bpo-32981: Fix catastrophic backtracking vulns #5955

davisjam · 2018-03-01T21:50:22Z

Fix catastrophic backtracking vulns disclosed by email.

CVE-2018-1060
CVE-2018-1061

https://bugs.python.org/issue32981

the-knights-who-say-ni · 2018-03-01T21:50:25Z

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davisjam · 2018-03-01T21:56:49Z

I have now signed the CLA.

davisjam · 2018-03-01T21:57:23Z

Because this is a security issue no issue was previously created to track it. Perhaps someone might create one now?

[Update: I created bpo-32981.]

davisjam · 2018-03-01T22:12:40Z

Travis failure:

Fixing Python file whitespace ... 2 files:
  Lib/test/test_difflib.py
  Lib/test/test_poplib.py

Took a look, I can't see what's wrong. Is there a command (or flag to configure) I can run on my end to get a more verbose error message?

gvanrossum · 2018-03-01T23:47:06Z

That error comes from Tools/scripts/patchcheck.py.

davisjam · 2018-03-02T00:13:44Z

Thank you. Fixed.

davisjam · 2018-03-02T00:39:07Z

I created bpo-32981 for this problem.

I skimmed through the git log. Commit messages for security seem to follow the 'XXX (CVE-...)' format rather than the 'bpo-XXX: ...' format. Let me know if I should change the commit messages though.

serhiy-storchaka

Please add a news entry and add your name into Misc/ACKS.

serhiy-storchaka · 2018-03-02T06:11:27Z

Lib/difflib.py

@@ -1083,7 +1083,7 @@ 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):


Use non-capturing group: (?:#\s*).

serhiy-storchaka · 2018-03-02T06:11:27Z

Lib/test/test_difflib.py

+    def test_is_line_junk_true(self):
+        for line in ['#', '  ', ' #', '# ', ' # ', '']:
+            is_junk = difflib.IS_LINE_JUNK(line)
+            self.assertEqual(is_junk, True)


Why not assertTrue()?

Fixed this and others, thanks.

serhiy-storchaka · 2018-03-02T06:11:27Z

Lib/test/test_difflib.py

+        elapsed = end - start
+
+        self.assertEqual(is_junk, False)
+        self.assertEqual(elapsed.seconds, 0)


Timing test is very fragile. It can fail randomly when tests are paused for some reasons. I suggest to remove the timing test and increase the size of input so that it would take a hour with non-fixed code.

Then the test will pass if you wait long enough. Since the test suite already takes a long time to run and does so in parallel (so one hanging test might not be visible), this seems undesirable to me. Thoughts?

Same thing in Lib/test/test_poplib.py

If the test takes too long time for running it will be failed on buildbots due to total timeout.

davisjam · 2018-03-02T15:33:39Z

@serhiy-storchaka Thank you! I think I have addressed all of your comments so far.

serhiy-storchaka

Thank you James. Here yet few comments.

serhiy-storchaka · 2018-03-02T16:21:20Z

Misc/NEWS.d/next/Security/2018-03-02-10-24-52.bpo-32981.O_qDyj.rst

@@ -0,0 +1,3 @@
+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.


Add "Patch by your name."

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_difflib.py

+    def test_is_character_junk_true(self):
+        for char in [' ', '\t']:
+            is_junk = difflib.IS_CHARACTER_JUNK(char)
+            self.assertEqual(is_junk, True)


assertTrue()?

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_difflib.py

+    def test_is_line_junk_true(self):
+        for line in ['#', '  ', ' #', '# ', ' # ', '']:
+            is_junk = difflib.IS_LINE_JUNK(line)
+            self.assertTrue(is_junk)


Why not just

self.assertTrue(difflib.IS_LINE_JUNK(line))

?

And you can add the second argument of assertTrue() for getting more information if the test fails:

self.assertTrue(difflib.IS_LINE_JUNK(line), rept(line))

But don't do this in test_is_line_junk_REDOS.

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_poplib.py

+        # At this length, evil input makes each apop call take
+        # on the order of milliseconds instead of microseconds.
+        evil_welcome = bytes('+OK' + ('<' * 500000), self.client.encoding)
+        self.client.welcome = evil_welcome


You can use a special helper swap_attr() for temporary setting the attribute:

with test.support.swap_attr(self.client, 'welcome', evil_welcome):

Ah, nice, thanks for the tip.

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_poplib.py

+        except poplib.error_proto:
+            threw = True
+        # The evil welcome is invalid, so apop should throw.
+        self.assertTrue(threw)


Why not use assertRaises()?

The reason for the poor choices of asserts is that I didn't know what asserts are available and made do with what I could find. I really appreciate your help.

https://docs.python.org/3/library/unittest.html#test-cases

assertRaises() can be used as a function:

self.assertRaises(poplib.error_proto, self.client.apop, 'foo', 'dummypassword')

or as a context manager:

with self.assertRaises(poplib.error_proto): self.client.apop('foo', 'dummypassword')

Yup I have at last found the documentation for unittest.

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_poplib.py

+        # 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 = bytes('+OK' + ('<' * 500000), self.client.encoding)


What is the advantage over just b'+OK' + b'<' * 500000?

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_poplib.py

@@ -10,6 +10,7 @@
 import os
 import errno
 import threading
+import datetime


No longer used.

serhiy-storchaka · 2018-03-02T16:21:20Z

Lib/test/test_difflib.py

@@ -3,6 +3,7 @@
 import unittest
 import doctest
 import sys
+import datetime


No longer used.

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. Co-authored-by: Tim Peters <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>

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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>

davisjam · 2018-03-02T17:44:04Z

The latest version uses 1-million character attack strings. These take at least 3 minutes to evaluate on the master branch. I didn't want to wait for the full timeout.

serhiy-storchaka · 2018-03-02T17:55:50Z

Lib/test/test_difflib.py

+
+    def test_is_line_junk_false(self):
+        for line in ['##', ' ##', '## ', 'abc ', 'abc #', 'abc # ', 'Mr. Moose is up!']:
+            self.assertFalse(difflib.IS_LINE_JUNK(line), 'should not be junk: {}'.format(line))


The length of this line exceeds the 79 character limit allowed by PEP 8.

And since testing strings contain spaces it is hard to distinguish the following cases:

should not be junk: abc #

and

should not be junk: abc #

This is why I suggested repr().

The length of this line exceeds the 79 character limit allowed by PEP 8.

Didn't realize this was being enforced, there are other lines over this limit as well. Will do.

This is why I suggested repr().

Got it, I misread that as rept earlier and couldn't understand what you meant.

serhiy-storchaka · 2018-03-02T17:55:50Z

Lib/test/test_poplib.py

+        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, 'arthur', 'kingofbritons')


Since this line is too long, a context manager form of assertRaises() would be more preferable.

serhiy-storchaka · 2018-03-02T17:57:26Z

And please make your changes as new commits rather of editing old commits. This would make reviewing easier.

davisjam · 2018-03-02T19:41:23Z

@serhiy-storchaka Acknowledged. I believe I've addressed all of the concerns you've raised.

davisjam · 2018-03-02T19:48:45Z

Should I rebase or will these be squashed during merge anyway?

serhiy-storchaka · 2018-03-02T20:03:20Z

Will be squashed during merge.

miss-islington · 2018-03-04T05:33:34Z

Thanks @davisjam for the PR, and @benjaminp for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7, 3.6, 3.7.
🐍🍒⛏🤖

bedevere-bot · 2018-03-04T05:33:34Z

@benjaminp: Please replace # with GH- in the commit message next time. Thanks!

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org> (cherry picked from commit 0e6c8ee) Co-authored-by: Jamie Davis <davisjam@vt.edu>

bedevere-bot · 2018-03-04T05:33:43Z

GH-5969 is a backport of this pull request to the 3.7 branch.

miss-islington · 2018-03-04T05:34:36Z

Sorry, @davisjam and @benjaminp, I could not cleanly backport this to 3.6 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 0e6c8ee2358a2e23117501826c008842acb835ac 3.6

miss-islington · 2018-03-04T05:35:36Z

Sorry, @davisjam and @benjaminp, I could not cleanly backport this to 2.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 0e6c8ee2358a2e23117501826c008842acb835ac 2.7

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>. (cherry picked from commit 0e6c8ee)

bedevere-bot · 2018-03-04T05:44:45Z

GH-5970 is a backport of this pull request to the 2.7 branch.

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>. (cherry picked from commit 0e6c8ee)

bedevere-bot · 2018-03-04T05:47:23Z

GH-5971 is a backport of this pull request to the 3.6 branch.

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org> Co-authored-by: Jamie Davis <davisjam@vt.edu> (cherry picked from commit 0e6c8ee)

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>. (cherry picked from commit 0e6c8ee)

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>.

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>. (cherry picked from commit 0e6c8ee)

* 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 <tim.peters@gmail.com> Co-authored-by: Christian Heimes <christian@python.org>

davisjam requested a review from python/email-team as a code owner Mar 1, 2018

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bpo-32981: Fix catastrophic backtracking vulns #5955

bpo-32981: Fix catastrophic backtracking vulns #5955

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