Block open redirect via backslash in redirect Location

datasette/utils/asgi.py

@@ -330,9 +330,11 @@ async def asgi_send_html(send, html, status=200, headers=None):
 
 
 async def asgi_send_redirect(send, location, status=302):
-    # Prevent open redirect vulnerability: strip multiple leading slashes
-    # //example.com would be interpreted as a protocol-relative URL (e.g., https://example.com/)
-    location = re.sub(r"^/+", "/", location)
+    # Prevent open redirect vulnerability: collapse leading slashes and
+    # backslashes into a single slash. Browsers treat a backslash as a slash,
+    # so //example.com, /\example.com and /\/example.com would all otherwise be
+    # interpreted as a protocol-relative URL (e.g. https://example.com/).
+    location = re.sub(r"^[/\\]+", "/", location)
     await asgi_send(
         send,
         "",

tests/test_custom_pages.py

@@ -99,8 +99,18 @@ def test_custom_route_pattern_404(custom_pages_client):
     assert ">Oh no</" in response.text
 
 
-def test_custom_route_pattern_with_slash_slash_302(custom_pages_client):
-    # https://github.com/simonw/datasette/issues/2429
-    response = custom_pages_client.get("//example.com/")
+@pytest.mark.parametrize(
+    "path",
+    (
+        # https://github.com/simonw/datasette/issues/2429
+        "//example.com/",
+        # https://github.com/simonw/datasette/issues/2680
+        # Browsers treat backslashes as slashes, so these are also open redirects
+        "/\\example.com/",
+        "/\\/example.com/",
+    ),
+)
+def test_custom_route_pattern_open_redirect_302(custom_pages_client, path):
+    response = custom_pages_client.get(path)
     assert response.status == 302
     assert response.headers["location"] == "/example.com"