diff --git a/Lib/email/header.py b/Lib/email/header.py
index aaca18a..ce55d61 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -46,6 +46,10 @@
 # For use with .match()
 fcre = re.compile(r'[\041-\176]+:$')
 
+# Find a header embeded in a putative header value.  Used to check for
+# header injection attack.
+_embeded_header = re.compile(r'\n[^ \t]+:')
+
 
 
 # Helpers
@@ -305,7 +309,11 @@
                 if len(lines) > 1:
                     formatter.newline()
             formatter.add_transition()
-        return str(formatter)
+        value = str(formatter)
+        if _embeded_header.search(value):
+            raise HeaderParseError("header value appears to contain "
+                "an embedded header: {!r}".format(value))
+        return value
 
     def _normalize(self):
         # Step 1: Normalize the chunks so that all runs of identical charsets
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index 05eb6a7..5222bab 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -540,6 +540,19 @@
             msg['Content-Disposition'])
 
 
+    # Issue 5871: reject an attempt to embed a header inside a header value
+    # (header injection attack).
+    def test_embeded_header_via_Header_rejected(self):
+        msg = Message()
+        msg['Dummy'] = Header('dummy\nX-Injected-Header: test')
+        self.assertRaises(errors.HeaderParseError, msg.as_string)
+
+    def test_embeded_header_via_string_rejected(self):
+        msg = Message()
+        msg['Dummy'] = 'dummy\nX-Injected-Header: test'
+        self.assertRaises(errors.HeaderParseError, msg.as_string)
+
+
 # Test the email.encoders module
 class TestEncoders(unittest.TestCase):
     def test_encode_empty_payload(self):