From 43fd2cb0ad204cb4dfd69dc1354c003595a18b67 Mon Sep 17 00:00:00 2001
From: Jared Dreyer <jared.dreyer@silverstripe.com>
Date: Wed, 28 Feb 2024 12:04:52 +1300
Subject: [PATCH] Fix BackURL redirect with strict or lax session cookie
 security.

See #55 PR for more context.

This is merely a cherry pick / copy of that commit but for the 2 branch so in theory should work in SS 4.x
---
 src/Control/SAMLController.php | 10 ++++++++++
 src/Helpers/SAMLHelper.php     |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/Control/SAMLController.php b/src/Control/SAMLController.php
index e8ca519..8222f4f 100644
--- a/src/Control/SAMLController.php
+++ b/src/Control/SAMLController.php
@@ -260,6 +260,16 @@ protected function getRedirect()
             return $this->redirect($this->getRequest()->getSession()->get('BackURL'));
         }
 
+        // In SAMLHelper, we use RelayState to convey BackURL because in a HTTP POST flow
+        // with lax or strict cookie security the session will not be there for us. RelayState
+        // will be reflected back in the acs POST request.
+        // Note if only assertion is signed, RelayState cannot be trusted. Prevent open relay
+        // as in https://github.com/SAML-Toolkits/php-saml#avoiding-open-redirect-attacks
+        $relayState = $this->owner->getRequest()->postVar('RelayState');
+        if ($relayState && Director::is_site_url($relayState)) {
+            return $this->redirect($relayState);
+        }
+        
         // Spoofing attack, redirect to homepage instead of spoofing url
         if ($this->getRequest()->getSession()->get('BackURL')
             && !Director::is_site_url($this->getRequest()->getSession()->get('BackURL'))) {
diff --git a/src/Helpers/SAMLHelper.php b/src/Helpers/SAMLHelper.php
index 0249e7b..28ddaae 100644
--- a/src/Helpers/SAMLHelper.php
+++ b/src/Helpers/SAMLHelper.php
@@ -75,7 +75,8 @@ public function redirect(RequestHandler $requestHandler = null, HTTPRequest $req
         $additionalGetQueryParams = $this->getAdditionalGETQueryParameters();
 
         try {
-            $auth->login(Director::absoluteBaseURL() . 'saml/', $additionalGetQueryParams);
+            /** Use RelayState to convey BackURL (will be handled in {@see SAMLController}). */
+            $auth->login($backURL, $additionalGetQueryParams);
         } catch (Exception $e) {
             /** @var LoggerInterface $logger */
             $logger = Injector::inst()->get(LoggerInterface::class);