Reference page — cumulative record through Monday, October 19, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.

CVE-2021-3733

CVE-2021-3733MEDIUM

n/a python — There's a flaw in urllib's AbstractBasicAuthHandler class.

  AV  AC  PR  UI  S  C  I  A   CVSS    EPSS   %ile   KEV
   N   L   L   N  U  N  N  H    6.5   .0480   91.8     —

AFFECTED
  Product  Versions                                                                   Fixed
  python   Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5. –  —

TIMELINE
  Aug 23  Reserved by redhat
  Mar 7   Published (CNA: redhat)
  Oct 8   ENRICHED — CVE-2021-3733 (python). Received CVSS 6.5 and CPE data from NVD.
  Oct 8   EXPLOIT PUBLISHED — CVE-2021-3733 (python). Public exploit reference added.
  Oct 14  EXPLOIT PUBLISHED — CVE-2021-3733 (python). Public exploit reference added.

CWE-400 · CNA: redhat · CVSS v3.1 · 10 references · NVD status: Modified

Description

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.

Lifecycle

Complete event history — 5 events, chronological
Date	Event	Detail
August 23, 2021	Reserved	Reserved by redhat
March 7, 2022	Published	Published (CNA: redhat)
October 8, 2026	ENRICHED	ENRICHED — CVE-2021-3733 (python). Received CVSS 6.5 and CPE data from NVD.
October 8, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2021-3733 (python). Public exploit reference added.
October 14, 2026	EXPLOIT PUBLISHED	EXPLOIT PUBLISHED — CVE-2021-3733 (python). Public exploit reference added.

Affected

Affected products and packages — 1 row
Vendor	Product / Package	Ecosystem	Version introduced	Fixed
n/a	python	—	Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.	—

Weaknesses

CWE-400

References (10)

https://bugs.python.org/issue43075 [Exploit, Issue Tracking, Patch, Vendor Advisory]
https://bugzilla.redhat.com/show_bug.cgi?id=1995234 [Issue Tracking, Third Party Advisory]
https://cert-portal.siemens.com/productcert/html/ssa-398330.html
https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb [Patch, Third Party Advisory]
https://github.com/python/cpython/pull/24391 [Patch, Third Party Advisory]
https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html
https://lists.debian.org/debian-lts-announce/2024/12/msg00000.html
https://security.netapp.com/advisory/ntap-20220407-0001/ [Third Party Advisory]
https://ubuntu.com/security/CVE-2021-3733 [Patch, Third Party Advisory]

Authoritative record: CVE-2021-3733 at cve.org

Weaknesses: CWE-400

About this page

This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2021-3733 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Monday, October 19, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.