diff --git a/requirements/doc.txt b/requirements/doc.txt
index 2d2245cd83a..c3415992a38 100644
--- a/requirements/doc.txt
+++ b/requirements/doc.txt
@@ -5,3 +5,4 @@ numpydoc>=1.1
 pillow>=8.0
 nb2plots>=0.6
 texext>=0.6.6
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability