEST = array(
-		'action'  => $_GET['action'] ?? '', // 1.2 only supports GET requests
+		'action'  => $requested_action,
 		'request' => array_diff_key( $_GET, [ 'action' => false, 'callback' => false ] ),
 	);
 }
diff --git a/api.wordpress.org/public_html/themes/theme-directory/1.0/index.php b/api.wordpress.org/public_html/themes/theme-directory/1.0/index.php
index 24d8d86a2f..7c5d9c82e4 100644
--- a/api.wordpress.org/public_html/themes/theme-directory/1.0/index.php
+++ b/api.wordpress.org/public_html/themes/theme-directory/1.0/index.php
@@ -16,15 +16,19 @@
 require dirname( dirname( dirname( __DIR__ ) ) ) . '/wp-init.php';
 
 function api_send_json( $data ) {
+	$origin = isset( $_SERVER['HTTP_ORIGIN'] ) ? sanitize_text_field( wp_unslash( $_SERVER['HTTP_ORIGIN'] ) ) : '';
+
 	// Allow cross-domain calls from *.wordpress.org
-	if ( isset( $_SERVER['HTTP_ORIGIN'] ) && preg_match( '!^https?://([^.]+\.)?wordpress\.org/?$!i', $_SERVER['HTTP_ORIGIN'] ) ) {
-		header( 'Access-Control-Allow-Origin: ' . $_SERVER['HTTP_ORIGIN'] );
+	if ( $origin && preg_match( '!^https?://([^.]+\.)?wordpress\.org/?$!i', $origin ) ) {
+		header( 'Access-Control-Allow-Origin: ' . $origin );
 		header( 'Access-Control-Allow-Credentials: true' ); // Allow cookies to be used.
 	}
 
-	if ( isset( $_GET['callback'] ) ) {
-		$callback = preg_replace( '/[^a-z0-9_]/i', '', $_GET['callback'] );
-	} else {
+	// phpcs:ignore WordPress.Security.NonceVerification.Recommended -- The callback name doesn't change any state; the actions below verify a nonce.
+	$callback = isset( $_GET['callback'] ) ? sanitize_text_field( wp_unslash( $_GET['callback'] ) ) : '';
+
+	// A callback that is not a valid JavaScript identifier falls back to plain JSON.
+	if ( ! preg_match( '/^[a-z_][a-z0-9_]*\z/i', $callback ) ) {
 		$callback = false;
 	}
 
@@ -32,9 +36,11 @@ function api_send_json( $data ) {
 
 	if ( $callback ) {
 		header( 'Content-Type:application/javascript; charset=' . get_option( 'blog_charset' ) );
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- validated callback, JSON payload.
 		echo "$callback( $json );";
 	} else {
 		header( 'Content-Type: application/json; charset=' . get_option( 'blog_charset' ) );
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON payload served as application/json.
 		echo $json;
 	}
 	die();
@@ -46,18 +52,45 @@ function api_send_json( $data ) {
 	) );
 }
 
-switch ( $_REQUEST['action'] ) {
+// Reject rather than transform: a malformed action must not become a valid one.
+$requested_action = filter_var(
+	wp_unslash( $_REQUEST['action'] ?? '' ),
+	FILTER_VALIDATE_REGEXP,
+	array(
+		'options' => array(
+			'regexp'  => '/^[a-z0-9_-]+\z/',
+			'default' => '',
+		),
+	)
+);
+
+switch ( $requested_action ) {
 	case 'add-favorite':
 	case 'remove-favorite':
-		if ( ! isset( $_REQUEST['theme'] ) || ! isset( $_REQUEST['_wpnonce'] ) || ! wp_verify_nonce( $_REQUEST['_wpnonce'], 'modify-theme-favorite' ) ) {
+		$nonce = isset( $_REQUEST['_wpnonce'] ) ? sanitize_text_field( wp_unslash( $_REQUEST['_wpnonce'] ) ) : '';
+
+		if ( ! isset( $_REQUEST['theme'] ) || ! wp_verify_nonce( $nonce, 'modify-theme-favorite' ) ) {
 			api_send_json( array(
 				'error' => 'bad_request'
 			) );
 		}
 
-		$theme_slug = wp_unslash( $_REQUEST['theme'] );
+		$theme_slug = filter_var(
+			wp_unslash( $_REQUEST['theme'] ),
+			FILTER_VALIDATE_REGEXP,
+			array(
+				'options' => array(
+					'regexp'  => '/^[a-z0-9_-]+\z/',
+					'default' => '',
+				),
+			)
+		);
+
+		if ( ! $theme_slug ) {
+			api_send_json( array( 'error' => 'bad_request' ) );
+		}
 
-		if ( 'add-favorite' == $_REQUEST['action'] ) {
+		if ( 'add-favorite' == $requested_action ) {
 			$result = wporg_themes_add_favorite( $theme_slug );
 		} else {
 			$result = wporg_themes_remove_favorite( $theme_slug );