diff --git a/Lib/cgi.py b/Lib/cgi.py
index 77ab703cc036004..8aa176774220c06 100755
--- a/Lib/cgi.py
+++ b/Lib/cgi.py
@@ -667,7 +667,7 @@ def read_multi(self, environ, keep_blank_values, strict_parsing):
     def read_single(self):
         """Internal: read an atomic part."""
         if self.length >= 0:
-            self.read_binary()
+            self.read_content()
             self.skip_lines()
         else:
             self.read_lines()
@@ -675,31 +675,31 @@ def read_single(self):
 
     bufsize = 8*1024            # I/O buffering size for copy to file
 
-    def read_binary(self):
-        """Internal: read binary data."""
-        self.file = self.make_file()
+    def read_content(self):
+        """Internal: read data up until content-length len."""
+        if self.length > 1000:
+            self.file = self.make_file()
+        else:
+            self.file = self.io_object()
+        self.__file = self.file
         todo = self.length
         if todo >= 0:
             while todo > 0:
                 data = self.fp.read(min(todo, self.bufsize)) # bytes
-                if not isinstance(data, bytes):
-                    raise ValueError("%s should return bytes, got %s"
-                                     % (self.fp, type(data).__name__))
                 self.bytes_read += len(data)
                 if not data:
                     self.done = -1
                     break
-                self.file.write(data)
+                self.__write(data)
                 todo = todo - len(data)
 
     def read_lines(self):
         """Internal: read lines until EOF or outerboundary."""
-        if self._binary_file:
-            self.file = self.__file = BytesIO() # store data as bytes for files
-        else:
-            self.file = self.__file = StringIO() # as strings for other fields
+        self.file = self.__file = self.io_object()
         if self.outerboundary:
             self.read_lines_to_outerboundary()
+        elif self.length >= 0:
+            self.read_content()
         else:
             self.read_lines_to_eof()
 
@@ -721,7 +721,7 @@ def __write(self, line):
     def read_lines_to_eof(self):
         """Internal: read lines until EOF."""
         while 1:
-            line = self.fp.readline(1<<16) # bytes
+            line = self.fp.read(1<<16) # bytes
             self.bytes_read += len(line)
             if not line:
                 self.done = -1
@@ -830,6 +830,12 @@ def make_file(self):
             return tempfile.TemporaryFile("w+",
                 encoding=self.encoding, newline = '\n')
 
+    def io_object(self):
+        if self._binary_file:
+            return BytesIO() # store data as bytes for files
+        else:
+            return StringIO() # as strings for other fields
+
 
 # Test/debug code
 # ===============
diff --git a/Lib/test/test_cgi.py b/Lib/test/test_cgi.py
index 101942de947fb46..8afdd6ea05db6a6 100644
--- a/Lib/test/test_cgi.py
+++ b/Lib/test/test_cgi.py
@@ -215,22 +215,22 @@ def test_log(self):
             self.addCleanup(cgi.closelog)
             cgi.log("Testing log 4")
 
-    def test_fieldstorage_readline(self):
-        # FieldStorage uses readline, which has the capacity to read all
-        # contents of the input file into memory; we use readline's size argument
-        # to prevent that for files that do not contain any newlines in
+    def test_fieldstorage_read(self):
+        # FieldStorage uses read, which has the capacity to read all contents
+        # of the input file into memory; we use read's size argument to
+        # prevent that for files that do not contain any newlines in
         # non-GET/HEAD requests
-        class TestReadlineFile:
+        class TestReadFile:
             def __init__(self, file):
                 self.file = file
                 self.numcalls = 0
 
-            def readline(self, size=None):
+            def read(self, size=None):
                 self.numcalls += 1
                 if size:
-                    return self.file.readline(size)
+                    return self.file.read(size)
                 else:
-                    return self.file.readline()
+                    return self.file.read()
 
             def __getattr__(self, name):
                 file = self.__dict__['file']
@@ -239,14 +239,14 @@ def __getattr__(self, name):
                     setattr(self, name, a)
                 return a
 
-        f = TestReadlineFile(tempfile.TemporaryFile("wb+"))
+        f = TestReadFile(tempfile.TemporaryFile("wb+"))
         self.addCleanup(f.close)
         f.write(b'x' * 256 * 1024)
         f.seek(0)
         env = {'REQUEST_METHOD':'PUT'}
         fs = cgi.FieldStorage(fp=f, environ=env)
         self.addCleanup(fs.file.close)
-        # if we're not chunking properly, readline is only called twice
+        # if we're not chunking properly, read is only called twice
         # (by read_binary); if we are chunking properly, it will be called 5 times
         # as long as the chunksize is 1 << 16.
         self.assertGreater(f.numcalls, 2)
@@ -366,6 +366,18 @@ def test_fieldstorage_part_content_length(self):
         self.assertEqual(fs.list[0].name, 'submit-name')
         self.assertEqual(fs.list[0].value, 'Larry')
 
+    def test_content_length_no_content_disposition(self):
+        body = b'{"test":123}'
+        env = {
+            'CONTENT_LENGTH': len(body),
+            'REQUEST_METHOD': 'POST',
+            'CONTENT_TYPE': 'application/json',
+            'wsgi.input': BytesIO(body),
+        }
+
+        form = cgi.FieldStorage(fp=env['wsgi.input'], environ=env)
+        self.assertEqual(form.file.read(), body.decode(form.encoding))
+
     def test_field_storage_multipart_no_content_length(self):
         fp = BytesIO(b"""--MyBoundary
 Content-Disposition: form-data; name="my-arg"; filename="foo"
diff --git a/Misc/NEWS.d/next/Library/2019-02-05-16-57-27.bpo-27777.AKUxDR.rst b/Misc/NEWS.d/next/Library/2019-02-05-16-57-27.bpo-27777.AKUxDR.rst
new file mode 100644
index 000000000000000..9e8513dd208a30c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-02-05-16-57-27.bpo-27777.AKUxDR.rst
@@ -0,0 +1,2 @@
+Fixes a bug in :mod:`cgi` :class:`cgi.FieldStorage` when a request has a "Content-Length" header but
+without "Content-Disposition" headers.