[2.0.x] Fixed CVE-2019-6975 -- Fixed memory exhaustion in utils.numberformat.format().

Thanks Sjoerd Job Postmus for the report and initial patch.
Thanks Michael Manfre, Tim Graham, and Florian Apolloner for review.

django/utils/numberformat.py

@@ -27,6 +27,19 @@ def format(number, decimal_sep, decimal_pos=None, grouping=0, thousand_sep='',
     # sign
     sign = ''
     if isinstance(number, Decimal):
+        # Format values with more than 200 digits (an arbitrary cutoff) using
+        # scientific notation to avoid high memory usage in {:f}'.format().
+        _, digits, exponent = number.as_tuple()
+        if abs(exponent) + len(digits) > 200:
+            number = '{:e}'.format(number)
+            coefficient, exponent = number.split('e')
+            # Format the coefficient.
+            coefficient = format(
+                coefficient, decimal_sep, decimal_pos, grouping,
+                thousand_sep, force_grouping, use_l10n,
+            )
+            return '{}e{}'.format(coefficient, exponent)
+        else:
             str_number = '{:f}'.format(number)
     else:
         str_number = str(number)

docs/releases/1.11.19.txt

@@ -5,3 +5,15 @@ Django 1.11.19 release notes
 *February 11, 2019*
 
 Django 1.11.19 fixes a security issue in 1.11.18.
+
+CVE-2019-6975: Memory exhaustion in ``django.utils.numberformat.format()``
+--------------------------------------------------------------------------
+
+If ``django.utils.numberformat.format()`` -- used by ``contrib.admin`` as well
+as the the ``floatformat``, ``filesizeformat``, and ``intcomma`` templates
+filters -- received a ``Decimal`` with a large number of digits or a large
+exponent, it could lead to significant memory usage due to a call to
+``'{:f}'.format()``.
+
+To avoid this, decimals with more than 200 digits are now formatted using
+scientific notation.

docs/releases/2.0.11.txt

@@ -5,3 +5,15 @@ Django 2.0.11 release notes
 *February 11, 2019*
 
 Django 2.0.11 fixes a security issue in 2.0.10.
+
+CVE-2019-6975: Memory exhaustion in ``django.utils.numberformat.format()``
+--------------------------------------------------------------------------
+
+If ``django.utils.numberformat.format()`` -- used by ``contrib.admin`` as well
+as the the ``floatformat``, ``filesizeformat``, and ``intcomma`` templates
+filters -- received a ``Decimal`` with a large number of digits or a large
+exponent, it could lead to significant memory usage due to a call to
+``'{:f}'.format()``.
+
+To avoid this, decimals with more than 200 digits are now formatted using
+scientific notation.

tests/utils_tests/test_numberformat.py

@@ -75,6 +75,25 @@ class TestNumberFormat(TestCase):
         )
         self.assertEqual(nformat(Decimal('3.'), '.'), '3')
         self.assertEqual(nformat(Decimal('3.0'), '.'), '3.0')
+        # Very large & small numbers.
+        tests = [
+            ('9e9999', None, '9e+9999'),
+            ('9e9999', 3, '9.000e+9999'),
+            ('9e201', None, '9e+201'),
+            ('9e200', None, '9e+200'),
+            ('1.2345e999', 2, '1.23e+999'),
+            ('9e-999', None, '9e-999'),
+            ('1e-7', 8, '0.00000010'),
+            ('1e-8', 8, '0.00000001'),
+            ('1e-9', 8, '0.00000000'),
+            ('1e-10', 8, '0.00000000'),
+            ('1e-11', 8, '0.00000000'),
+            ('1' + ('0' * 300), 3, '1.000e+300'),
+            ('0.{}1234'.format('0' * 299), 3, '1.234e-300'),
+        ]
+        for value, decimal_pos, expected_value in tests:
+            with self.subTest(value=value):
+                self.assertEqual(nformat(Decimal(value), '.', decimal_pos), expected_value)
 
     def test_decimal_subclass(self):
         class EuroDecimal(Decimal):