diff --git a/Lib/email/header.py b/Lib/email/header.py
index 9e91dc2..702adf1 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -47,6 +47,10 @@
 # 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
@@ -403,7 +407,11 @@
             newchunks += self._split(s, charset, targetlen, splitchars)
             lastchunk, lastcharset = newchunks[-1]
             lastlen = lastcharset.encoded_header_len(lastchunk)
-        return self._encode_chunks(newchunks, maxlinelen)
+        value = self._encode_chunks(newchunks, maxlinelen)
+        if _embeded_header.search(value):
+            raise HeaderParseError("header value appears to contain "
+                "an embedded header: {!r}".format(value))
+        return value
 
 
 
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index 4aac500..5c9a725 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -553,6 +553,17 @@
         msg.set_charset(u'us-ascii')
         self.assertEqual('us-ascii', msg.get_content_charset())
 
+    # 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