MEDIUM

CVE-2021-3733

Q: What is CVE-2021-3733?

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.

Q: Is CVE-2021-3733 actively exploited?

Active exploitation of CVE-2021-3733 has not been confirmed. Its EPSS score was 4.7% on 2026-10-12, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-3733?

CVE-2021-3733 has a CVSS v3 base score of 6.5 (MEDIUM severity).

CVSS v3

6.5

MEDIUM

EPSS Score

4.7 %

exploit probability, as of 2026-10-12

CISA KEV

known exploited

Exploitation

—

SSVC status

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Affected and fixed versions

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

pythonBitnamiall versions · fixed in 3.6.14and 2 other packages
babelAlmaLinux:8all versions · fixed in 2.5.1-10.module_el8.6.0+2781+fed64c13and 1 other package
python-nose-docsAlmaLinux:8all versions · fixed in 1.3.7-31.module_el8.5.0+2569+5c5719bc
python-sqlalchemy-docAlmaLinux:8all versions · fixed in 1.3.2-2.module_el8.5.0+2569+5c5719bc
python2AlmaLinux:8all versions · fixed in 2.7.18-10.module_el8.6.0+2781+fed64c13.almaand 6 other packages
python2-attrsAlmaLinux:8all versions · fixed in 17.4.0-10.module_el8.6.0+2781+fed64c13
python2-backportsAlmaLinux:8all versions · fixed in 1.0-16.module_el8.6.0+2781+fed64c13
python2-backports-ssl_match_hostnameAlmaLinux:8all versions · fixed in 3.5.0.1-12.module_el8.6.0+2781+fed64c13

104 more package ranges on OSV.dev

Vendor advisories

CVE-2021-3733 Red Hat Product Security948 products fixed · 57 known affected
python: urllib: Regular expression DoS in AbstractBasicAuthHandler
python as a component of Red Hat Enterprise Linux 6 · python-devel as a component of Red Hat Enterprise Linux 6 · python-libs as a component of Red Hat Enterprise Linux 6
ICSA-23-348-10 CISA5 known affected
Siemens SIMATIC S7-1500
SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) · SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) · SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0)
SSA-398330 Siemens ProductCERT5 known affected
SSA-398330: Vulnerabilities in the additional GNU/Linux subsystem of the SIMATIC S7-1500 CPU 1518(F)-4 PN/DP MFP >= V3.1.0 and < V3.1.5
SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) · SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) · SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0)
RHSA-2022:1821 Red Hat Product Security264 products fixed
Red Hat Security Advisory: python27:2.7 security update
RHSA-2022:1764 Red Hat Product Security239 products fixed
Red Hat Security Advisory: python38:3.8 and python38-devel:3.8 security update
RHSA-2022:1663 Red Hat Product Security34 products fixed
Red Hat Security Advisory: python27-python and python27-python-pip security update

3 more advisories

Full record on 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.

Technical details

Published: 2022-03-10

Frequently asked questions

What is CVE-2021-3733?

Is CVE-2021-3733 actively exploited?

Active exploitation of CVE-2021-3733 has not been confirmed. Its EPSS score was 4.7% on 2026-10-12, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-3733?

CVE-2021-3733 has a CVSS v3 base score of 6.5 (MEDIUM severity).

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