diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 8d98b80..36580d5 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -1660,6 +1660,9 @@
         with self.assertRaises(ValueError):
             ctx.wrap_bio(ssl.MemoryBIO(), ssl.MemoryBIO(),
                          server_hostname=".example.org")
+        with self.assertRaises(TypeError):
+            ctx.wrap_bio(ssl.MemoryBIO(), ssl.MemoryBIO(),
+                         server_hostname="example.org\x00evil.com")
 
 
 class MemoryBIOTests(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Security/2018-03-25-12-05-43.bpo-33136.TzSN4x.rst b/Misc/NEWS.d/next/Security/2018-03-25-12-05-43.bpo-33136.TzSN4x.rst
new file mode 100644
index 0000000..c350516
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2018-03-25-12-05-43.bpo-33136.TzSN4x.rst
@@ -0,0 +1,3 @@
+Harden ssl module against LibreSSL CVE-2018-8970.
+X509_VERIFY_PARAM_set1_host() is called with an explicit namelen. A new test
+ensures that NULL bytes are not allowed.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 30c3403..4baabd5 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -852,7 +852,8 @@
     if (self->ctx->check_hostname) {
         X509_VERIFY_PARAM *param = SSL_get0_param(self->ssl);
         if (ip == NULL) {
-            if (!X509_VERIFY_PARAM_set1_host(param, server_hostname, 0)) {
+            if (!X509_VERIFY_PARAM_set1_host(param, server_hostname,
+                                             strlen(server_hostname))) {
                 _setSSLError(NULL, 0, __FILE__, __LINE__);
                 goto error;
             }
@@ -4025,7 +4026,7 @@
     PyObject *res;
 
     /* server_hostname is either None (or absent), or to be encoded
-       as IDN A-label (ASCII str). */
+       as IDN A-label (ASCII str) without NULL bytes. */
     if (hostname_obj != Py_None) {
         if (!PyArg_Parse(hostname_obj, "es", "ascii", &hostname))
             return NULL;
@@ -4063,7 +4064,7 @@
     PyObject *res;
 
     /* server_hostname is either None (or absent), or to be encoded
-       as IDN A-label (ASCII str). */
+       as IDN A-label (ASCII str) without NULL bytes. */
     if (hostname_obj != Py_None) {
         if (!PyArg_Parse(hostname_obj, "es", "ascii", &hostname))
             return NULL;