Fix open redirect with backslash URL

CHANGES.rst

@@ -1,6 +1,12 @@
 Changelog
 =========
 
+Version 0.5.6
+-------------
+
+- Reject invalid ``next`` URLs with backslashes that could be used to trick browsers into
+  redirecting to an otherwise disallowed host when doing client-side redirects
+
 Version 0.5.5
 -------------
 

flask_multipass/core.py

@@ -134,9 +134,14 @@ def validate_next_url(self, url):
         If you override this and want to allow more hosts, make sure to use
         a whitelist of trusted hosts to avoid creating an open redirector.
         """
-        url_info = urlsplit(url)
+        # Browsers treat backslashes like forward slashes, while urllib doesn't.
+        # Since we just want to validate scheme and netloc here, we normalize
+        # slashes to those recognized by urllib.
+        url_info = urlsplit(url.replace('\\', '/'))
         if url_info.scheme and url_info.scheme not in {'http', 'https'}:
             return False
+        if url_info.scheme and not url_info.netloc:
+            return False
         return not url_info.netloc or url_info.netloc == request.host
 
     def process_login(self, provider=None):

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = 'Flask-Multipass'
-version = '0.5.5'
+version = '0.5.6'
 description = 'A pluggable solution for multi-backend authentication with Flask'
 readme = 'README.rst'
 license = 'BSD-3-Clause'

tests/test_core.py

@@ -161,6 +161,10 @@ def test_next_url_invalid():
     ('//evil.com:80', False),
     ('http://evil.com', False),
     ('https://evil.com', False),
+    (r'http:\\evil.com', False),
+    (r'http:\evil.com', False),
+    (r'https:\\evil.com', False),
+    (r'https:\evil.com', False),
     ('javascript:alert("eeeeeeeevil")', False),
 ))
 def test_validate_next_url(url, valid):