diff --git a/docs/requirements.txt b/docs/requirements.txt
index 7d6b993..4d81626 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -11,4 +11,5 @@ numpydoc
 pdflatex
 ipywidgets
 scalecast
-tqdm
\ No newline at end of file
+tqdm
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability
\ No newline at end of file