diff --git a/tests/e2e.nix b/tests/e2e.nix
index 0b37c96..cb2dc8c 100644
--- a/tests/e2e.nix
+++ b/tests/e2e.nix
@@ -7,14 +7,35 @@
 { mailmig, nixos-mailserver }:
 { pkgs, lib, ... }:
 let
+  # Python 3.13 verifies strictly (VERIFY_X509_STRICT): the CA needs keyUsage,
+  # leaves need SKI/AKI.
   certs = pkgs.runCommand "mailmig-test-certs" { nativeBuildInputs = [ pkgs.openssl ]; } ''
     mkdir $out && cd $out
-    openssl req -x509 -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -nodes \
-      -keyout ca.key -out ca.crt -days 3650 -subj "/CN=mailmig test CA"
+    cat > ca.cnf <<EOF
+    [req]
+    distinguished_name = dn
+    x509_extensions = v3_ca
+    prompt = no
+    [dn]
+    CN = mailmig test CA
+    [v3_ca]
+    basicConstraints = critical,CA:TRUE
+    keyUsage = critical,keyCertSign,cRLSign
+    subjectKeyIdentifier = hash
+    EOF
+    openssl req -x509 -config ca.cnf -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -nodes \
+      -keyout ca.key -out ca.crt -days 3650
     for h in old new; do
       openssl req -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -nodes \
         -keyout $h.key -out $h.csr -subj "/CN=$h.test"
-      printf 'subjectAltName=DNS:%s.test\nextendedKeyUsage=serverAuth\nkeyUsage=digitalSignature\nbasicConstraints=CA:FALSE\n' $h > $h.ext
+      cat > $h.ext <<EOF
+    subjectAltName = DNS:$h.test
+    extendedKeyUsage = serverAuth
+    keyUsage = critical,digitalSignature
+    basicConstraints = critical,CA:FALSE
+    subjectKeyIdentifier = hash
+    authorityKeyIdentifier = keyid
+    EOF
       openssl x509 -req -in $h.csr -CA ca.crt -CAkey ca.key -CAcreateserial \
         -out $h.crt -days 3650 -extfile $h.ext
     done