API Add option to disable user-agent header session validation

docs/en/02_Developer_Guides/18_Cookies_And_Sessions/02_Sessions.md

@@ -110,6 +110,19 @@ SilverStripe\Control\Session:
 
 This uses the session_name `SECSESSID` for `https` connections instead of the default `PHPSESSID`. Doing so adds an extra layer of security to your session cookie since you no longer share `http` and `https` sessions.
 
+## Relaxing checks around user agent strings
+
+Out of the box, SilverStripe will invalidate a user's session if the `User-Agent` header changes. This provides some supplemental protection against session high-jacking attacks.
+
+It is possible to disable the user agent header session validation. However, it is not recommended.
+
+To disable the user agent session check, add the following code snippet to your project's YML configuration.
+
+```yml
+SilverStripe\Control\Session:
+  strict_user_agent_check: false
+```
+
 
 ## API Documentation
 

src/Control/Session.php

@@ -144,6 +144,14 @@ class Session
      */
     private static $sessionCacheLimiter = '';
 
+    /**
+     * Invalidate the session if user agent header changes between request. Defaults to true. Disabling this checks is
+     * not recommended.
+     * @var bool
+     * @config
+     */
+    private static $strict_user_agent_check = true;
+
     /**
      * Session data.
      * Will be null if session has not been started
@@ -223,7 +231,7 @@ class Session
         }
 
         // Funny business detected!
-        if (isset($this->data['HTTP_USER_AGENT'])) {
+        if (self::config()->get('strict_user_agent_check') && isset($this->data['HTTP_USER_AGENT'])) {
             if ($this->data['HTTP_USER_AGENT'] !== $this->userAgent($request)) {
                 $this->clearAll();
                 $this->destroy();

tests/php/Control/SessionTest.php

@@ -285,7 +285,31 @@ class SessionTest extends SapphireTest
         // Verify the new session reset our values
         $s2 = new Session($s);
         $s2->init($req2);
-        $this->assertNotEquals($s2->get('val'), 123);
+        $this->assertEmpty($s2->get('val'));
+    }
+
+    public function testDisabledUserAgentLockout()
+    {
+        Session::config()->set('strict_user_agent_check', false);
+
+        // Set a user agent
+        $req1 = new HTTPRequest('GET', '/');
+        $req1->addHeader('User-Agent', 'Test Agent');
+
+        // Generate our session
+        $s = new Session([]);
+        $s->init($req1);
+        $s->set('val', 123);
+        $s->finalize($req1);
+
+        // Change our UA
+        $req2 = new HTTPRequest('GET', '/');
+        $req2->addHeader('User-Agent', 'Fake Agent');
+
+        // Verify the new session reset our values
+        $s2 = new Session($s);
+        $s2->init($req2);
+        $this->assertEquals($s2->get('val'), 123);
     }
 
     public function testSave()