remote_server: Improve uuid validation at the server/register endpoint.

As explained in the comments in the code, just doing UUID(string) and
catching ValueError is not enough, because the uuid library sometimes
tries to modify the string to convert it into a valid UUID:

>>> a = '18cedb98-5222-5f34-50a9-fc418e1ba972'
>>> uuid.UUID(a, version=4)
UUID('18cedb98-5222-4f34-90a9-fc418e1ba972')

zerver/tests/test_push_notifications.py

@@ -2470,6 +2470,14 @@ class PushBouncerSignupTest(ZulipTestCase):
         result = self.client_post("/api/v1/remotes/server/register", request)
         self.assert_json_error(result, "Invalid UUID")
 
+        # This looks mostly like a proper UUID, but isn't actually a valid UUIDv4,
+        # which makes it slip past a basic validation via initializing uuid.UUID with it.
+        # Thus we should test this scenario separately.
+        zulip_org_id = "18cedb98-5222-5f34-50a9-fc418e1ba972"
+        request["zulip_org_id"] = zulip_org_id
+        result = self.client_post("/api/v1/remotes/server/register", request)
+        self.assert_json_error(result, "Invalid UUID")
+
     def test_push_signup_success(self) -> None:
         zulip_org_id = str(uuid.uuid4())
         zulip_org_key = get_random_string(64)

zilencer/views.py

@@ -53,7 +53,13 @@ def validate_entity(entity: Union[UserProfile, RemoteZulipServer]) -> RemoteZuli
 
 def validate_uuid(uuid: str) -> None:
     try:
-        UUID(uuid, version=4)
+        uuid_object = UUID(uuid, version=4)
+        # The UUID initialization under some circumstances will modify the uuid
+        # string to create a valid UUIDv4, instead of raising a ValueError.
+        # The submitted uuid needing to be modified means it's invalid, so
+        # we need to check for that condition.
+        if str(uuid_object) != uuid:
+            raise ValidationError(err_("Invalid UUID"))
     except ValueError:
         raise ValidationError(err_("Invalid UUID"))