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Vulnerability record · CVE-2021-3733 · published 10 March 2022

CVE-2021-3733: Python uncontrolled resource consumption vulnerability

Python · Python

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.

6.5 CVSS 3.1 Medium EPSS 4.7% · top 8.5% CWE-400 · Uncontrolled resource consumption
6.5CVSS 3.1 base score, v2 4.0
4.7%EPSS exploitation probability, 30 days
NoNot in CISA KEV
20Affected product versions listed by NVD
17References, 2 tagged exploit
17 Jun 2026Last modified by NVD

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.

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

Affected products

20 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bugs.python.org/issue43075 ExploitIssue TrackingPatchVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1995234 Issue TrackingThird Party Advisory
https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb PatchThird Party Advisory
https://github.com/python/cpython/pull/24391 PatchThird 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://security.netapp.com/advisory/ntap-20220407-0001/ Third Party Advisory
https://ubuntu.com/security/CVE-2021-3733 PatchThird Party Advisory
https://bugs.python.org/issue43075 ExploitIssue TrackingPatchVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1995234 Issue TrackingThird Party Advisory
https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb PatchThird Party Advisory
https://github.com/python/cpython/pull/24391 PatchThird 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 PatchThird Party Advisory

Track CVE-2021-3733 inside VULONE

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Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2021-3733), CISA KEV, FIRST EPSS (scores of 2026-09-29). This page is refreshed as NVD updates the record.