diff --git a/docs/requirements.txt b/docs/requirements.txt
index ff7be839..04cf9190 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -20,3 +20,4 @@ dill
 sphinx-book-theme
 nctoolkit
 
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability