CVE-2021-23393

Name: CVE-2021-23393
Creator: NVD / NIST
Published: 2021-06-11T00:15:09.273+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 5.4/10EPSS 0.72%

Last modified Jun 17, 2026

CVE-2021-23393 is a medium-severity vulnerability rated 5.4/10 on the CVSS scale. This affects the package Flask-Unchained before 0.9.0. When using the the _validate_redirect_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. EPSS estimates a 0.72% chance of exploitation in the next 30 days.

Description

This affects the package Flask-Unchained before 0.9.0. When using the the _validate_redirect_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. 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

5.4/10

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

EPSS Probability

0.72%

49.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-601

Affected Software

Vendor	Product	Versions
Flask Unchained Project	Flask Unchained	< 0.9.0

References

https://github.com/briancappello/flask-unchained/commit/71e36b28166f9ffbe0a991f51127f0984f7e6a40Patch, Third Party Advisory
https://snyk.io/vuln/SNYK-PYTHON-FLASKUNCHAINED-1293189Patch, Third Party Advisory
https://github.com/briancappello/flask-unchained/commit/71e36b28166f9ffbe0a991f51127f0984f7e6a40Patch, Third Party Advisory
https://snyk.io/vuln/SNYK-PYTHON-FLASKUNCHAINED-1293189Patch, Third Party Advisory

Timeline

Published: Jun 11, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-23393?

How severe is CVE-2021-23393?

CVE-2021-23393 has a CVSS score of 5.4/10 (MEDIUM severity). The EPSS model estimates a 0.72% probability of exploitation in the next 30 days.

How do I fix CVE-2021-23393?

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-2021-23393?

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

Scan your code now

Source: NVD / NIST