diff --git a/docs/requirements.txt b/docs/requirements.txt
index 87001dfb..2d931d89 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -19,3 +19,4 @@ plotnine
 dill
 sphinx-book-theme
 
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability