CVE-2021-3733 Denial of service in python

Medium 6.5 Published 7 Mar 2022 Updated 14 Oct 2026

A malicious HTTP server can cause a Regular Expression Denial of Service in Python's urllib authentication handler, impacting application availability.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2021-3733?

CVE-2021-3733 is a medium-severity denial of service vulnerability in Fedoraproject Extra Packages for Enterprise Linux, published on 7 Mar 2022. Any signed-in user can exploit it over the network, with no action from the victim.

Official 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.

Weakness: CWE-400

Which versions are affected by CVE-2021-3733?

3 open source packages ship an affected version, including libpython, python and python-min. It affects 20 products.

Package	Affected	Fixed in
`libpython` Bitnami	All versions	`3.6.14, 3.7.11, 3.8.10, 3.9.5`
`python` Bitnami	All versions	`3.6.14, 3.7.11, 3.8.10, 3.9.5`
`python-min` Bitnami	All versions	`3.6.14, 3.7.11, 3.8.10, 3.9.5, 3.10.1`

Linux distribution packages (14)

`platform-python` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`platform-python-debug` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`platform-python-devel` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`python3-debuginfo` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`python3-debugsource` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`python3-idle` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`python3-test` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`python3-tkinter` Red Hat:enterprise_linux:8::appstream	All versions	`0:3.6.8-39.el8_4`
`platform-python` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`
`python3` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`
`python3-debuginfo` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`
`python3-debugsource` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`
`python3-libs` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`
`python3-test` Red Hat:enterprise_linux:8::baseos	All versions	`0:3.6.8-39.el8_4`

Products include Fedoraproject Extra Packages for Enterprise Linux, Fedoraproject Fedora, Netapp Hci Compute Node Firmware, Netapp Management Services for Element Software and Netapp Hci, Netapp Ontap Select Deploy Administration Utility, Netapp Solidfire\, Enterprise Sds \& Hci Storage Node and 14 more.

How do I fix CVE-2021-3733?

Upgrade libpython to a fixed version for your release line: 3.6.14, 3.7.11, 3.8.10 or 3.9.5.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2021-3733 being exploited?

There is no known exploitation of CVE-2021-3733 in the wild and no public exploit code.

All Fedoraproject Extra Packages for Enterprise Linux vulnerabilities

Sources

Also known as AZL-109638, BIT-libpython-2021-3733, BIT-python-2021-3733, BIT-python-min-2021-3733, CLSA-2024-1722527236, CLSA-2024-1723146030 and 6 more.