CVE-2021-3733

MEDIUMNVD 6.56.5—
EchelonGraph scoreMEDIUM confidence

Score 6.5 from GitHub Security Advisory published 2022-03-11. NVD baseline CVSS 6.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
6.5
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 5%CVSS: 6.5Exploit: NoneExposed: 0

A fix is available — apply it.

There's a flaw in urllib's AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is sent by the server to the client. The greatest threat that this flaw poses is to application availability.

CVSS v3
6.5
EG Score
6.5(medium)
EPSS
90.8%
KEV
Not listed

Published

March 10, 2022

Last Modified

November 3, 2025

References (17)

Vendor Advisories for CVE-2021-3733(6)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(10)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntupython3.4-venv (3.4.3-1ubuntu1~14.04.7+esm11) @ trusty2026-05-26ubuntu
ubuntulibpython3.6-minimal (3.6.9-1~18.04ubuntu1.6) @ bionic2026-05-26ubuntu
ubuntulibpython3.7 (3.7.5-2ubuntu1~18.04.2) @ bionic2026-05-26ubuntu
ubuntupython3.8-venv (3.8.0-3ubuntu1~18.04.2+esm2) @ bionic2026-05-26ubuntu
redhatpython38-devel:3.8-8060020220120164031.5294be162022-05-10redhat
redhatpython27:2.7-8060020220210185952.8cdc22682022-05-10redhat
redhatpython27-python-0:2.7.18-4.el72022-05-02redhat
redhatpython39-devel:3.9-8050020210811100211.d428a79b2021-11-09redhat
redhatpython3-0:3.6.8-39.el8_42021-11-02redhat
redhatrh-python38-python-urllib3-0:1.25.7-7.el72021-08-24redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Additional Vendor Advisories

(5)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 12× in last 7d / 48× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 104 total refreshes for this CVE.

  1. 2026-07-21 15:22 UTCEPSS rescore
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Frequently asked(5)

What is CVE-2021-3733?
CVE-2021-3733 is a medium vulnerability published on March 10, 2022. There's a flaw in urllib's AbstractBasicAuthHandler class. An attacker who controls a malicious HTTP server that an HTTP client (such as web browser) connects to, could trigger a Regular Expression Denial of Service (ReDOS) during an authentication request with a specially crafted payload that is…
When was CVE-2021-3733 disclosed?
CVE-2021-3733 was first published in the National Vulnerability Database on March 10, 2022, with the most recent update on November 3, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-3733 actively exploited?
CVE-2021-3733 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 90.8% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-3733?
CVE-2021-3733 has a CVSS v3 base score of 6.5 (NVD).
How do I remediate CVE-2021-3733?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-3733, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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