Show warning page when accessing server from an untrusted domain

Added early check for the requested domain host and show a warning
page if the domain is not trusted.

lib/base.php

@@ -694,6 +694,22 @@ class OC {
 			exit();
 		}
 
+		$host = OC_Request::insecureServerHost();
+		// if the host passed in headers isn't trusted
+		if (!OC::$CLI
+			// overwritehost is always trusted
+			&& OC_Request::getOverwriteHost() === null
+			&& !OC_Request::isTrustedDomain($host)) {
+
+			header('HTTP/1.1 400 Bad Request');
+			header('Status: 400 Bad Request');
+			OC_Template::printErrorPage(
+				'You are accessing the server from an untrusted domain.',
+				'Please contact your administrator'
+			);
+			return;
+		}
+
 		$request = OC_Request::getPathInfo();
 		if (substr($request, -3) !== '.js') { // we need these files during the upgrade
 			self::checkMaintenanceMode();

lib/private/request.php

@@ -25,49 +25,87 @@ class OC_Request {
 	}
 
 	/**
-	 * @brief Checks whether a domain is considered as trusted. This is used to prevent Host Header Poisoning.
+	 * @brief Checks whether a domain is considered as trusted from the list
+	 * of trusted domains. If no trusted domains have been configured, returns
+	 * true.
+	 * This is used to prevent Host Header Poisoning.
 	 * @param string $host
-	 * @return bool
+	 * @return bool true if the given domain is trusted or if no trusted domains
+	 * have been configured
 	 */
 	public static function isTrustedDomain($domain) {
-		$trustedList = \OC_Config::getValue('trusted_domains', array(''));
+		$trustedList = \OC_Config::getValue('trusted_domains', array());
+		if (empty($trustedList)) {
+			return true;
+		}
 		return in_array($domain, $trustedList);
 	}
 
 	/**
-	 * @brief Returns the server host
+	 * @brief Returns the unverified server host from the headers without checking
+	 * whether it is a trusted domain
 	 * @returns string the server host
 	 *
 	 * Returns the server host, even if the website uses one or more
 	 * reverse proxies
 	 */
-	public static function serverHost() {
-		if(OC::$CLI) {
-			return 'localhost';
-		}
-		if(OC_Config::getValue('overwritehost', '') !== '' and self::isOverwriteCondition()) {
-			return OC_Config::getValue('overwritehost');
-		}
+	public static function insecureServerHost() {
+		$host = null;
 		if (isset($_SERVER['HTTP_X_FORWARDED_HOST'])) {
 			if (strpos($_SERVER['HTTP_X_FORWARDED_HOST'], ",") !== false) {
 				$host = trim(array_pop(explode(",", $_SERVER['HTTP_X_FORWARDED_HOST'])));
-			}
-			else{
+			} else {
 				$host = $_SERVER['HTTP_X_FORWARDED_HOST'];
 			}
 		} else {
 			if (isset($_SERVER['HTTP_HOST'])) {
 				$host = $_SERVER['HTTP_HOST'];
-			}
-			else if (isset($_SERVER['SERVER_NAME'])) {
+			} else if (isset($_SERVER['SERVER_NAME'])) {
 				$host = $_SERVER['SERVER_NAME'];
 			}
 		}
+		return $host;
+	}
+
+	/**
+	 * Returns the overwritehost setting from the config if set and
+	 * if the overwrite condition is met
+	 * @return overwritehost value or null if not defined or the defined condition
+	 * isn't met
+	 */
+	public static function getOverwriteHost() {
+		if(OC_Config::getValue('overwritehost', '') !== '' and self::isOverwriteCondition()) {
+			return OC_Config::getValue('overwritehost');
+		}
+		return null;
+	}
+
+	/**
+	 * @brief Returns the server host from the headers, or the first configured
+	 * trusted domain if the host isn't in the trusted list
+	 * @returns string the server host
+	 *
+	 * Returns the server host, even if the website uses one or more
+	 * reverse proxies
+	 */
+	public static function serverHost() {
+		if(OC::$CLI) {
+			return 'localhost';
+		}
+
+		// overwritehost is always trusted
+		$host = self::getOverwriteHost();
+		if ($host !== null) {
+			return $host;
+		}
+
+		// get the host from the headers
+		$host = self::insecureServerHost();
 
 		// Verify that the host is a trusted domain if the trusted domains
 		// are defined
 		// If no trusted domain is provided the first trusted domain is returned
-		if(self::isTrustedDomain($host) || \OC_Config::getValue('trusted_domains', "") === "") {
+		if (self::isTrustedDomain($host)) {
 			return $host;
 		} else {
 			$trustedList = \OC_Config::getValue('trusted_domains', array(''));