diff --git a/docs/requirements.txt b/docs/requirements.txt
index 8acece5..845ef1a 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -4,7 +4,7 @@ filelock==3.0.8
 future==0.16.0
 futures==3.1.1
 ipyparallel==6.2.2
-lxml==4.6.3
+lxml==4.9.1
 matplotlib==2.2.3
 numexpr==2.6.8
 numpy==1.15.2
diff --git a/rtd.requirements.txt b/rtd.requirements.txt
index e28bd36..9556f66 100644
--- a/rtd.requirements.txt
+++ b/rtd.requirements.txt
@@ -6,7 +6,7 @@ filelock==3.0.8
 Flask==1.0.2
 future==0.17.1
 ipyparallel==6.2.2
-lxml==4.2.5
+lxml==4.9.1
 matplotlib==2.2.3
 numexpr==2.6.8
 numpy==1.15.2