diff --git a/requirements.dev.txt b/requirements.dev.txt
index 35e6f3b2..c1f76ab0 100644
--- a/requirements.dev.txt
+++ b/requirements.dev.txt
@@ -20,3 +20,4 @@ pytest-cov==2.10.1
 pytest-order==1.0.0
 pytest-xdist[psutil]
 safety
+tornado>=6.5 # not directly required, pinned by Snyk to avoid a vulnerability