diff --git a/.python-version b/.python-version
new file mode 100644
index 00000000..902b2c90
--- /dev/null
+++ b/.python-version
@@ -0,0 +1 @@
+3.11
\ No newline at end of file
diff --git a/requirements.in b/requirements.in
index d0fbd1e2..06b36183 100644
--- a/requirements.in
+++ b/requirements.in
@@ -6,3 +6,4 @@ sphinxcontrib-opendataservices-jsonschema
 sphinx-intl
 sphinx_rtd_theme
 myst-parser
+certifi>=2026.10.19         # Response to eMudhra/emSign CA certificate security incident.
\ No newline at end of file
diff --git a/requirements.txt b/requirements.txt
index ed3d3b37..4d151a68 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -16,8 +16,10 @@ backports-datetime-fromisoformat==2.0.3
     # via flattentool
 btrees==6.1
     # via zodb
-certifi==2025.6.15
-    # via requests
+certifi==2026.10.19
+    # via
+    #   -r requirements.in
+    #   requests
 cffi==1.17.1
     # via persistent
 charset-normalizer==3.4.2