Escape invalid host when displayed in html

Use html.escape

Use web.HTTPError to safely return errors to browser

jupyter_server_proxy/handlers.py

@@ -307,10 +307,11 @@ async def proxy(self, host, port, proxied_path):
         '''
 
         if not self._check_host_allowlist(host):
-            self.set_status(403)
-            self.write("Host '{host}' is not allowed. "
-                       "See https://jupyter-server-proxy.readthedocs.io/en/latest/arbitrary-ports-hosts.html for info.".format(host=host))
-            return
+            raise web.HTTPError(
+                403,
+                f"Host '{host}' is not allowed. "
+                "See https://jupyter-server-proxy.readthedocs.io/en/latest/arbitrary-ports-hosts.html for info.",
+            )
 
         # Remove hop-by-hop headers that don't necessarily apply to the request we are making
         # to the backend. See https://github.com/jupyterhub/jupyter-server-proxy/pull/328
@@ -360,9 +361,7 @@ async def proxy(self, host, port, proxied_path):
             # Ref: https://www.tornadoweb.org/en/stable/httpclient.html#tornado.httpclient.AsyncHTTPClient.fetch
             if err.code == 599:
                 self._record_activity()
-                self.set_status(599)
-                self.write(str(err))
-                return
+                raise web.HTTPError(599, str(err))
             else:
                 raise
 
@@ -371,8 +370,7 @@ async def proxy(self, host, port, proxied_path):
 
         # For all non http errors...
         if response.error and type(response.error) is not httpclient.HTTPError:
-            self.set_status(500)
-            self.write(str(response.error))
+            raise web.HTTPError(500, str(response.error))
         else:
             # Represent the original response as a RewritableResponse object.
             original_response = RewritableResponse(orig_response=response)

tests/test_proxies.py

@@ -187,6 +187,15 @@ def test_server_proxy_host_absolute():
     assert 'X-Forwarded-Context' not in s
     assert 'X-Proxycontextpath' not in s
 
+
+@pytest.mark.parametrize("absolute", ["", "/absolute"])
+def test_server_proxy_host_invalid(absolute: str) -> None:
+    r = request_get(PORT, f"/proxy{absolute}/<invalid>:54321/", TOKEN)
+    assert r.code == 403
+    s = r.read().decode("ascii")
+    assert "Host &#39;&lt;invalid&gt;&#39; is not allowed." in s
+
+
 def test_server_proxy_port_non_service_rewrite_response():
     """Test that 'hello' is replaced by 'foo'."""
     r = request_get(PORT, '/proxy/54321/hello', TOKEN)