CVE-2021-33503: Urllib3 Catastrophic Backtracking Vulnerability
HIGHCVE-2021-33503 is a HIGH (CVSS 7.5) catastrophic backtracking vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Urllib3 libraries in the PyPI ecosystem. The issue involves catastrophic backtracking in url authority parser when passed url containing many @ characters. Remediation is available by upgrading affected packages to version 1.26.5.
CVE-2021-33503 at a Glance
Security Impact
CVSS: 7.5
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).
How to Fix CVE-2021-33503
To remediate this vulnerability, upgrade all affected Urllib3 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
pip install urllib3>=1.26.5Run the following command in your project directory to verify that all resolved dependencies use the patched release:
pip show urllib3How to Check if You Are Affected
To determine whether your Python environment is affected by CVE-2021-33503, inspect your installed packages using pip:
pip show urllib3Analyze the terminal output to locate any version of Urllib3 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.26.5), your application is affected.
Note: The pip show urllib3 command checks currently installed packages in your active Python environment.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Urllib3, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
CVE-2021-33503 Affected Urllib3 Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
urllib3
|
PyPI |
≥ 1.25.4
< 1.26.5
|
1.26.5
|
References
Frequently Asked Questions
What is CVE-2021-33503?
Which Urllib3 versions are affected?
What version fixes CVE-2021-33503?
Is urllib3 affected?
What is the CVSS score of CVE-2021-33503?
What CWE is associated with CVE-2021-33503?
How can I check PyPI dependencies for CVE-2021-33503?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-q2q7-5pp4-w6pg",
"modified" : "2026-06-09T13:01:45Z",
"published" : "2021-06-01T21:19:32Z",
"aliases" : [ "CVE-2021-33503" ],
"summary" : "Catastrophic backtracking in URL authority parser when passed URL containing many @ characters",
"details" : "### Impact\n\nWhen 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.\n\n\n### Patches\n\nThe issue has been fixed in urllib3 v1.26.5.\n\n### References\n\n- [CVE-2021-33503](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-33503)\n- [JVNVU#92413403 (English)](https://jvn.jp/en/vu/JVNVU92413403/)\n- [JVNVU#92413403 (Japanese)](https://jvn.jp/vu/JVNVU92413403/)\n- [urllib3 v1.26.5](https://github.com/urllib3/urllib3/releases/tag/1.26.5)\n\n### For more information\nIf you have any questions or comments about this advisory:\n* Ask in our [community Discord](https://discord.gg/urllib3)\n* Email [sethmichaellarson@gmail.com](mailto:sethmichaellarson@gmail.com)",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
}, {
"type" : "CVSS_V4",
"score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "PyPI",
"name" : "urllib3"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "1.25.4"
}, {
"fixed" : "1.26.5"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/urllib3/urllib3/security/advisories/GHSA-q2q7-5pp4-w6pg"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-33503"
}, {
"type" : "WEB",
"url" : "https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ec"
}, {
"type" : "WEB",
"url" : "https://github.com/urllib3/urllib3/commit/5b047b645f5f93900d5e2fc31230848c25eb1f5f#diff-52026d639119bf1e0364836b4e8a18bd9ed3c95c6ba39b26534a5057a65e35bbR65"
}, {
"type" : "ADVISORY",
"url" : "https://github.com/advisories/GHSA-q2q7-5pp4-w6pg"
}, {
"type" : "WEB",
"url" : "https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2021-108.yaml"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/urllib3/urllib3"
}, {
"type" : "WEB",
"url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W"
}, {
"type" : "WEB",
"url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73"
}, {
"type" : "WEB",
"url" : "https://security.gentoo.org/glsa/202107-36"
}, {
"type" : "WEB",
"url" : "https://www.oracle.com/security-alerts/cpuoct2021.html"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-400" ],
"severity" : "HIGH",
"github_reviewed" : true,
"github_reviewed_at" : "2021-05-28T22:09:12Z",
"nvd_published_at" : "2021-06-29T11:15:00Z"
}
}