diff --git a/docs/requirements.txt b/docs/requirements.txt
index 1a7842c7..caae9619 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -7,3 +7,4 @@ sphinx-reredirects==0.0.1
 sphinxcontrib-images==0.9.2
 sphinxcontrib-spelling==5.4.0
 docutils<0.18  # see https://github.com/sphinx-doc/sphinx/issues/9807
+tornado>=6.3.2 # not directly required, pinned by Snyk to avoid a vulnerability