diff -r 1756beed417c Lib/nntplib.py
--- a/Lib/nntplib.py	Sat Dec 17 09:19:11 2016 +0100
+++ b/Lib/nntplib.py	Sat Dec 17 16:04:33 2016 +0100
@@ -85,11 +85,9 @@
            "decode_header",
            ]
 
-# maximal line length when calling readline(). This is to prevent
-# reading arbitrary length lines. RFC 3977 limits NNTP line length to
-# 512 characters, including CRLF. We have selected 2048 just to be on
-# the safe side.
-_MAXLINE = 2048
+# A response from the server exceeding _MAXBYTES is considered a malicious Dos
+# attack.
+_MAXBYTES = 100 * 1024 * 1024
 
 
 # Exceptions raised when an error or invalid response is received
@@ -413,6 +411,8 @@
     def _putline(self, line):
         """Internal: send one line to the server, appending CRLF.
         The `line` must be a bytes-like object."""
+        if not hasattr(self, 'file'):
+            raise NNTPPermanentError('The session is closed.')
         line = line + _CRLF
         if self.debugging > 1: print('*put*', repr(line))
         self.file.write(line)
@@ -429,9 +429,13 @@
         """Internal: return one line from the server, stripping _CRLF.
         Raise EOFError if the connection is closed.
         Returns a bytes object."""
-        line = self.file.readline(_MAXLINE +1)
-        if len(line) > _MAXLINE:
-            raise NNTPDataError('line too long')
+        if not hasattr(self, 'file'):
+            raise NNTPPermanentError('The session is closed.')
+        line = self.file.readline(_MAXBYTES + 1)
+        if len(line) > _MAXBYTES:
+            self._close()
+            raise NNTPPermanentError('Too many bytes received from the'
+                                     ' server, closing the session.')
         if self.debugging > 1:
             print('*get*', repr(line))
         if not line: raise EOFError
@@ -477,26 +481,26 @@
             if resp[:3] not in _LONGRESP:
                 raise NNTPReplyError(resp)
 
+            count = len(resp)
             lines = []
-            if file is not None:
-                # XXX lines = None instead?
-                terminators = (b'.' + _CRLF, b'.\n')
-                while 1:
-                    line = self._getline(False)
-                    if line in terminators:
-                        break
-                    if line.startswith(b'..'):
-                        line = line[1:]
+            file_is_None = (file is None)
+            terminators = (b'.', ) if file_is_None else (b'.' + _CRLF, b'.\n')
+            while 1:
+                line = self._getline(file_is_None)
+                count += len(line)
+                if count > _MAXBYTES:
+                    self._close()
+                    raise NNTPPermanentError('Too many bytes received from'
+                                         ' the server, closing the session.')
+                if line in terminators:
+                    break
+                if line.startswith(b'..'):
+                    line = line[1:]
+                if file_is_None:
+                    lines.append(line)
+                else:
                     file.write(line)
-            else:
-                terminator = b'.'
-                while 1:
-                    line = self._getline()
-                    if line == terminator:
-                        break
-                    if line.startswith(b'..'):
-                        line = line[1:]
-                    lines.append(line)
+
         finally:
             # If this method created the file, then it must close it
             if openedFile:
@@ -888,6 +892,8 @@
         return resp, _parse_datetime(date, None)
 
     def _post(self, command, f):
+        if not hasattr(self, 'file'):
+            raise NNTPPermanentError('The session is closed.')
         resp = self._shortcmd(command)
         # Raises a specific exception if posting is not allowed
         if not resp.startswith('3'):
@@ -933,9 +939,12 @@
         - resp: server response if successful"""
         try:
             resp = self._shortcmd('QUIT')
+        except NNTPPermanentError:
+            pass
+        else:
+            return resp
         finally:
             self._close()
-        return resp
 
     def login(self, user=None, password=None, usenetrc=True):
         if self.authenticated:
diff -r 1756beed417c Lib/test/test_nntplib.py
--- a/Lib/test/test_nntplib.py	Sat Dec 17 09:19:11 2016 +0100
+++ b/Lib/test/test_nntplib.py	Sat Dec 17 16:04:33 2016 +0100
@@ -132,8 +132,6 @@
         self.assertLessEqual(art_num, last)
         self._check_art_dict(art_dict)
 
-    @unittest.skipIf(True, 'temporarily skipped until a permanent solution'
-                           ' is found for issue #28971')
     def test_over(self):
         resp, count, first, last, name = self.server.group(self.GROUP_NAME)
         start = last - 10
@@ -1183,8 +1181,13 @@
 
     def test_too_long_lines(self):
         dt = datetime.datetime(2010, 1, 1, 9, 0, 0)
-        self.assertRaises(nntplib.NNTPDataError,
+        oldv = nntplib._MAXBYTES
+        nntplib._MAXBYTES = 2048
+        try:
+            self.assertRaises(nntplib.NNTPPermanentError,
                           self.server.newnews, "comp.lang.python", dt)
+        finally:
+            nntplib._MAXBYTES = oldv
 
 
 class NNTPv1Tests(NNTPv1v2TestsMixin, MockedNNTPTestsMixin, unittest.TestCase):