diff -r ca8e86711403 Lib/cgi.py
--- a/Lib/cgi.py	Tue Jun 11 22:13:17 2013 -0500
+++ b/Lib/cgi.py	Wed Jun 12 13:24:29 2013 +0300
@@ -697,6 +697,9 @@
             if not line:
                 self.done = -1
                 break
+            if delim == "\r":
+                line = delim + line
+                delim = ""
             if line[:2] == "--" and last_line_lfend:
                 strippedline = line.strip()
                 if strippedline == next:
@@ -713,6 +716,12 @@
                 delim = "\n"
                 line = line[:-1]
                 last_line_lfend = True
+            elif line[-1] == "\r":
+                # We may interrupt \r\n sequences if they span the 2**16
+                # byte boundary
+                delim = "\r"
+                line = line[:-1]
+                last_line_lfend = False
             else:
                 delim = ""
                 last_line_lfend = False
diff -r ca8e86711403 Lib/test/test_cgi.py
--- a/Lib/test/test_cgi.py	Tue Jun 11 22:13:17 2013 -0500
+++ b/Lib/test/test_cgi.py	Wed Jun 12 13:24:29 2013 +0300
@@ -266,6 +266,29 @@
                 got = getattr(fs.list[x], k)
                 self.assertEqual(got, exp)
 
+    def test_fieldstorage_multipart_maxline(self):
+        # Issue #18167
+        maxline = 1 << 16
+        self.maxDiff = None
+        def check(content):
+            data = """
+---123
+Content-Disposition: form-data; name="upload"; filename="fake.txt"
+Content-Type: text/plain
+
+%s
+---123--
+""".replace('\n', '\r\n') % content
+            environ = {
+                'CONTENT_LENGTH':   str(len(data)),
+                'CONTENT_TYPE':     'multipart/form-data; boundary=-123',
+                'REQUEST_METHOD':   'POST',
+            }
+            self.assertEqual(gen_result(data, environ), {'upload': content})
+        check('x' * (maxline - 1))
+        check('x' * (maxline - 1) + '\r')
+        check('x' * (maxline - 1) + '\r' + 'y' * (maxline - 1))
+
     _qs_result = {
         'key1': 'value1',
         'key2': ['value2x', 'value2y'],