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[2.7] bpo-32981: Fix catastrophic backtracking vulns (GH-5955)

* Prevent low-grade poplib REDOS (CVE-2018-1060)

The regex to test a mail server's timestamp is susceptible to
catastrophic backtracking on long evil responses from the server.

Happily, the maximum length of malicious inputs is 2K thanks
to a limit introduced in the fix for CVE-2013-1752.

A 2KB evil response from the mail server would result in small slowdowns
(milliseconds vs. microseconds) accumulated over many apop calls.
This is a potential DOS vector via accumulated slowdowns.

Replace it with a similar non-vulnerable regex.

The new regex is RFC compliant.
The old regex was non-compliant in edge cases.

* Prevent difflib REDOS (CVE-2018-1061)

The default regex for IS_LINE_JUNK is susceptible to
catastrophic backtracking.
This is a potential DOS vector.

Replace it with an equivalent non-vulnerable regex.

Also introduce unit and REDOS tests for difflib.

Co-authored-by: Tim Peters <>
Co-authored-by: Christian Heimes <>.
(cherry picked from commit 0e6c8ee)
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benjaminp committed Mar 4, 2018
1 parent 20003f9 commit e052d40cea15f582b50947f7d906b39744dc62a2
@@ -1103,7 +1103,7 @@ def _qformat(self, aline, bline, atags, btags):

import re

def IS_LINE_JUNK(line, pat=re.compile(r"\s*#?\s*$").match):
def IS_LINE_JUNK(line, pat=re.compile(r"\s*(?:#\s*)?$").match):
Return 1 for ignorable line: iff `line` is blank or contains a single '#'.
@@ -274,7 +274,7 @@ def rpop(self, user):
return self._shortcmd('RPOP %s' % user)

timestamp = re.compile(r'\+OK.*(<[^>]+>)')
timestamp = re.compile(br'\+OK.[^<]*(<.*>)')

def apop(self, user, secret):
@@ -269,13 +269,33 @@ def test_range_format_context(self):
self.assertEqual(fmt(3,6), '4,6')
self.assertEqual(fmt(0,0), '0')

class TestJunkAPIs(unittest.TestCase):
def test_is_line_junk_true(self):
for line in ['#', ' ', ' #', '# ', ' # ', '']:
self.assertTrue(difflib.IS_LINE_JUNK(line), repr(line))

def test_is_line_junk_false(self):
for line in ['##', ' ##', '## ', 'abc ', 'abc #', 'Mr. Moose is up!']:
self.assertFalse(difflib.IS_LINE_JUNK(line), repr(line))

def test_is_line_junk_REDOS(self):
evil_input = ('\t' * 1000000) + '##'

def test_is_character_junk_true(self):
for char in [' ', '\t']:
self.assertTrue(difflib.IS_CHARACTER_JUNK(char), repr(char))

def test_is_character_junk_false(self):
for char in ['a', '#', '\n', '\f', '\r', '\v']:
self.assertFalse(difflib.IS_CHARACTER_JUNK(char), repr(char))

def test_main():
difflib.HtmlDiff._default_prefix = 0
Doctests = doctest.DocTestSuite(difflib)
TestWithAscii, TestAutojunk, TestSFpatches, TestSFbugs,
TestOutputFormat, Doctests)
TestOutputFormat, TestJunkAPIs)

if __name__ == '__main__':
@@ -211,6 +211,16 @@ def test_noop(self):
def test_rpop(self):

def test_apop_REDOS(self):
# Replace welcome with very long evil welcome.
# NB The upper bound on welcome length is currently 2048.
# At this length, evil input makes each apop call take
# on the order of milliseconds instead of microseconds.
evil_welcome = b'+OK' + (b'<' * 1000000)
with test_support.swap_attr(self.client, 'welcome', evil_welcome):
# The evil welcome is invalid, so apop should throw.
self.assertRaises(poplib.error_proto, self.client.apop, 'a', 'kb')

def test_top(self):
expected = ('+OK 116 bytes',
['From:', 'Content-Type: text/plain',
@@ -318,6 +318,8 @@ Jonathan Dasteel
Pierre-Yves David
A. Jesse Jiryu Davis
Jake Davis
Jamie (James C.) Davis
Ratnadeep Debnath
Merlijn van Deen
John DeGood
Ned Deily
@@ -0,0 +1,4 @@
Regexes in difflib and poplib were vulnerable to catastrophic backtracking.
These regexes formed potential DOS vectors (REDOS). They have been
refactored. This resolves CVE-2018-1060 and CVE-2018-1061.
Patch by Jamie Davis.

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