diff --git a/docs/requirements.txt b/docs/requirements.txt
index 258d249..460c5ff 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -11,3 +11,4 @@ sphinxcontrib-fulltoc
 sphinxcontrib-napoleon
 sphinxcontrib-websupport
 stringcase
+tornado>=6.4.1 # not directly required, pinned by Snyk to avoid a vulnerability