CVE-2021-3733: ReDoS in urllib.request #87241

yetingli · 2021-01-30T08:11:46Z

BPO	43075
Nosy	@orsenthil, @vstinner, @ned-deily, @ambv, @serhiy-storchaka, @miss-islington, @yetingli, @StayPirate
PRs	bpo-43075: Fix ReDoS in request #24391 [3.9] bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) #25247 [3.8] bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) #25248 [3.7] bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) #25249 [3.6] bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) #25250
Files	redos_python.py redos_python2.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-05-06.17:00:47.929>
created_at = <Date 2021-01-30.08:11:46.452>
labels = ['type-security', 'easy', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'CVE-2021-3733: ReDoS in urllib.request'
updated_at = <Date 2021-09-07.20:12:23.266>
user = 'https://github.com/yetingli'

bugs.python.org fields:

activity = <Date 2021-09-07.20:12:23.266>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-06.17:00:47.929>
closer = 'ned.deily'
components = ['Library (Lib)']
creation = <Date 2021-01-30.08:11:46.452>
creator = 'yetingli'
dependencies = []
files = ['49778', '49938']
hgrepos = []
issue_num = 43075
keywords = ['patch', 'easy', 'newcomer friendly']
message_count = 16.0
messages = ['385974', '385987', '385993', '386009', '388358', '388665', '390415', '390417', '390419', '390420', '390425', '390441', '392885', '393109', '400130', '401341']
nosy_count = 8.0
nosy_names = ['orsenthil', 'vstinner', 'ned.deily', 'lukasz.langa', 'serhiy.storchaka', 'miss-islington', 'yetingli', 'crazybyte']
pr_nums = ['24391', '25247', '25248', '25249', '25250']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue43075'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

yetingli · 2021-01-30T08:11:46Z

Hi,

I find this regex '(?:^|,)[ \t]*([^ \\t]+)[ \t]+' may be stucked by input.

The vulnerable regex is located in

cpython/Lib/urllib/request.py

Line 946 in 5c5a938

rx = re.compile('(?:^|,)' # start of the string or ','

The ReDOS vulnerability of the regex is mainly due to the sub-pattern ',([^ \\t]+)' and can be exploited with the following string
attack_str = "," * 10000

You can execute redos_python.py to reproduce the ReDos vulnerability.

I am willing to suggest that you replace '(?:^|,)[ \t]([^ \\t]+)[ \t]+' with '(?:^|,)[ \t]([^ \\t,]+)[ \t]+'

Looking forward for your response!

Best,
Yeting Li

serhiy-storchaka · 2021-01-30T16:32:06Z

I agree. There is no catastrophic backtracking here (it was fixed in bpo-39503), but the complexity of matching the regular expression is linear. Searching the pattern in a sequence of commas has quadratic complexity, because every step has linear complexity and we advance only one character at every attempt.

The proposed solution looks correct to me and fixes the issue. Yeting Li, do you mind to create a pull request for it? I can do it myself, but since you have found the problem and the solution, it would be better if the commit be attributed to you.

orsenthil · 2021-01-30T18:59:10Z

+1. The suggested fix looks good to me.

yetingli · 2021-01-31T05:44:12Z

Thank you for your quick reply!

I agree. Catastrophic backtracking is typically regarded as a regex with exponential worst-case matching time. Besides regexes with exponential worst-case time complexity, ReDoS also includes ones with other super-linear (e.g., quadratic) worst-case time complexity.

Thanks again for your reply, I'm trying to create a pull request for it.

vstinner · 2021-03-09T11:51:40Z

I see that you attached a redos_python.py benchmark (which looks like a benchmark that I wrote recently ;-)) but you didn't give results. Can you please show that your fix is effective to avoid catastrophic performances?

Is this issue related to the CVE-2020-8492? Is it the same issue or is it different?
https://python-security.readthedocs.io/vuln/urllib-basic-auth-regex.html

yetingli · 2021-03-14T08:50:54Z

Sorry for the delay. I analyzed the performance of the current version '(?:^|,)[ \t]([^ \\t]+)[ \t]+' and the fixed version '(?:^|,)[ \t]([^ \\t,]+)[ \t]+'. I ran the following HTTP header ten times:

header = '' + ',' * (10 ** 5)

The current version takes about 139.178s-140.946s, while the repaired version takes about 0.006s.

You can analyze them with the code below.

    from time import perf_counter
    for _ in range(0, 10):
        BEGIN = perf_counter()
        header = repeat_10_5_simple
        headers = Headers(header)
        handler.http_error_auth_reqed("WWW-Authenticate", host, req, Headers(header))
        DURATION = perf_counter() - BEGIN
        print(f"took {DURATION} seconds!")

For CVE-2020-8492, it is the backtracking performance caused by some ambiguity during the matching, and this issue is caused by the regex engine constantly moves the matching regex across the malicious string that does not have a match for the regex.

Because the locations of the vulnerabilities are the same, so I refer to your code. Thanks for the code ;-)!

vstinner · 2021-04-07T10:59:29Z

header = '' + ',' * (10 ** 5)

I guess that a more generic protection against future attacks would be to limit the maximum length of a HTTP header. 100,000 characters for a HTTP Basic authentification does not sound reasonable.

But for now, let's fix the regex.

yetingli · 2021-04-07T11:14:28Z

For a regex has polynomial worst-case complexity, limiting the maximum input length is indeed a very effective method.

As shown below, as the input length becomes smaller, the matching time becomes significantly smaller.

header = '' + ',' * (10 ** 4) 1.617s
header = '' + ',' * (10 ** 3) 0.014s
header = '' + ',' * (10 ** 2) 0.00017s

vstinner · 2021-04-07T11:25:55Z

redos_python2.py: Updated benchmark.

I confirm that PR 24391 fix a worst case performance, starting with 100 characters.

Since the complexity is quadratic, strings longer 10^4 characters are likely to hang a client for several minutes.

== Reference (vulnerable) ==

simple: Mean +- std dev: 2.10 us +- 0.05 us
repeat 10: Mean +- std dev: 3.85 us +- 0.13 us
repeat 10^2: Mean +- std dev: 133 us +- 3 us
repeat 10^4: Mean +- std dev: 1.23 sec +- 0.05 sec

== With the PR 24391 fix ==

simple: Mean +- std dev: 2.15 us +- 0.15 us
repeat 10: Mean +- std dev: 2.44 us +- 0.04 us
repeat 10^2: Mean +- std dev: 7.45 us +- 0.17 us
repeat 10^4: Mean +- std dev: 574 us +- 28 us

== Comparison ==

simple: Mean +- std dev: [ref] 2.10 us +- 0.05 us -> [fix] 2.15 us +- 0.15 us: 1.02x slower
repeat 10: Mean +- std dev: [ref] 3.85 us +- 0.13 us -> [fix] 2.44 us +- 0.04 us: 1.58x faster
repeat 10^2: Mean +- std dev: [ref] 133 us +- 3 us -> [fix] 7.45 us +- 0.17 us: 17.80x faster
repeat 10^4: Mean +- std dev: [ref] 1.23 sec +- 0.05 sec -> [fix] 574 us +- 28 us: 2152.36x faster

Geometric mean: 15.59x faster

vstinner · 2021-04-07T11:27:50Z

New changeset 7215d1a by Yeting Li in branch 'master':
bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391)
7215d1a

miss-islington · 2021-04-07T11:45:13Z

New changeset e7654b6 by Miss Islington (bot) in branch '3.8':
bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391)
e7654b6

vstinner · 2021-04-07T15:58:07Z

New changeset a21d4fb by Miss Islington (bot) in branch '3.9':
bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) (GH-25247)
a21d4fb

ambv · 2021-05-04T12:46:44Z

New changeset ada1499 by Miss Islington (bot) in branch '3.7':
bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) (bpo-25249)
ada1499

ned-deily · 2021-05-06T17:00:15Z

New changeset 3fbe961 by Miss Islington (bot) in branch '3.6':
bpo-43075: Fix ReDoS in urllib AbstractBasicAuthHandler (GH-24391) (GH-25250)
3fbe961

staypirate · 2021-08-23T11:13:05Z

RedHat has now assigned CVE-2021-3733 to this security bug.

vstinner · 2021-09-07T20:12:23Z

I created https://python-security.readthedocs.io/vuln/urllib-basic-auth-regex2.html to track this vulnerability.

OmkarPatil11 · 2022-06-27T12:20:48Z

is there any affect of CVE-2021-3733 this security bug on python 2.7.18?

arhadthedev · 2022-06-27T14:01:07Z

@OmkarPatil11 Unfortunately yes. Here how a fix for 3.11 looks like:

 class AbstractBasicAuthHandler:
     # XXX this allows for multiple auth-schemes, but will stupidly pick
     # the last one with a realm specified.
     # allow for double- and single-quoted realm values
     # (single quotes are a violation of the RFC, but appear in the wild)
     rx = re.compile('(?:^|,)'   # start of the string or ','
                     '[ \t]*'    # optional whitespaces
-                    '([^ \t]+)' # scheme like "Basic"
+                    '([^ \t,]+)' # scheme like "Basic"
                     '[ \t]+'    # mandatory whitespaces
                     # realm=xxx
                     # realm='xxx'
                     # realm="xxx"
                     'realm=(["\']?)([^"\']*)\\2',
                     re.I)
     # XXX could pre-emptively send auth info already accepted (RFC 2617,
     # end of section 2, and section 1.2 immediately after "credentials"
     # production).

And here is the same fragment from v2.7.18, unfixed:

cpython/Lib/urllib2.py

Lines 852 to 864 in 8d21aa2

    
           class AbstractBasicAuthHandler: 
        
               # XXX this allows for multiple auth-schemes, but will stupidly pick 
        
               # the last one with a realm specified. 
        
               # allow for double- and single-quoted realm values 
        
               # (single quotes are a violation of the RFC, but appear in the wild) 
        
               rx = re.compile('(?:.*,)*[ \t]*([^ \t]+)[ \t]+' 
        
                               'realm=(["\']?)([^"\']*)\\2', re.I) 
        
               # XXX could pre-emptively send auth info already accepted (RFC 2617, 
        
               # end of section 2, and section 1.2 immediately after "credentials" 
        
               # production).

orsenthil · 2022-06-27T14:03:56Z

@OmkarPatil11 , Python 2.7.x is not supported. It doesn't receive security fixes too. Time to upgrade applications to Python 3.8 +

vstinner · 2022-06-27T21:49:28Z

Contact your Linux distribution to get a fix on Python 2.7. On Fedora, there are:

fedora-python@78a8416 : the fix
https://github.com/fedora-python/cpython/commits/fedora-2.7 : Python 2.7 fork with security fixes on top of it

yetingli mannequin added 3.7 (EOL) end of life 3.8 only security fixes 3.9 only security fixes stdlib Python modules in the Lib dir labels Jan 30, 2021

tirkarthi added type-security A security issue labels Jan 30, 2021

serhiy-storchaka added 3.10 only security fixes labels Jan 30, 2021

orsenthil added easy labels Mar 3, 2021

merwok changed the title ~~ReDoS in request~~ ReDoS in urllib.request Mar 3, 2021

zware added easy and removed easy labels Mar 3, 2021

taleinat added easy labels Mar 3, 2021

ned-deily closed this as completed May 6, 2021

vstinner changed the title ~~ReDoS in urllib.request~~ CVE-2021-3733: ReDoS in urllib.request Sep 7, 2021

ezio-melotti transferred this issue from another repository Apr 10, 2022

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CVE-2021-3733: ReDoS in urllib.request #87241

CVE-2021-3733: ReDoS in urllib.request #87241

yetingli mannequin commented Jan 30, 2021

yetingli mannequin commented Jan 30, 2021

serhiy-storchaka commented Jan 30, 2021

orsenthil commented Jan 30, 2021

yetingli mannequin commented Jan 31, 2021

vstinner commented Mar 9, 2021

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vstinner commented Apr 7, 2021

yetingli mannequin commented Apr 7, 2021

vstinner commented Apr 7, 2021

vstinner commented Apr 7, 2021

miss-islington commented Apr 7, 2021

vstinner commented Apr 7, 2021

ambv commented May 4, 2021

ned-deily commented May 6, 2021

staypirate mannequin commented Aug 23, 2021

vstinner commented Sep 7, 2021

OmkarPatil11 commented Jun 27, 2022

arhadthedev commented Jun 27, 2022

orsenthil commented Jun 27, 2022

vstinner commented Jun 27, 2022

CVE-2021-3733: ReDoS in urllib.request #87241

CVE-2021-3733: ReDoS in urllib.request #87241

Comments

yetingli mannequin commented Jan 30, 2021

yetingli mannequin commented Jan 30, 2021

serhiy-storchaka commented Jan 30, 2021

orsenthil commented Jan 30, 2021

yetingli mannequin commented Jan 31, 2021

vstinner commented Mar 9, 2021

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vstinner commented Apr 7, 2021

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vstinner commented Sep 7, 2021

OmkarPatil11 commented Jun 27, 2022

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