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Fencer Vulns
high 7.5

CVE-2021-33503 json

Catastrophic backtracking in URL authority parser when passed URL containing many @ characters

published 2021-06-01 5 y ago modified 2026-10-13 26 d ago also GHSA-q2q7-5pp4-w6pg , PYSEC-2021-108 , EUVD-2021-0456

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
3.3%
EPSS 3.3% chance of exploitation in 30 days, percentile 88%
public exploits
0
none known

Summary

An issue was discovered in urllib3 before 1.26.5. When provided with a URL containing many @ characters in the authority component, the authority regular expression exhibits catastrophic backtracking, causing a denial of service if a URL were passed as a parameter or redirected to via an HTTP redirect.

Patch / fix

  • pypi urllib3 1.26.5
  • python urllib3 1.26.5
  • python-urllib3 Debian 14, Debian 13, Debian 12 1.26.5-1~exp1

debian advisories DEBIAN-CVE-2021-33503

  • python-pip Ubuntu 20.04 LTS 20.0.2-5ubuntu1.7
  • python-urllib3 Ubuntu 20.04 LTS 1.25.8-2ubuntu0.2

ubuntu advisories UBUNTU-CVE-2021-33503

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • pytz RHEL 8 2019.3-3.module+el8.4.0+8888+89bc7e79
  • babel RHEL 8 2.7.0-11.module+el8.5.0+11015+9c1c7c42
  • numpy RHEL 8 1.17.3-6.module+el8.5.0+12205+a865257a
  • scipy RHEL 8 1.3.1-4.module+el8.4.0+8888+89bc7e79
  • Cython RHEL 8 0.29.14-4.module+el8.4.0+8888+89bc7e79
  • pytest RHEL 8 4.6.6-3.module+el8.4.0+8888+89bc7e79
  • PyYAML RHEL 8 5.4.1-1.module+el8.5.0+10721+14d8e0d5
  • mod_wsgi RHEL 8 4.6.8-3.module+el8.4.0+8888+89bc7e79

red hat advisories RHSA-2021:4162RHSA-2021:3254RHSA-2021:4702RHSA-2021:4160RHSA-2021:3473

  • python-requests OpenStack Cloud Crowbar 8, OpenStack Cloud 8, HPE Helion OpenStack 8 2.24.0-3.3.1
  • python-urllib3 OpenStack Cloud Crowbar 8, OpenStack Cloud 8, HPE Helion OpenStack 8 1.25.10-5.19.1
  • aws-cli SLE Module for Public Cloud 12, OpenStack Cloud Crowbar 8, OpenStack Cloud 8 +1 more 1.19.9-22.24.2
  • azure-cli SLE Module for Public Cloud 12 2.14.2-2.10.2
  • libsodium SLE Module for Web and Scripting 12, SLE Module for Public Cloud 12 1.0.16-1.7.1
  • python-scp SLE Module for Public Cloud 12 0.13.2-2.8.2
  • python-six SLES for SAP 12 SP3, SLES 12 SP3-LTSS, SLES 12 SP3-BCL +5 more 1.14.0-13.10.3
  • python-adal SLE Module for Public Cloud 12 1.2.4-5.10.2

suse advisories SUSE-SU-2021:2195-1SUSE-RU-2021:2194-1SUSE-SU-2021:2012-1

  • python-urllib3 openSUSE Tumbleweed 2.8.0-1.1
  • python-urllib3_1 openSUSE Tumbleweed 1.26.20-7.2
  • python-urllib3 openSUSE Tumbleweed 2.1.0-3.1
  • python-urllib3 openSUSE Tumbleweed 1.26.6-1.2
  • python-urllib3 openSUSE Leap 15.3 1.25.10-4.3.1

opensuse advisories openSUSE-SU-2026:12044-1openSUSE-SU-2026:12045-1openSUSE-SU-2024:14055-1openSUSE-SU-2024:11277-1openSUSE-SU-2021:2012-1

  • python38-py AlmaLinux 8 1.8.0-8.module_el8.6.0+2778+cd494b30
  • python38-ply AlmaLinux 8 3.11-10.module_el8.6.0+2778+cd494b30
  • python38-six AlmaLinux 8 1.12.0-10.module_el8.6.0+2778+cd494b30
  • python38-cffi AlmaLinux 8 1.13.2-3.module_el8.6.0+2778+cd494b30
  • python38-idna AlmaLinux 8 2.8-6.module_el8.6.0+2778+cd494b30
  • python38-pytz AlmaLinux 8 2019.3-3.module_el8.6.0+2778+cd494b30
  • python38-attrs AlmaLinux 8 19.3.0-3.module_el8.6.0+2778+cd494b30
  • python38-babel AlmaLinux 8 2.7.0-11.module_el8.6.0+2778+cd494b30

almalinux advisories ALSA-2021:4162ALSA-2021:4160

  • pytz Rocky Linux 8 2019.3-3.module+el8.4.0+570+c2eaf144
  • babel Rocky Linux 8 2.7.0-11.module+el8.5.0+672+ab6eb015
  • numpy Rocky Linux 8 1.17.3-6.module+el8.5.0+672+ab6eb015
  • scipy Rocky Linux 8 1.3.1-4.module+el8.5.0+672+ab6eb015
  • scipy Rocky Linux 8 1.3.1-4.module+el8.4.0+570+c2eaf144
  • Cython Rocky Linux 8 0.29.14-4.module+el8.4.0+570+c2eaf144
  • pytest Rocky Linux 8 4.6.6-3.module+el8.4.0+570+c2eaf144
  • PyYAML Rocky Linux 8 5.4.1-1.module+el8.5.0+672+ab6eb015

rocky linux advisories RLSA-2021:4162RLSA-2021:4160

  • python-urllib3 CBL-Mariner 2.0 1.25.9-3

azure linux advisories AZL-6821

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

  • python-urllib3 Arch Linux 1.26.5-1

arch linux advisories AVG-2038

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-33503 updated cvss 7.5

python-urllib3: ReDoS in the parsing of authority part of URL

  • Red Hat OpenShift Container Platform 4 python-urllib3, python2-urllib3 Red Hat OpenStack Platform 10 (Newton) python-urllib3
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Quay 3 quay/quay-rhel8
    affected: workaround, no fix available
  • Red Hat Automation Hub 4.2 for RHEL 7 python3-click 7.1.2-3.el7pc, python3-gnupg 0.4.6-3.el7pc, automation-hub 4.2.6-1.el7pc, python-urllib3 1.26.5-1.el7pc, python3-chardet 3.0.4-3.el7pc, python3-jinja2 2.11.2-3.el7pc, +5 more Red Hat Automation Hub 4.2 for RHEL 8 python3-click 7.1.2-3.el8pc, python3-gnupg 0.4.6-3.el8pc, automation-hub 4.2.6-1.el8pc, python-urllib3 1.26.5-1.el8pc, python3-jinja2 2.11.2-3.el8pc, python-galaxy-ng 4.2.6-1.el8pc, +4 more
    fixed: vendor fix RHSA-2021:3473
  • Red Hat CodeReady Linux Builder (v. 8) pytest 4.6.6-3.module+el8.4.0+8888+89bc7e79, pytest 6.0.2-2.module+el8.4.0+9822+20bf1249, Cython 0.29.21-5.module+el8.4.0+9822+20bf1249, pybind11 2.6.1-2.module+el8.4.0+9822+20bf1249, python-py 1.8.0-8.module+el8.4.0+8888+89bc7e79, python-py 1.10.0-1.module+el8.4.0+9822+20bf1249, +34 more Red Hat Enterprise Linux AppStream (v. 8) pytz 2019.3-3.module+el8.4.0+8888+89bc7e79, scipy 1.3.1-4.module+el8.4.0+8888+89bc7e79, scipy 1.5.4-3.module+el8.4.0+9822+20bf1249, babel 2.7.0-11.module+el8.5.0+11015+9c1c7c42, numpy 1.17.3-6.module+el8.5.0+12205+a865257a, numpy 1.19.4-3.module+el8.5.0+12204+54860423, +114 more
    fixed: vendor fix RHSA-2021:4162
  • Red Hat Satellite 6.10 python-urllib3 1.26.5-1.el7pc, python3-urllib3 1.26.5-1.el7pc Red Hat Satellite Capsule 6.10 python-urllib3 1.26.5-1.el7pc, python3-urllib3 1.26.5-1.el7pc
    fixed: vendor fix RHSA-2021:4702
  • Red Hat Software Collections for Red Hat Enterprise Linux Server (v. 7) rh-python38-babel 2.7.0-12.el7, rh-python38-python 3.8.11-2.el7, rh-python38-python-lxml 4.4.1-7.el7, rh-python38-python-pip 19.3.1-2.el7, rh-python38-python-idle 3.8.11-2.el7, rh-python38-python-libs 3.8.11-2.el7, +9 more Red Hat Software Collections for Red Hat Enterprise Linux Server EUS (v. 7.7) rh-python38-babel 2.7.0-12.el7, rh-python38-python 3.8.11-2.el7, rh-python38-python-lxml 4.4.1-7.el7, rh-python38-python-pip 19.3.1-2.el7, rh-python38-python-idle 3.8.11-2.el7, rh-python38-python-libs 3.8.11-2.el7, +9 more Red Hat Software Collections for Red Hat Enterprise Linux Workstation (v. 7) rh-python38-babel 2.7.0-12.el7, rh-python38-python 3.8.11-2.el7, rh-python38-python-lxml 4.4.1-7.el7, rh-python38-python-pip 19.3.1-2.el7, rh-python38-python-idle 3.8.11-2.el7, rh-python38-python-libs 3.8.11-2.el7, +9 more
    fixed: vendor fix RHSA-2021:3254
  • Red Hat Automation Hub 4.2 for RHEL 7 python-requests 2.25.1-1.el7pc, python3-requests 2.25.1-1.el7pc Red Hat Automation Hub 4.2 for RHEL 8 python-requests 2.25.1-1.el8pc, python3-requests 2.25.1-1.el8pc Red Hat Enterprise Linux 6 python-urllib3 Red Hat Enterprise Linux 7 python-urllib3 Red Hat Enterprise Linux 8 python-pip, python2-pip, python3-pip, python-urllib3, python3-urllib3, python2-pip-wheel, +2 more Red Hat Enterprise Linux 9 python-urllib3, python3-urllib3
    +5 more products
    Red Hat OpenShift Container Platform 3.11 python-urllib3, python2-urllib3 Red Hat OpenStack Platform 13 (Queens) python2-urllib3 Red Hat Satellite 6.10 tfm 7.0-1.el7sat, gofer 2.12.5-7.el7sat, katello 4.1.1-2.el7sat, libsolv 0.7.20-1.el7pc, receptor 0.6.4-2.el7ar, libcomps 0.1.15-1.el7pc, +580 more Red Hat Satellite Capsule 6.10 tfm 7.0-1.el7sat, katello 4.1.1-2.el7sat, libsolv 0.7.20-1.el7pc, libcomps 0.1.15-1.el7pc, python-rq 1.8.1-1.el7pc, python-idna 2.10-1.el7pc, +353 more Red Hat Storage 3 python-urllib3, python2-urllib3
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

3.27% chance of exploitation in the next 30 days, higher than 88% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
pypi urllib3
>=1.25.4, <1.26.5
1.26.5

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.