CVE-2021-33503

Name: CVE-2021-33503
Creator: NVD / NIST
Published: 2021-06-29T11:15:07.847+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 3.27%

Last modified Jun 17, 2026

CVE-2021-33503 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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.. EPSS estimates a 3.27% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

3.27%

86.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-400

Affected Software

Vendor	Product	Versions
Python	Urllib3	>= 1.25.4, < 1.26.5
Fedoraproject	Fedora	33
Fedoraproject	Fedora	34
Oracle	Enterprise Manager Ops Center	12.4.0.0
Oracle	Instantis Enterprisetrack	17.1
Oracle	Instantis Enterprisetrack	17.2
Oracle	Instantis Enterprisetrack	17.3
Oracle	Zfs Storage Appliance Kit	8.8

References

https://github.com/advisories/GHSA-q2q7-5pp4-w6pgThird Party Advisory
https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ecPatch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73/
https://security.gentoo.org/glsa/202107-36Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory
https://github.com/advisories/GHSA-q2q7-5pp4-w6pgThird Party Advisory
https://github.com/urllib3/urllib3/commit/2d4a3fee6de2fa45eb82169361918f759269b4ecPatch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/6SCV7ZNAHS3E6PBFLJGENCDRDRWRZZ6W/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FMUGWEAUYGGHTPPXT6YBD53WYXQGVV73/
https://security.gentoo.org/glsa/202107-36Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory

Timeline

Published: Jun 29, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-33503?

How severe is CVE-2021-33503?

CVE-2021-33503 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 3.27% probability of exploitation in the next 30 days.

How do I fix CVE-2021-33503?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-33503?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST