Index: install/english.lang
===================================================================
--- install/english.lang	(revision 13470)
+++ install/english.lang	(working copy)
@@ -250,6 +250,7 @@
 			<PHRASE Label="la_config_ResizableFrames" Module="Core" Type="1">RnJhbWVzIGluIGFkbWluaXN0cmF0aXZlIGNvbnNvbGUgYXJlIHJlc2l6YWJsZQ==</PHRASE>
 			<PHRASE Label="la_config_Search_MinKeyword_Length" Module="Core" Type="1">TWluaW1hbCBTZWFyY2ggS2V5d29yZCBMZW5ndGg=</PHRASE>
 			<PHRASE Label="la_config_SessionBrowserSignatureCheck" Module="Core" Type="1">U2Vzc2lvbiBTZWN1cml0eSBDaGVjayBiYXNlZCBvbiBCcm93c2VyIFNpZ25hdHVyZQ==</PHRASE>
+			<PHRASE Label="la_config_SessionCookieDomains" Module="Core" Type="1">U2Vzc2lvbiBDb29raWUgRG9tYWlucyAoc2luZ2xlIGRvbWFpbiBwZXIgbGluZSk=</PHRASE>
 			<PHRASE Label="la_config_SessionIPAddressCheck" Module="Core" Type="1">U2Vzc2lvbiBTZWN1cml0eSBDaGVjayBiYXNlZCBvbiBJUA==</PHRASE>
 			<PHRASE Label="la_config_SiteNameSubTitle" Module="Core" Type="1">V2Vic2l0ZSBTdWJ0aXRsZQ==</PHRASE>
 			<PHRASE Label="la_config_site_zone" Module="Core" Type="1">VGltZSB6b25lIG9mIHRoZSBzaXRl</PHRASE>
Index: install/install_data.sql
===================================================================
--- install/install_data.sql	(revision 13476)
+++ install/install_data.sql	(working copy)
@@ -43,6 +43,7 @@
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'UseVisitorTracking', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsWebsite', 'la_config_UseVisitorTracking', 'checkbox', '', '', 10.09, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'CookieSessions', '2', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_prompt_session_management', 'select', NULL, '0=lu_opt_QueryString||1=lu_opt_Cookies||2=lu_opt_AutoDetect', 20.01, 0, 1, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'SessionCookieName', 'sid', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_prompt_session_cookie_name', 'text', '', '', 20.02, 0, 1, NULL);
+INSERT INTO ConfigurationValues VALUES(DEFAULT, 'SessionCookieDomains', '', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_config_SessionCookieDomains', 'textarea', '', 'rows="5" cols="40"', 20.021, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'KeepSessionOnBrowserClose', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_config_KeepSessionOnBrowserClose', 'checkbox', '', '', 20.03, 0, 0, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'SessionBrowserSignatureCheck', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_config_SessionBrowserSignatureCheck', 'checkbox', NULL, NULL, 20.04, 0, 1, NULL);
 INSERT INTO ConfigurationValues VALUES(DEFAULT, 'SessionIPAddressCheck', '0', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_config_SessionIPAddressCheck', 'checkbox', NULL, NULL, 20.05, 0, 1, NULL);
Index: install/upgrades.sql
===================================================================
--- install/upgrades.sql	(revision 13476)
+++ install/upgrades.sql	(working copy)
@@ -1810,3 +1810,5 @@
 UPDATE Phrase
 SET l<%PRIMARY_LANGUAGE%>_Translation = 'Enable SEO-friendly URLs mode (MOD-REWRITE)'
 WHERE Phrase = 'la_config_use_modrewrite' AND l<%PRIMARY_LANGUAGE%>_Translation = 'Use MOD REWRITE';
+
+INSERT INTO ConfigurationValues VALUES(DEFAULT, 'SessionCookieDomains', '', 'In-Portal', 'in-portal:configure_advanced', 'la_section_SettingsSession', 'la_config_SessionCookieDomains', 'textarea', '', 'rows="5" cols="40"', 20.021, 0, 0, NULL);
Index: kernel/session/session.php
===================================================================
--- kernel/session/session.php	(revision 13462)
+++ kernel/session/session.php	(working copy)
@@ -549,9 +549,63 @@
 	 */
 	function SetCookieDomain($domain)
 	{
-		$this->CookieDomain = substr_count($domain, '.') ? '.'.ltrim($domain, '.') : false;
+		// 1. localhost or other like it without "." in domain name
+		if (!substr_count($domain, '.')) {
+			// don't use cookie domain at all
+			$this->CookieDomain = false;
+			return ;
+		}
+
+		// 2. match using predefined cookie domains from configuration
+		$cookie_domains = $this->Application->ConfigValue('SessionCookieDomains');
+
+		if ($cookie_domains) {
+			$cookie_domains = array_map('trim', explode("\n", $cookie_domains));
+
+			foreach ($cookie_domains as $cookie_domain) {
+				if (ltrim($cookie_domain, '.') == $domain) {
+					$this->CookieDomain = $cookie_domain; // as defined in configuration
+					return ;
+				}
+			}
+		}
+
+		// 3. only will execute, when none of domains were matched at previous step
+		$this->CookieDomain = $this->_autoGuessDomain($domain);
 	}
 
+	/**
+	 * Auto-guess cookie domain based on $_SERVER['HTTP_HOST']
+	 *
+	 * @param $domain
+	 * @return string
+	 */
+	function _autoGuessDomain($domain)
+	{
+		static $cache = Array ();
+
+		if (!array_key_exists($domain, $cache)) {
+			switch ( substr_count($domain, '.') ) {
+				case 2:
+					// 3rd level domain (3 parts)
+					$cache[$domain] = substr($domain, strpos($domain, '.')); // with leading "."
+					break;
+
+				case 1:
+					// 2rd level domain (2 parts)
+					$cache[$domain] = '.' . $domain; // with leading "."
+					break;
+
+				default:
+					// more then 3rd level
+					$cache[$domain] = ltrim($domain, '.'); // without leading "."
+					break;
+			}
+		}
+
+		return $cache[$domain];
+	}
+
 	function SetGETName($get_name)
 	{
 		$this->GETName = $get_name;
@@ -715,6 +769,21 @@
 			$this->Application->HttpQuery->Cookie[$name] = $value;
 		}
 
+		$old_style_domains = Array (
+			// domain like in pre 5.1.0 versions
+			'.' . SERVER_NAME,
+
+			// auto-guessed domain (when user specified other domain in configuration variable)
+			$this->_autoGuessDomain(SERVER_NAME)
+		);
+
+		foreach ($old_style_domains as $old_style_domain) {
+			if ($this->CookieDomain != $old_style_domain) {
+				// new style cookie domain -> delete old style cookie to prevent infinite redirect
+				setcookie($name, $value, adodb_mktime() - 3600, $this->CookiePath, $old_style_domain, $this->CookieSecure);
+			}
+		}
+
 		setcookie($name, $value, $expires, $this->CookiePath, $this->CookieDomain, $this->CookieSecure);
 	}
 
Index: kernel/utility/unit_config_reader.php
===================================================================
--- kernel/utility/unit_config_reader.php	(revision 13473)
+++ kernel/utility/unit_config_reader.php	(working copy)
@@ -82,6 +82,7 @@
 			// session related
 			'SessionTimeout',
 			'SessionCookieName',
+			'SessionCookieDomains',
 			'SessionBrowserSignatureCheck',
 			'SessionIPAddressCheck',
 			'CookieSessions',