diff --git a/doc/requirements.txt b/doc/requirements.txt
index ff8238184df..d072bc153a9 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -36,3 +36,4 @@ nbconvert==5.6.1
 orjson
 dash-bio
 jinja2<3.1
+tornado>=6.4.2 # not directly required, pinned by Snyk to avoid a vulnerability