diff --git a/requirements-dev.txt b/requirements-dev.txt
index 4df8412..9f05ced 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -13,3 +13,4 @@ tox
 twine
 vcrpy
 wheel
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability