Catastrophic backtracking in poplib (CVE-2018-1060) and difflib (CVE-2018-1061)

[davisjam]

BPO	32981
Nosy	@tim-one, @vstinner, @larryhastings, @benjaminp, @ned-deily, @davisjam
PRs	bpo-32981: Fix catastrophic backtracking vulns #5955 [3.7] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) #5969 [2.7] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) #5970 [3.6] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) #5971 [3.5] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) #6034 [3.4] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) #6035

^{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 2018-03-14.05:23:09.730>
created_at = <Date 2018-03-02.00:36:19.854>
labels = ['type-security', '3.8', '3.7', 'library']
title = 'Catastrophic backtracking in poplib (CVE-2018-1060) and difflib (CVE-2018-1061)'
updated_at = <Date 2019-05-10.18:09:12.303>
user = 'https://github.com/davisjam'

bugs.python.org fields:

activity = <Date 2019-05-10.18:09:12.303>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2018-03-14.05:23:09.730>
closer = 'ned.deily'
components = ['Library (Lib)']
creation = <Date 2018-03-02.00:36:19.854>
creator = 'davisjam'
dependencies = []
files = []
hgrepos = []
issue_num = 32981
keywords = ['patch', 'security_issue']
message_count = 10.0
messages = ['313119', '313198', '313200', '313202', '313203', '313609', '313610', '313611', '313803', '316124']
nosy_count = 6.0
nosy_names = ['tim.peters', 'vstinner', 'larry', 'benjamin.peterson', 'ned.deily', 'davisjam']
pr_nums = ['5955', '5969', '5970', '5971', '6034', '6035']
priority = 'critical'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue32981'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[davisjam]

Hi Python security team,

My name is James Davis. I'm a security researcher at Virginia Tech.

The python core (cpython) has 2 regular expressions vulnerable to catastrophic backtracking that look like potential DOS vectors.
The vulnerable expressions are listed below.

Each vulnerability has the following keys, explained in more detail below:

• pattern
• filesIn (as of December/January; I excluded any appearances in irrelevant-looking dirs, and in '.min' files)
• stringLenFor10Sec
• nPumpsFor10Sec
• attackFormat
• blowupCurve

The attack format describes how to generate an attack string.
On my machine, an attack string generated using nPumpsFor10Sec repetitions ("pumps") of the pump string(s)
blocks the python regex engine for 10 seconds, though this will vary based on your hardware.

Compose an attack string like this:
'prefix 1' + 'pump 1' X times + 'prefix 2' + 'pump 2' X times + ... + suffix
Example:
With pumpPairs: [{'prefix': 'a', 'pump': 'b'}], suffix: 'c', an attack string with three pumps would be:
abbbc

Catastrophic backtracking blows up at either an exponential rate or a super-linear (power law) rate.
The blowupCurve indicates how severe the blow-up is.
The 'type' is either EXP(onential) or POW(er law) in the number of pumps (x).
The 'parms' are the parameters for the two curve types. The second parameter is more important, because:
EXP: f(x) = parms[0] * parms[1]^x
POW: f(x) = parms[0] * x^parms[1]

JSON formatted:

Vuln 1:

{
"attackFormat" : {
"pumpPairs" : [
{
"pump" : "<a",
"prefix" : "+OKa"
}
],
"suffix" : "+"
},
"blowupCurve" : {
"parms" : [
2.71096268836868e-08,
1.83422078906374
],
"type" : "POWER",
"r2" : 0.997503282766243
},
"stringLenFor10Sec" : 96655,
"nPumpsFor10Sec" : "48325",
"pattern" : "\\+OK.*(<[^\>]+>)",
"filesIn" : [
[
"Lib/poplib.py"
]
]
}

Vuln 2:

{
"blowupCurve" : {
"parms" : [
1.31911634447601e-08,
1.89691808610459
],
"r2" : 0.998387790742004,
"type" : "POWER"
},
"stringLenFor10Sec" : 48328,
"attackFormat" : {
"pumpPairs" : [
{
"pump" : "\t",
"prefix" : "\t"
}
],
"suffix" : "##"
},
"pattern" : "\\s*#?\\s*$",
"filesIn" : [
[
"Lib/difflib.py"
]
],
"nPumpsFor10Sec" : "48325"
}

[benjaminp]

New changeset 0e6c8ee by Benjamin Peterson (Jamie Davis) in branch 'master':
bpo-32981: Fix catastrophic backtracking vulns (bpo-5955)
0e6c8ee

[benjaminp]

New changeset 0902a2d by Benjamin Peterson (Miss Islington (bot)) in branch '3.7':
bpo-32981: Fix catastrophic backtracking vulns (GH-5955)
0902a2d

[benjaminp]

New changeset e052d40 by Benjamin Peterson in branch '2.7':
[2.7] bpo-32981: Fix catastrophic backtracking vulns (GH-5955)
e052d40

[benjaminp]

New changeset c951675 by Benjamin Peterson in branch '3.6':
[3.6] bpo-32981: Fix catastrophic backtracking vulns (GH-5955)
c951675

[larryhastings]

New changeset 942cc04 by larryhastings (Ned Deily) in branch '3.4':
[3.4] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) (bpo-6035)
942cc04

[larryhastings]

New changeset 937ac1f by larryhastings (Ned Deily) in branch '3.5':
[3.5] bpo-32981: Fix catastrophic backtracking vulns (GH-5955) (bpo-6034)
937ac1f

[larryhastings]

Is this ready to close?

[ned-deily]

Is this ready to close?

The fixes are now available from the cpython repo for all current security and maintenance branches (3.4 to 3.7 plus 2.7). They are now released in 3.6.5rc1 and will be available in the next releases of other branches: 3.7.0, 3.5.6, 3.4.9, and 2.7.15.

Thanks again for reporting the issues, James, and helping to resolve them!