# CVE-2021-33503: Catastrophic backtracking in URL authority parser when passed URL containing many @ characters > Attackers can cause denial of service in urllib3 by providing URLs with many @ characters, triggering catastrophic backtracking in the authority parser. - **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 29 Jun 2021 - **Updated:** 3 Aug 2024 - **Also known as:** AZL-6821, DEBIAN-CVE-2021-33503, GHSA-q2q7-5pp4-w6pg, MINI-r83f-v2rq-2cj6, OESA-2021-1260, PYSEC-2021-108, RHSA-2021:3473, ROOT-APP-PYPI-CVE-2021-33503, SUSE-RU-2021:2194-1, SUSE-SU-2021:2012-1, SUSE-SU-2021:2195-1, UBUNTU-CVE-2021-33503, USN-5812-1, openSUSE-SU-2021:2012-1 ## What is CVE-2021-33503? CVE-2021-33503 is a high-severity denial of service vulnerability in Fedoraproject Fedora, published on 29 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description 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. Weakness: CWE-400 (Uncontrolled Resource Consumption) ## Which versions are affected by CVE-2021-33503? 3 open source packages ship an affected version, including urllib3. It affects 5 products. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `urllib3` | PyPI | From 1.25.4 | 1.26.5 | | `urllib3` | PyPI | From 1.25.4 | 1.26.5 | | `urllib3` | PyPI | From 1.25.4 | 1.26.5 | | `automation-hub` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:4.2.6-1.el7pc | | `python-galaxy-ng` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:4.2.6-1.el7pc | | `python-urllib3` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:1.26.5-1.el7pc | | `python3-chardet` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:3.0.4-3.el7pc | | `python3-click` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:7.1.2-3.el7pc | | `python3-galaxy-ng` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:4.2.6-1.el7pc | | `python3-gnupg` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:0.4.6-3.el7pc | | `python3-jinja2` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:2.11.2-3.el7pc | | `python3-markupsafe` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:1.1.1-4.el7pc | | `python3-markupsafe-debuginfo` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:1.1.1-4.el7pc | | `python3-semantic-version` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:2.8.5-3.el7pc | | `python3-urllib3` | Red Hat:ansible_automation_platform:4.2::el7 | All versions | 0:1.26.5-1.el7pc | | `automation-hub` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:4.2.6-1.el8pc | | `python-galaxy-ng` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:4.2.6-1.el8pc | | `python-urllib3` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:1.26.5-1.el8pc | | `python3-click` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:7.1.2-3.el8pc | | `python3-galaxy-ng` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:4.2.6-1.el8pc | | `python3-gnupg` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:0.4.6-3.el8pc | | `python3-jinja2` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:2.11.2-3.el8pc | | `python3-markupsafe` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:1.1.1-4.el8pc | | `python3-markupsafe-debuginfo` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:1.1.1-4.el8pc | | `python3-markupsafe-debugsource` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:1.1.1-4.el8pc | | `python3-semantic-version` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:2.8.5-3.el8pc | | `python3-urllib3` | Red Hat:ansible_automation_platform:4.2::el8 | All versions | 0:1.26.5-1.el8pc | | `aws-cli` | SUSE:HPE Helion OpenStack 8 | All versions | 1.19.9-22.24.2 | | `python-PyYAML` | SUSE:HPE Helion OpenStack 8 | All versions | 5.3.1-28.4.3 | | `python-botocore` | SUSE:HPE Helion OpenStack 8 | All versions | 1.20.9-28.30.2 | | `python-colorama` | SUSE:HPE Helion OpenStack 8 | All versions | 0.4.4-4.4.2 | | `python-pexpect` | SUSE:HPE Helion OpenStack 8 | All versions | 4.3.1-3.9.2 | | `python-ptyprocess` | SUSE:HPE Helion OpenStack 8 | All versions | 0.5.2-2.8.2 | | `python-requests` | SUSE:HPE Helion OpenStack 8 | All versions | 2.24.0-3.3.1 | | `python-six` | SUSE:HPE Helion OpenStack 8 | All versions | 1.14.0-13.10.3 | | `python-urllib3` | SUSE:HPE Helion OpenStack 8 | All versions | 1.25.10-3.29.1 | | `python-urllib3` | SUSE:HPE Helion OpenStack 8 | All versions | 1.25.10-5.19.1 | | `python3-requests` | SUSE:HPE Helion OpenStack 8 | All versions | 2.24.0-8.14.3 | | `python-pexpect` | SUSE:Linux Enterprise High Availability Extension 12 SP3 | All versions | 4.3.1-3.9.2 | | `python-ptyprocess` | SUSE:Linux Enterprise High Availability Extension 12 SP3 | All versions | 0.5.2-2.8.2 | Products: Fedoraproject Fedora, Oracle Enterprise Manager Ops Center, Oracle Instantis Enterprisetrack, Oracle Zfs Storage Appliance Kit, Python Urllib3. ## How do I fix CVE-2021-33503? Upgrade urllib3 to 1.26.5 or later. ## Is CVE-2021-33503 being exploited? There is no known exploitation of CVE-2021-33503 in the wild and no public exploit code. ## Other vulnerabilities in Fedoraproject Fedora - [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild) - [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild) - [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild) - [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild) - [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2021-33503 and fix it if it is affected. What it is: CVE-2021-33503 is a high-severity denial of service vulnerability in Fedoraproject Fedora, published on 29 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2021-33503 in the wild and no public exploit code. Affected packages and fixed versions: - urllib3 (PyPI): affected from 1.25.4; fixed in 1.26.5 - urllib3 (PyPI): affected from 1.25.4; fixed in 1.26.5 - urllib3 (PyPI): affected from 1.25.4; fixed in 1.26.5 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2021-33503/ (Markdown: https://insights.sbomify.com/CVE-2021-33503/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2021-33503 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-33503 - OSV: https://osv.dev/vulnerability/CVE-2021-33503 --- From sbomify Insights: https://insights.sbomify.com/CVE-2021-33503/ Summaries are generated and may contain mistakes; the official description above is the record.