diff --git a/requirements/requirements_doc.txt b/requirements/requirements_doc.txt
index 3f7c2f24a40..3198dcf2cee 100644
--- a/requirements/requirements_doc.txt
+++ b/requirements/requirements_doc.txt
@@ -12,4 +12,5 @@ sphinx-gallery==0.11.1
 sphinx-notfound-page==0.8.3
 sphinxcontrib-websupport==1.2.4
 sphinxemoji==0.2.0
-autodocsumm==0.2.10
\ No newline at end of file
+autodocsumm==0.2.10
+tornado>=6.4.1 # not directly required, pinned by Snyk to avoid a vulnerability
\ No newline at end of file