diff --git a/doc/requirements.txt b/doc/requirements.txt
index 87a8eacb3c9e..592b2b649f7d 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -13,3 +13,4 @@ sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-osexample==0.1.1
 sphinxcontrib-qthelp==1.0.3
 sphinxcontrib-serializinghtml==1.1.4
+tornado>=6.4.1 # not directly required, pinned by Snyk to avoid a vulnerability