diff --git a/requirements.txt b/requirements.txt
index 270db1576..21a6b46fc 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ sphinx==3.2.0
 sphinx-autobuild==0.7.1
 git+https://github.com/python/python-docs-theme.git#egg=python-docs-theme
 git+https://github.com/pypa/pypa-docs-theme.git#egg=pypa-docs-theme
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability