diff --git a/requirements-dev.txt b/requirements-dev.txt
index bffca55..f268405 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -11,3 +11,4 @@ pytest-repeat
 jupyter
 ipykernel
 -r docs/requirements.txt
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability