Validate redirect target in TrailingSlashHandler

Fixes open redirect vulnerability GHSA-c7vm-f5p4-8fqh

notebook/base/handlers.py

@@ -853,13 +853,18 @@ class APIVersionHandler(APIHandler):
 
 class TrailingSlashHandler(web.RequestHandler):
     """Simple redirect handler that strips trailing slashes
-    
+
     This should be the first, highest priority handler.
     """
-    
+
     def get(self):
-        self.redirect(self.request.uri.rstrip('/'))
-    
+        path, *rest = self.request.uri.partition("?")
+        # trim trailing *and* leading /
+        # to avoid misinterpreting repeated '//'
+        path = "/" + path.strip("/")
+        new_uri = "".join([path, *rest])
+        self.redirect(new_uri)
+
     post = put = get
 
 
@@ -910,6 +915,7 @@ class RedirectWithParams(web.RequestHandler):
         url = sep.join([self._url, self.request.query])
         self.redirect(url, permanent=self._permanent)
 
+
 class PrometheusMetricsHandler(IPythonHandler):
     """
     Return prometheus metrics for this notebook server

notebook/tests/test_paths.py

@@ -2,10 +2,13 @@
 import re
 
 from notebook.base.handlers import path_regex
+from notebook.utils import url_path_join
+from .launchnotebook import NotebookTestBase
 
 # build regexps that tornado uses:
 path_pat = re.compile('^' + '/x%s' % path_regex + '$')
 
+
 def test_path_regex():
     for path in (
         '/x',
@@ -29,3 +32,18 @@ def test_path_regex_bad():
         '/y/x/foo',
     ):
         assert not re.match(path_pat, path)
+
+
+class RedirectTestCase(NotebookTestBase):
+    def test_trailing_slash(self):
+        for uri, expected in (
+            ("/notebooks/mynotebook/", "/notebooks/mynotebook"),
+            ("////foo///", "/foo"),
+            ("//example.com/", "/example.com"),
+            ("/has/param/?hasparam=true", "/has/param?hasparam=true"),
+        ):
+            r = self.request("GET", uri, allow_redirects=False)
+            print(uri, expected)
+            assert r.status_code == 302
+            assert "Location" in r.headers
+            assert r.headers["Location"] == url_path_join(self.url_prefix, expected)