diff --git a/api.wordpress.org/public_html/core/browse-happy/1.0/index.php b/api.wordpress.org/public_html/core/browse-happy/1.0/index.php
index b307bc2e58..0f9c624f7f 100644
--- a/api.wordpress.org/public_html/core/browse-happy/1.0/index.php
+++ b/api.wordpress.org/public_html/core/browse-happy/1.0/index.php
@@ -1,38 +1,60 @@
 <?php
+/**
+ * WordPress.org Browse Happy API endpoint.
+ *
+ * This is a standalone, unauthenticated, stateless API endpoint: WordPress is not loaded,
+ * so request data is never slashed, and there is no session or nonce infrastructure.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package BrowseHappy
+ */
 
 require dirname( __FILE__ ) . '/parse.php';
 
+// A JSONP callback is a plain JavaScript identifier; anything else is discarded.
+$jsonp_filter_args = array(
+	'options' => array(
+		'regexp'  => '/^[a-zA-Z_][a-zA-Z0-9_]*\z/',
+		'default' => '',
+	),
+	'flags'   => FILTER_REQUIRE_SCALAR,
+);
+
 $jsonp = '';
 if ( ! empty( $_GET['jsonp'] ) ) {
-	$jsonp = preg_replace( '/[^a-zA-Z0-9_]/', '', $_GET['jsonp'] );
+	$jsonp = filter_var( $_GET['jsonp'], FILTER_VALIDATE_REGEXP, $jsonp_filter_args );
 	header( 'Content-Type: application/javascript' );
 } else if ( ! empty( $_GET['callback'] ) ) {
-	$jsonp = preg_replace( '/[^a-zA-Z0-9_]/', '', $_GET['callback'] );
+	$jsonp = filter_var( $_GET['callback'], FILTER_VALIDATE_REGEXP, $jsonp_filter_args );
 	header( 'Content-Type: application/javascript' );
 }
 
-if ( empty( $_REQUEST['useragent'] ) ) {
+if ( empty( $_REQUEST['useragent'] ) || ! is_string( $_REQUEST['useragent'] ) ) {
 	return;
 }
 
-$user_agent = $_REQUEST['useragent'];
+$user_agent = filter_var( $_REQUEST['useragent'], FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW );
 $data = browsehappy_parse_user_agent( $user_agent );
 
 // Collect a sample: One out of every 25.
-if ( 0 === strpos( $_SERVER['HTTP_USER_AGENT'], 'WordPress/' ) && 1 === rand( 1, 25 ) ) {
+// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized -- Only used for a prefix comparison, never output or stored.
+if ( 0 === strpos( $_SERVER['HTTP_USER_AGENT'] ?? '', 'WordPress/' ) && 1 === rand( 1, 25 ) ) {
 	require( dirname( dirname( dirname( dirname( __FILE__ ) ) ) ) . '/includes/hyperdb/bb-10-hyper-db.php' );
 	bh_record_data( $user_agent, $data );
 }
 
 if ( $jsonp ) {
 	header( 'Access-Control-Allow-Origin: *' );
-	echo $jsonp.'('.json_encode($data).')';
+	// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- validated callback, JSON-encoded payload.
+	echo $jsonp . '(' . json_encode( $data ) . ')';
 } elseif ( defined( 'JSON_RESPONSE' ) ) {
 	header( 'Access-Control-Allow-Origin: *' );
 	header( 'Content-Type: application/json' );
 	echo json_encode( $data );
 } else {
 	header( 'Content-Type: text/plain' );
+	// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- serialized payload served as text/plain.
 	echo serialize( $data );
 }
 
@@ -46,7 +68,12 @@
 function bh_record_data( $ua, $data ) {
 	global $wpdb;
 
-	list( $wp_ver, $url ) = explode( ';', $_SERVER['HTTP_USER_AGENT'], 2 );
+	// The requesting client's own user agent, which is recorded alongside the reported one.
+	$client_ua = filter_var( $_SERVER['HTTP_USER_AGENT'] ?? '', FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW );
+
+	/* Core sends `WordPress/{version}; {site url}`, but the URL may be absent. */
+	list( $wp_ver, $url ) = array_pad( explode( ';', $client_ua, 2 ), 2, '' );
+
 	$wp_ver = substr( $wp_ver, 10, 64 );
 	$url = rtrim( strtolower( trim( $url ) ), '/' );
 	$pk = md5( $url . '|' . $ua );
diff --git a/api.wordpress.org/public_html/core/browse-happy/1.0/test.php b/api.wordpress.org/public_html/core/browse-happy/1.0/test.php
index 18aeb00ce3..c775c8a3e5 100644
--- a/api.wordpress.org/public_html/core/browse-happy/1.0/test.php
+++ b/api.wordpress.org/public_html/core/browse-happy/1.0/test.php
@@ -1,10 +1,24 @@
 <?php
-
-echo htmlspecialchars( $_SERVER['HTTP_USER_AGENT'], ENT_QUOTES ) . "<br/><br/>";
+/**
+ * Browse Happy user agent parser test page.
+ *
+ * This is a standalone diagnostic page: WordPress is not loaded, so request data is
+ * never slashed and the `esc_*()` escaping helpers are unavailable.
+ *
+ * phpcs:disable WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package BrowseHappy
+ */
 
 include dirname( __FILE__ ) . '/parse.php';
+$user_agent = filter_var( $_SERVER['HTTP_USER_AGENT'] ?? '', FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW );
+
+// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- htmlspecialchars() escapes; no esc_html() here.
+echo htmlspecialchars( $user_agent, ENT_QUOTES ) . '<br/><br/>';
 
-$output = browsehappy_parse_user_agent( $_SERVER['HTTP_USER_AGENT'] );
+$output = browsehappy_parse_user_agent( $user_agent );
 
-foreach ( $output as $k => $v )
-	echo htmlspecialchars( $k . ' = ' . ( is_bool( $v ) ? (int) $v : $v ), ENT_QUOTES ) . "<br/>";
+foreach ( $output as $k => $v ) {
+	// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- htmlspecialchars() escapes; no esc_html() here.
+	echo htmlspecialchars( $k . ' = ' . ( is_bool( $v ) ? (int) $v : $v ), ENT_QUOTES ) . '<br/>';
+}
diff --git a/api.wordpress.org/public_html/core/credits/index.php b/api.wordpress.org/public_html/core/credits/index.php
index d554a0e7e5..30cbb05340 100644
--- a/api.wordpress.org/public_html/core/credits/index.php
+++ b/api.wordpress.org/public_html/core/credits/index.php
@@ -1,4 +1,14 @@
 <?php
+/**
+ * WordPress.org Credits API endpoint.
+ *
+ * This is a standalone, unauthenticated, stateless API endpoint: WordPress is not loaded,
+ * so request data is never slashed, and there is no session or nonce infrastructure.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Credits
+ */
 
 $api_root = dirname( dirname( __DIR__ ) );
 
@@ -32,17 +42,42 @@ function like_escape( $text ) {
 endif;
 
 if ( ! empty( $_GET['version'] ) ) {
-	$version = preg_replace( '/^([.0-9]+).*/', '$1', $_GET['version'] );
+	$version = preg_replace(
+		'/^([.0-9]+).*/s',
+		'$1',
+		filter_var(
+			$_GET['version'],
+			FILTER_VALIDATE_REGEXP,
+			array(
+				'options' => array(
+					'regexp'  => '/^[0-9][.0-9]*/',
+					'default' => '',
+				),
+			)
+		)
+	);
 } elseif ( 'cli' == php_sapi_name() && isset( $argv[1] ) ) {
 	$version = preg_replace( '/^([.0-9]+).*/', '$1', $argv[1] );
 } else {
 	$version = WP_CORE_LATEST_RELEASE;
 }
 
+// A WP locale, e.g. `de_DE_formal` or `es_419`.
+$requested_locale = isset( $_GET['locale'] ) ? filter_var(
+	$_GET['locale'],
+	FILTER_VALIDATE_REGEXP,
+	array(
+		'options' => array(
+			'regexp'  => '/^[A-Za-z0-9_-]+\z/',
+			'default' => '',
+		),
+	)
+) : '';
+
 if (
 	! is_string( $version ) ||
 	version_compare( $version, '3.2', '<' ) ||
-	( isset( $_GET['locale'] ) && ! is_string( $_GET['locale'] ) )
+	( isset( $_GET['locale'] ) && ! is_string( $requested_locale ) )
 ) {
 	header( 'HTTP/1.0 400 Bad Request', true, 400 );
 	die( 'Bad request.' );
@@ -50,10 +85,13 @@ function like_escape( $text ) {
 
 $locale = false;
 // Convert a locale from a WP locale to a GP locale.
-if ( ( isset( $_GET['locale'] ) && 'en_US' != $_GET['locale'] ) || ( 'cli' == php_sapi_name() && isset( $argv[2] ) ) ) {
+if (
+	( isset( $_GET['locale'] ) && 'en_US' != $requested_locale ) ||
+	( 'cli' == php_sapi_name() && isset( $argv[2] ) )
+) {
 	require GLOTPRESS_LOCALES_PATH;
 
-	$gp_locale = GP_Locales::by_field( 'wp_locale', isset( $argv[2] ) ? $argv[2] : $_GET['locale'] );
+	$gp_locale = GP_Locales::by_field( 'wp_locale', isset( $argv[2] ) ? $argv[2] : $requested_locale );
 	if ( $gp_locale ) {
 		$locale = $gp_locale;
 	}
diff --git a/api.wordpress.org/public_html/core/credits/wp-credits.php b/api.wordpress.org/public_html/core/credits/wp-credits.php
index 3638294178..3ba85e0af0 100644
--- a/api.wordpress.org/public_html/core/credits/wp-credits.php
+++ b/api.wordpress.org/public_html/core/credits/wp-credits.php
@@ -447,6 +447,7 @@ final public function execute() {
 		} elseif ( defined( 'JSON_RESPONSE' ) && JSON_RESPONSE ) {
 			echo json_encode( $results );
 		} else {
+			// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- serialized payload served as text/plain.
 			echo serialize( $results );
 		}
 	}
diff --git a/api.wordpress.org/public_html/core/importers/1.0/index.php b/api.wordpress.org/public_html/core/importers/1.0/index.php
index 0258dfa6d9..8bd2df4855 100644
--- a/api.wordpress.org/public_html/core/importers/1.0/index.php
+++ b/api.wordpress.org/public_html/core/importers/1.0/index.php
@@ -1,4 +1,14 @@
 <?php
+/**
+ * WordPress.org Importers API endpoint.
+ *
+ * This is a standalone, unauthenticated, stateless API endpoint: WordPress is not loaded,
+ * so request data is never slashed, and there is no session or nonce infrastructure.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Importers
+ */
 
 // Allow playground access.
 header( 'Access-Control-Allow-Origin: *' );
@@ -6,7 +16,8 @@
 
 $version = '';
 if ( isset( $_REQUEST['version'] ) ) { // Introduced in WordPress 4.6.
-	$version = str_replace( '-src', '', $_REQUEST['version'] );
+	$requested_version = filter_var( $_REQUEST['version'], FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW );
+	$version           = is_string( $requested_version ) ? str_replace( '-src', '', $requested_version ) : '';
 }
 
 if ( version_compare( $version, '5.4-beta', '>=' ) ) {
@@ -21,6 +32,7 @@
 }
 
 $response = array( 'importers' => $popular_importers, 'translated' => false );
+// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON or serialized payload, not HTML.
 echo defined( 'JSON_RESPONSE' ) ? json_encode( $response ) : serialize( $response );
 
 function __( $string ) { return $string; }
diff --git a/api.wordpress.org/public_html/core/serve-happy/1.0/include.php b/api.wordpress.org/public_html/core/serve-happy/1.0/include.php
index 5bcde61f33..481bd6722f 100644
--- a/api.wordpress.org/public_html/core/serve-happy/1.0/include.php
+++ b/api.wordpress.org/public_html/core/serve-happy/1.0/include.php
@@ -1,4 +1,15 @@
 <?php
+/**
+ * Serve Happy API request parsing and response functions.
+ *
+ * This is a standalone, unauthenticated, stateless API endpoint: there is no session or
+ * nonce infrastructure to verify against.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification
+ *
+ * @package WordPressdotorg\API\Serve_Happy
+ */
+
 namespace WordPressdotorg\API\Serve_Happy;
 
 function determine_request( $request = false ) {
diff --git a/api.wordpress.org/public_html/core/serve-happy/1.0/index.php b/api.wordpress.org/public_html/core/serve-happy/1.0/index.php
index a197f03995..8e13d3a54a 100644
--- a/api.wordpress.org/public_html/core/serve-happy/1.0/index.php
+++ b/api.wordpress.org/public_html/core/serve-happy/1.0/index.php
@@ -1,4 +1,15 @@
 <?php
+/**
+ * WordPress.org Serve Happy API endpoint: PHP version recommendations.
+ *
+ * This is a standalone, unauthenticated, stateless API endpoint: WordPress is not loaded,
+ * so request data is never slashed, and there is no session or nonce infrastructure.
+ *
+ * phpcs:disable WordPress.Security.NonceVerification, WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Serve_Happy
+ */
+
 namespace WordPressdotorg\API\Serve_Happy;
 
 define( 'API_VERSION', '1.0' );
@@ -15,9 +26,26 @@
 	)
 );
 
-// Output functions
-function bail( $error_code, $error_text, $http_code = 400, $http_code_text = false ) {
-	$server_protocol = $_SERVER['SERVER_PROTOCOL'] ?? 'HTTP/1.1';
+/**
+ * Sends an error response and halts.
+ *
+ * @param string      $error_code     Machine-readable error code.
+ * @param string      $error_text     Human-readable error description.
+ * @param int         $http_code      Optional. HTTP status code. Default 400.
+ * @param string|null $http_code_text Optional. HTTP status reason phrase. Default derived from the status code.
+ */
+function bail( $error_code, $error_text, $http_code = 400, $http_code_text = null ) {
+	// Only a well-formed protocol version is echoed back into the status header.
+	$server_protocol = filter_var(
+		$_SERVER['SERVER_PROTOCOL'] ?? '',
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '#^HTTP/[0-9]+(\.[0-9]+)?\z#',
+				'default' => 'HTTP/1.1',
+			),
+		)
+	);
 	$http_code_texts = [
 		400 => 'Bad Request',
 	];
@@ -39,15 +67,28 @@ function output_response( $data ) {
 
 	header( 'Access-Control-Allow-Origin: *' );
 
-	if ( !empty( $_GET['callback'] ) ) {
+	// A JSONP callback is a JavaScript identifier, optionally namespaced; anything else is discarded.
+	$callback = filter_var(
+		$_GET['callback'] ?? '',
+		FILTER_VALIDATE_REGEXP,
+		array(
+			'options' => array(
+				'regexp'  => '/^[a-zA-Z_$][a-zA-Z0-9_$]*(\.[a-zA-Z_$][a-zA-Z0-9_$]*)*\z/',
+				'default' => '',
+			),
+			'flags'   => FILTER_REQUIRE_SCALAR,
+		)
+	);
+
+	if ( $callback ) {
 		call_headers( 'application/javascript' );
 
-		echo '/**/' .
-			preg_replace('/[^a-zA-Z0-9_.]/', '', $_GET['callback'] ) .
-			'(' . $json_data . ')';
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- validated callback, JSON-encoded payload.
+		echo '/**/' . $callback . '(' . $json_data . ')';
 	} else {
 		call_headers( 'application/json' );
 
+		// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- JSON-encoded payload served as application/json.
 		echo $json_data;
 	}
 }
\ No newline at end of file
diff --git a/api.wordpress.org/public_html/patterns/1.0/index.php b/api.wordpress.org/public_html/patterns/1.0/index.php
index 1c838833e7..1528c3a0f7 100644
--- a/api.wordpress.org/public_html/patterns/1.0/index.php
+++ b/api.wordpress.org/public_html/patterns/1.0/index.php
@@ -1,4 +1,13 @@
 <?php
+/**
+ * WordPress.org Block Pattern Directory API endpoint.
+ *
+ * The request is read here, before `main()` loads WordPress, so it has not been slashed.
+ *
+ * phpcs:disable WordPress.Security.ValidatedSanitizedInput.MissingUnslash
+ *
+ * @package WordPressdotorg\API\Patterns
+ */
 
 namespace WordPressdotorg\API\Patterns;
 
@@ -8,7 +17,7 @@
  * This is cached by nginx, so we don't have to worry about the performance costs of loading WP, and don't need to
  * do any any object caching.
  */
-main( $_SERVER['QUERY_STRING'] );
+main( filter_var( $_SERVER['QUERY_STRING'] ?? '', FILTER_UNSAFE_RAW, FILTER_FLAG_STRIP_LOW ) );
 
 /**
  * Last minute rewrite of headers, to correct URLs set by the internal API endpoint.
@@ -41,7 +50,6 @@ function flush_handler( $buffer ) {
 	return false; // Original buffer will be output with no changes.
 }
 
-
 /**
  * Proxy w.org/patterns API endpoints for reliability.
  *