Index: src/wp-includes/class-http.php
===================================================================
--- src/wp-includes/class-http.php	(revision 26045)
+++ src/wp-includes/class-http.php	(working copy)
@@ -519,27 +519,42 @@
 		if ( $check['host'] == 'localhost' || $check['host'] == $home['host'] )
 			return apply_filters('block_local_requests', false);
 
-		if ( !defined('WP_ACCESSIBLE_HOSTS') )
-			return true;
+		static $wp_accessible_hosts;
+		if ( null == $wp_accessible_hosts ) {
+			if ( defined( 'WP_ACCESSIBLE_HOSTS' ) )
+				$wp_accessible_hosts = preg_split( '|,\s*|', WP_ACCESSIBLE_HOSTS );
+			else
+				$wp_accessible_hosts = array();
+		}
 
-		static $accessible_hosts;
-		static $wildcard_regex = false;
-		if ( null == $accessible_hosts ) {
-			$accessible_hosts = preg_split('|,\s*|', WP_ACCESSIBLE_HOSTS);
+		/**
+		 * Filter the list of accessible hosts.
+		 *
+		 * @since 3.8
+		 *
+		 * @param array $wp_accessible_hosts List of accessible hosts.
+		 * @param string $uri The URL being accessed.
+		 * @param array $check The pasesed URL being accessed.
+		 */
+		$accessible_hosts = apply_filters( 'wp_http_accessible_hosts', $wp_accessible_hosts, $uri, $check );
 
-			if ( false !== strpos(WP_ACCESSIBLE_HOSTS, '*') ) {
-				$wildcard_regex = array();
-				foreach ( $accessible_hosts as $host )
-					$wildcard_regex[] = str_replace( '\*', '.+', preg_quote( $host, '/' ) );
-				$wildcard_regex = '/^(' . implode('|', $wildcard_regex) . ')$/i';
-			}
+		// Check if there's an exact match
+		if ( in_array( $accessible_hosts, $check['host'] ) )
+			return false; // do not block
+
+		// Check to see if there's a wildcard match
+		foreach ( $accessible_hosts as $host ) {
+			// Skip non-wildcard entries
+			if ( false === strpos( $host, '*' ) )
+				continue;
+
+			$regex = str_replace( '\*', '.+', preg_quote( $host, '!' ) );
+			if ( preg_match( '!^(' . $regex . ')$!i', $check['host'] ) )
+				return false; // do not block
 		}
 
-		if ( !empty($wildcard_regex) )
-			return !preg_match($wildcard_regex, $check['host']);
-		else
-			return !in_array( $check['host'], $accessible_hosts ); //Inverse logic, If it's in the array, then we can't access it.
-
+		// Otherwise, block the request
+		return true;
 	}
 
 	static function make_absolute_url( $maybe_relative_path, $url ) {