diff --git a/docs/requirements.txt b/docs/requirements.txt
index fea7951..3fea4d1 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -5,3 +5,4 @@ nbsphinx>=0.8.9
 sphinx-copybutton>=0.5.0
 sphinx-toolbox>=3.1.2
 
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability