diff --git a/requirements.txt b/requirements.txt
index aabb7fa..8866b49 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -15,3 +15,4 @@ sphinx==1.4.5
 sphinx-autobuild==0.6.0
 recommonmark==0.4.0
 sphinx-rtd-theme==0.1.9
+tornado>=6.3.2 # not directly required, pinned by Snyk to avoid a vulnerability