added url check

app/routes.py

@@ -41,6 +41,7 @@ from .utils import (
     get_course,
     token_required,
     get_cached_deck_info,
+    is_safe_url
 )
 
 
@@ -131,6 +132,10 @@ def set_cookie(token):
         current_app.logger.debug("next_url not present redirecting to index")
         next_url = url_for("main.index")
 
+    if not is_safe_url(next_url):
+        current_app.logger.warning(f"Blocked unsafe redirect attempt to: {next_url}")
+        return abort(400)
+
     resp = make_response(redirect(next_url))
     resp.set_cookie(
         "access_token", serialize_token(token), httponly=True, samesite="Lax", path="/"
@@ -260,6 +265,10 @@ def admin_login():
 
             next_page = request.args.get("next")
 
+            if next_page and not is_safe_url(next_page):
+                current_app.logger.warning(f"Blocked unsafe redirect attempt to: {next_page}")
+                return abort(400)
+
             if next_page:
                 session["next_url"] = next_page
 

app/utils.py

@@ -6,6 +6,7 @@ from typing import cast
 from itsdangerous import BadSignature, URLSafeSerializer
 from xkcdpass import xkcd_password
 from functools import wraps, lru_cache
+from urllib.parse import urlparse, urljoin
 
 from .models import Deck, InfoBanner, Token, db, Course
 
@@ -209,3 +210,13 @@ def get_cached_deck_info(token_id, course_folder, deck_slug):
         "deck_name": deck_obj.name,
         "index_file": deck_obj.index_file
     }
+
+def is_safe_url(target):
+    """
+    Ensures that a redirect target will lead to the same server.
+    Handles both relative paths ('/admin') and absolute URLs ('https://yourdomain.com/admin').
+    """
+    ref_url = urlparse(request.host_url)
+    test_url = urlparse(urljoin(request.host_url, target))
+    
+    return test_url.scheme in ('http', 'https') and ref_url.netloc == test_url.netloc