Prevent URI traversal for GitHub refs

This defect exposes an oracle for existence of private repos or refs
through the Binder operator's github token.

Fixes
https://github.com/jupyterhub/binderhub/security/advisories/GHSA-q276-fxp7-xhx3.

Author: Yann-P

binderhub/repoproviders.py

@@ -1054,11 +1054,13 @@ async def get_resolved_ref(self):
         if hasattr(self, "resolved_ref"):
             return self.resolved_ref
 
+        # Encode the ref (safe="" so its slashes can't inject extra path
+        # segments) to prevent traversal, see GHSA-q276-fxp7-xhx3.
         api_url = "{api_base_path}/repos/{user}/{repo}/commits/{ref}".format(
             api_base_path=self.api_base_path.format(hostname=self.hostname),
             user=self.user,
             repo=self.repo,
-            ref=self.unresolved_ref,
+            ref=urllib.parse.quote(self.unresolved_ref, safe=""),
         )
         self.log.debug("Fetching %s", api_url)
         cached = self.cache.get(api_url)

binderhub/tests/test_repoproviders.py

@@ -1,6 +1,6 @@
 import re
 from unittest import TestCase
-from urllib.parse import quote
+from urllib.parse import quote, urlparse
 
 import pytest
 from tornado.ioloop import IOLoop
@@ -417,6 +417,26 @@ def test_github_missing_ref():
     assert ref is None
 
 
+async def test_github_ref_is_url_encoded(monkeypatch):
+    """An attacker-controlled ref must not inject extra GitHub API path segments."""
+    ref = "HEAD/../../../../owner/private/contents/README.md"
+    provider = GitHubRepoProvider(spec=f"user/repo/{ref}")
+
+    captured = {}
+
+    async def fake_request(api_url, etag=None):
+        captured["url"] = api_url
+        return None  # short-circuit before any network call
+
+    monkeypatch.setattr(provider, "github_api_request", fake_request)
+    await provider.get_resolved_ref()
+
+    path = urlparse(captured["url"]).path
+    # the ref's slashes are encoded, so the ".." cannot climb out of /commits/
+    assert path == f"/repos/user/repo/commits/{quote(ref, safe='')}"
+    assert "/../" not in path
+
+
 class TestSpecErrorHandling(TestCase):
     def test_too_short_spec(self):
         spec = "nothing_to_split"