diff --git a/requirements.txt b/requirements.txt
index cf1575e30..325e7449b 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -14,3 +14,4 @@ numexpr
 pandas-datareader
 basemap
 netcdf4
+tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability