[2.2.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.

Backport of 402c0caa851e265410fbcaa55318f22d2bf22ee2 from master

1 files changed, 14 insertions, 1 deletions

diff --git a/django/utils/numberformat.py b/django/utils/numberformat.py
index 9c0496342d..88b35fc435 100644
--- a/django/utils/numberformat.py
+++ b/django/utils/numberformat.py
@@ -27,7 +27,20 @@ def format(number, decimal_sep, decimal_pos=None, grouping=0, thousand_sep='',
     # sign
     sign = ''
     if isinstance(number, Decimal):
-        str_number = '{:f}'.format(number)
+        # 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)
     if str_number[0] == '-':