=== modified file 'includes/session.inc'
--- includes/session.inc	2010-10-15 04:15:41 +0000
+++ includes/session.inc	2010-10-29 22:32:39 +0000
@@ -88,7 +88,10 @@ function _drupal_session_read($sid) {
   // a HTTPS session or we are about to log in so we check the sessions table
   // for an anonymous session with the non-HTTPS-only cookie.
   if ($is_https) {
-    $user = db_query("SELECT u.*, s.* FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.ssid = :ssid", array(':ssid' => $sid))->fetchObject();
+    // Ensure that an empty secure session ID cannot be selected.
+    if ($sid) {
+      $user = db_query("SELECT u.*, s.* FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.ssid = :ssid", array(':ssid' => $sid))->fetchObject();
+    }
     if (!$user) {
       if (isset($_COOKIE[$insecure_session_name])) {
         $user = db_query("SELECT u.*, s.* FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.sid = :sid AND s.uid = 0", array(
@@ -180,21 +183,23 @@ function _drupal_session_write($sid, $va
         'timestamp' => REQUEST_TIME,
       );
 
-      // The "secure pages" setting allows a site to simultaneously use both
-      // secure and insecure session cookies. If enabled and both cookies are
-      // presented then use both keys. If not enabled but on HTTPS then use the
-      // PHP session id as 'ssid'. If on HTTP then use the PHP session id as
-      // 'sid'.
+      // Use the session ID as 'sid' and an empty string as 'ssid', which
+      // _drupal_session_read() will prevent from being selected as a valid
+      // secure session ID.
+      $key = array('sid' => $sid, 'ssid' => '');
+      // On HTTPS connections, use the session ID as both 'sid' and 'ssid'.
       if ($is_https) {
         $key['ssid'] = $sid;
-        $insecure_session_name = substr(session_name(), 1);
-        if (variable_get('https', FALSE) && isset($_COOKIE[$insecure_session_name])) {
-          $key['sid'] = $_COOKIE[$insecure_session_name];
+        // The "secure pages" setting allows a site to simultaneously use both
+        // secure and insecure session cookies. If enabled and both cookies are
+        // presented then use both keys. 
+        if (variable_get('https', FALSE)) {
+          $insecure_session_name = substr(session_name(), 1);
+          if (isset($_COOKIE[$insecure_session_name])) {
+            $key['sid'] = $_COOKIE[$insecure_session_name];
+          }
         }
       }
-      else {
-        $key['sid'] = $sid;
-      }
 
       db_merge('sessions')
         ->key($key)

=== modified file 'includes/update.inc'
--- includes/update.inc	2010-10-27 19:31:53 +0000
+++ includes/update.inc	2010-10-29 22:32:39 +0000
@@ -701,7 +701,7 @@ function update_fix_d7_requirements() {
     }
 
     // Add ssid column and index.
-    db_add_field('sessions', 'ssid', array('description' => "Secure session ID. The value is generated by PHP's Session API.", 'type' => 'varchar', 'length' => 128, 'not null' => TRUE, 'default' => ''));
+    db_add_field('sessions', 'ssid', array('description' => "Secure session ID. The value is generated by Drupal's session handlers.", 'type' => 'varchar', 'length' => 128, 'not null' => TRUE, 'default' => ''));
     db_add_index('sessions', 'ssid', array('ssid'));
     // Drop existing primary key.
     db_drop_primary_key('sessions');

=== added file 'modules/simpletest/tests/http.php'
--- modules/simpletest/tests/http.php	1970-01-01 00:00:00 +0000
+++ modules/simpletest/tests/http.php	2010-10-29 22:32:52 +0000
@@ -0,0 +1,29 @@
+<?php
+// $Id$
+
+/**
+ * @file
+ * Fake an HTTP request, for use during testing.
+ */
+
+// Negated copy of the condition in _drupal_bootstrap(). If the user agent is
+// not from simpletest then disallow access.
+if (!(isset($_SERVER['HTTP_USER_AGENT']) && (strpos($_SERVER['HTTP_USER_AGENT'], "simpletest") !== FALSE))) {
+  exit;
+}
+
+// Set a global variable to indicate a mock HTTP request.
+$is_http_mock = !empty($_SERVER['HTTPS']);
+
+// Change to http.
+$_SERVER['HTTPS'] = NULL;
+ini_set('session.cookie_secure', FALSE);
+
+// Change to index.php.
+chdir('../../..');
+foreach ($_SERVER as $key => $value) {
+  $_SERVER[$key] = str_replace('modules/simpletest/tests/http.php', 'index.php', $value);
+  $_SERVER[$key] = str_replace('https://', 'http://', $_SERVER[$key]);
+}
+
+require_once 'index.php';

=== modified file 'modules/simpletest/tests/session.test'
--- modules/simpletest/tests/session.test	2010-10-15 04:15:41 +0000
+++ modules/simpletest/tests/session.test	2010-10-29 22:32:39 +0000
@@ -316,7 +316,7 @@ class SessionHttpsTestCase extends Drupa
     // Check insecure cookie is not set.
     $this->assertFalse(isset($this->cookies[$insecure_session_name]));
     $ssid = $this->cookies[$secure_session_name]['value'];
-    $this->assertSessionIds('', $ssid, 'Session has NULL for SID and a correct secure SID.');
+    $this->assertSessionIds($ssid, $ssid, 'Session has a non-empty SID and a correct secure SID.');
     $cookie = $secure_session_name . '=' . $ssid;
 
     // Verify that user is logged in on secure URL.
@@ -326,12 +326,36 @@ class SessionHttpsTestCase extends Drupa
     $this->assertResponse(200);
 
     // Verify that user is not logged in on non-secure URL.
-    if (!$is_https) {
-      $this->curlClose();
-      $this->drupalGet('admin/config', array(), array('Cookie: ' . $cookie));
-      $this->assertNoText(t('Configuration'));
-      $this->assertResponse(403);
-    }
+    $this->curlClose();
+    $this->drupalGet($this->httpUrl('admin/config'), array(), array('Cookie: ' . $cookie));
+    $this->assertNoText(t('Configuration'));
+    $this->assertResponse(403);
+    
+    // Verify that empty SID cannot be used on the non-secure site.
+    $this->curlClose();
+    $cookie = $insecure_session_name . '=';
+    $this->drupalGet($this->httpUrl('admin/config'), array(), array('Cookie: ' . $cookie));
+    $this->assertResponse(403);
+
+    // Test HTTP session handling by altering the form action to submit the
+    // login form through http.php, which creates a mock HTTP request on HTTPS
+    // test environments.
+    $this->curlClose();
+    $this->drupalGet('user');
+    $form = $this->xpath('//form[@id="user-login"]');
+    $form[0]['action'] = $this->httpUrl('user');
+    $edit = array('name' => $user->name, 'pass' => $user->pass_raw);
+    $this->drupalPost(NULL, $edit, t('Log in'));
+    $this->drupalGet($this->httpUrl('admin/config'));
+    $this->assertResponse(200);
+    $sid = $this->cookies[$insecure_session_name]['value'];
+    $this->assertSessionIds($sid, '', 'Session has the correct SID and an empty secure SID.');
+
+    // Verify that empty secure SID cannot be used on the secure site.
+    $this->curlClose();
+    $cookie = $secure_session_name . '=';
+    $this->drupalGet($this->httpsUrl('admin/config'), array(), array('Cookie: ' . $cookie));
+    $this->assertResponse(403);
 
     // Clear browser cookie jar.
     $this->cookies = array();
@@ -458,9 +482,32 @@ class SessionHttpsTestCase extends Drupa
     return $this->assertTrue(db_query('SELECT timestamp FROM {sessions} WHERE sid = :sid AND ssid = :ssid', $args)->fetchField(), $assertion_text);
   }
 
+  /**
+   * Builds a URL for submitting a mock HTTPS request to HTTP test environments.
+   *
+   * @param $url
+   *   A Drupal path such as 'user'.
+   *
+   * @return
+   *   An absolute URL.
+   */
   protected function httpsUrl($url) {
     global $base_url;
     return $base_url . '/modules/simpletest/tests/https.php?q=' . $url;
   }
+
+  /**
+   * Builds a URL for submitting a mock HTTP request to HTTPS test environments.
+   *
+   * @param $url
+   *   A Drupal path such as 'user'.
+   *
+   * @return
+   *   An absolute URL.
+   */
+  protected function httpUrl($url) {
+    global $base_url;
+    return $base_url . '/modules/simpletest/tests/http.php?q=' . $url;
+  }
 }
 

=== modified file 'modules/simpletest/tests/upgrade/upgrade.test'
--- modules/simpletest/tests/upgrade/upgrade.test	2010-10-08 18:19:11 +0000
+++ modules/simpletest/tests/upgrade/upgrade.test	2010-10-29 22:32:39 +0000
@@ -113,7 +113,12 @@ abstract class UpgradePathTestCase exten
 
     // Force our way into the session of the child site.
     drupal_save_session(TRUE);
+    // A session cannot be written without the ssid column which is missing on
+    // Drupal 6 sites.
+    db_add_field('sessions', 'ssid', array('description' => "Secure session ID. The value is generated by Drupal's session handlers.", 'type' => 'varchar', 'length' => 128, 'not null' => TRUE, 'default' => ''));
     _drupal_session_write($sid, '');
+    // Remove the temporarily added ssid column.
+    db_drop_field('sessions', 'ssid');
     drupal_save_session(FALSE);
 
     // Restore necessary variables.

=== modified file 'modules/system/system.install'
--- modules/system/system.install	2010-10-20 00:47:43 +0000
+++ modules/system/system.install	2010-10-29 22:32:39 +0000
@@ -1469,14 +1469,13 @@ function system_schema() {
         'not null' => TRUE,
       ),
       'sid' => array(
-        'description' => "A session ID. The value is generated by PHP's Session API.",
+        'description' => "A session ID. The value is generated by Drupal's session handlers.",
         'type' => 'varchar',
         'length' => 128,
         'not null' => TRUE,
-        'default' => '',
       ),
       'ssid' => array(
-        'description' => "Secure session ID. The value is generated by PHP's Session API.",
+        'description' => "Secure session ID. The value is generated by Drupal's session handlers.",
         'type' => 'varchar',
         'length' => 128,
         'not null' => TRUE,
@@ -2902,6 +2901,19 @@ function system_update_7064() {
 }
 
 /**
+ * Remove the default value for sid.
+ */
+function system_update_7065() {
+  $spec = array(
+    'description' => "A session ID. The value is generated by Drupal's session handlers.",
+    'type' => 'varchar',
+    'length' => 128,
+    'not null' => TRUE,
+  );
+  db_change_field('sessions', 'sid', 'sid', $spec);
+}
+
+/**
  * @} End of "defgroup updates-6.x-to-7.x"
  * The next series of updates should start at 8000.
  */