--- title: "CVE-2021-3733\n | Ubuntu" description: Ubuntu is an open source software operating system that runs from the desktop, to the cloud, to all your internet connected things. url: https://ubuntu.com/security/CVE-2021-3733?format=md keywords: index, follow --- # CVE-2021-3733 Publication date 2 September 2021 Last updated 25 August 2025 --- Ubuntu priority **Medium** [Why this priority?](https://ubuntu.com/security/cves/about#priority ) ## Cvss 3 Severity Score **6.5 · Medium** [Score breakdown](https://ubuntu.com/security/CVE-2021-3733?format=md#impact-score) Toggle side navigation ## 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. [Read the notes from the security team](https://ubuntu.com/security/CVE-2021-3733?format=md#notes) ## Status Show unmaintained releases | Package | Ubuntu Release | Status | | --- | --- | --- | | python3.10 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not affected | | 22.04 LTS jammy | Not affected | | 21.10 impish | Not affected | | 21.04 hirsute | Fixed 3.10.0~b1-3~21.04 | | 20.04 LTS focal | Not in release | | 18.04 LTS bionic | Not in release | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Not in release | | python3.9 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not affected | | 21.04 hirsute | Not affected | | 20.04 LTS focal | Fixed 3.9.5-3~20.04.1 | | 18.04 LTS bionic | Not in release | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Not in release | | python3.8 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not in release | | 21.04 hirsute | Not in release | | 20.04 LTS focal | Fixed 3.8.10-0ubuntu1~20.04 | | 18.04 LTS bionic | Fixed 3.8.0-3ubuntu1~18.04.2 | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Not in release | | python3.7 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not in release | | 21.04 hirsute | Not in release | | 20.04 LTS focal | Not in release | | 18.04 LTS bionic | Fixed 3.7.5-2ubuntu1~18.04.2 | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Not in release | | python3.6 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not in release | | 21.04 hirsute | Not in release | | 20.04 LTS focal | Not in release | | 18.04 LTS bionic | Fixed 3.6.9-1~18.04ubuntu1.6 | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Not in release | | python3.4 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not in release | | 20.04 LTS focal | Not in release | | 18.04 LTS bionic | Not in release | | 16.04 LTS xenial | Not in release | | 14.04 LTS trusty | Fixed 3.4.3-1ubuntu1~14.04.7+esm11 Ubuntu Pro | | python3.5 | 24.10 oracular | Not in release | | 24.04 LTS noble | Not in release | | 23.10 mantic | Not in release | | 23.04 lunar | Not in release | | 22.10 kinetic | Not in release | | 22.04 LTS jammy | Not in release | | 21.10 impish | Not in release | | 21.04 hirsute | Not in release | | 20.04 LTS focal | Not in release | | 18.04 LTS bionic | Not in release | | 16.04 LTS xenial | Fixed 3.5.2-2ubuntu0~16.04.13+esm1 Ubuntu Pro | | 14.04 LTS trusty | Fixed 3.5.2-2ubuntu0~16.04.4~14.04.1+esm1 Ubuntu Pro | --- * [How can I get the fixes?](https://ubuntu.com/security/cves/about#security) * [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses) ### Get expanded security coverage with Ubuntu Pro Reduce your average CVE exposure time from 98 days to 1 day with expanded CVE patching, ten-years security maintenance and optional support for the full stack of open-source applications. 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[Get Ubuntu Pro](https://ubuntu.com/pro) [30-day free trial](https://ubuntu.com/pro/free-trial) ## Notes --- ### [leosilva](https://launchpad.net/~leosilva) code affected in hirsute and devel is already patched, so both releases in python3.9 are not affected. ## Severity score breakdown CVSS version: CVSS v3.0 **Base score** 6.5 · Medium * Base metrics | Parameter | Value | | --- | --- | | Attack vector | Network | | Attack complexity | Low | | Privileges required | Low | | User interaction | None | | Scope | Unchanged | | Confidentiality impact | None | | Integrity impact | None | | Availability impact | High | * Scores | Parameter | Value | | --- | --- | | Base score | 6.5 · Medium | | Exploitability score | - | | Impact score | - | **Vector:** CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H ## References * [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3733) * [NVD](https://nvd.nist.gov/vuln/detail/CVE-2021-3733) * [Launchpad](https://launchpad.net/bugs/cve/CVE-2021-3733) * [Debian](https://security-tracker.debian.org/tracker/CVE-2021-3733) ### Related Ubuntu Security Notices (USN) + [USN-5083-1](https://usn.ubuntu.com/USN-5083-1) + Python vulnerabilities + 16 September 2021 + [USN-5199-1](https://usn.ubuntu.com/USN-5199-1) + Python vulnerabilities + 17 December 2021 + [USN-5200-1](https://usn.ubuntu.com/USN-5200-1) + Python vulnerabilities + 17 December 2021 + [USN-6891-1](https://usn.ubuntu.com/USN-6891-1) + Python vulnerabilities + 11 July 2024 ### Other references * * * * * * * *