CVE-2021-33503

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.

Published: 2021-06-29 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-33503 is rated Moderate Risk (62.8/100): CVSS High severity, with medium exploitation likelihood (EPSS 3.27%). EPSS rose +2.41% over the last day, indicating growing attacker interest. Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-33503

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.86% 3.27% +2.41%
2 2025-03-30 12.93% 0.86% -12.06%
3 2025-03-29 — 12.93% —

Full EPSS history (15 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-33503

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-33503

GitHub Security Advisory for CVE-2021-33503

GHSA-q2q7-5pp4-w6pg · Severity: high · Ecosystem: pip — Catastrophic backtracking in URL authority parser when passed URL containing many @ characters

OS Trackers for CVE-2021-33503

vendor priority summary link
alpine high CVE-2021-33503: 1 source package rows (py3-urllib3); 10 state rows across 5 repos (3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 0, open 10. https://security.alpinelinux.org/vuln/CVE-2021-33503
debian not yet assigned CVE-2021-33503 not yet assigned priority: Debian including 1 source packages (python-urllib3), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-33503
gentoo normal CVE-2021-33503: 1 GLSA(s) (202107-36), 1 atom(s) (dev-python/urllib3); latest impact normal. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-33503
redhat medium — https://access.redhat.com/security/cve/CVE-2021-33503
suse high CVE-2021-33503 severity important: SUSE including 264 source package names (0.7.1-rev1.0.0-build2.2.1:python3-urllib3-1.25.10-4.3.1, 1.8.6.0.3.2.5:python3-urllib3-1.25.10-4.3.1, …), 622 product×package rows across 355 product lines (Container bci/kiwi, Container ses/7.1/cephcsi/cephcsi, … (355 product lines)): Fixed 439, Known Affected 169, Known Not Affected 14. https://www.suse.com/security/cve/CVE-2021-33503/
ubuntu low CVE-2021-33503 low priority: Ubuntu including 2 source packages (python-pip, python-urllib3), 20 status rows across 10 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, trusty, upstream, xenial): not-affected 12, ignored 4, released 4. https://ubuntu.com/security/CVE-2021-33503

Affected software / configurations for CVE-2021-33503

Vendor Product Version Raw CPE
python urllib3 >= 1.25.4, < 1.26.5 cpe:2.3:a:python:urllib3:*:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
oracle enterprise_manager_ops_center 12.4.0.0 cpe:2.3:a:oracle:enterprise_manager_ops_center:12.4.0.0:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.1 cpe:2.3:a:oracle:instantis_enterprisetrack:17.1:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.2 cpe:2.3:a:oracle:instantis_enterprisetrack:17.2:*:*:*:*:*:*:*
oracle instantis_enterprisetrack 17.3 cpe:2.3:a:oracle:instantis_enterprisetrack:17.3:*:*:*:*:*:*:*
oracle zfs_storage_appliance_kit 8.8 cpe:2.3:a:oracle:zfs_storage_appliance_kit:8.8:*:*:*:*:*:*:*

References for CVE-2021-33503

cvelogic Threat Intelligence