CVE-2021-23401

Name: CVE-2021-23401
Creator: NVD / NIST
Published: 2021-07-05T11:15:07.67+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 1.11%

Last modified Jun 17, 2026

CVE-2021-23401 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. 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. EPSS estimates a 1.11% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.1/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS Probability

1.11%

61.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-601

Affected Software

Vendor	Product	Versions
Flask-User Project	Flask-User	All versions

References

https://github.com/lingthio/Flask-UserProduct, Third Party Advisory
https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.pyExploit, Third Party Advisory
https://snyk.io/vuln/SNYK-PYTHON-FLASKUSER-1293188Third Party Advisory
https://github.com/lingthio/Flask-UserProduct, Third Party Advisory
https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.pyExploit, Third Party Advisory
https://snyk.io/vuln/SNYK-PYTHON-FLASKUSER-1293188Third Party Advisory

Timeline

Published: Jul 5, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23401?

How severe is CVE-2021-23401?

CVE-2021-23401 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 1.11% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23401?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-23401?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST