CVE-2021-23401: This affects all versions of package Flask-User

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MEDIUM 5.4MITIGATE

CVE-2021-23401 is a medium-severity flaw (CVSS 5.4) affecting flask-user. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.4 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.1% (top 34%)
CISA exploited list	No
Weakness	CWE-601
Published	2021-07-05
Last modified	2026-06-17

Publisher (CNA): Snyk · Vendor: flask-user project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-07-05 11:15 UTCPublished CVE record published
2026-06-17 03:38 UTCUpdated Record last modified

flask-user project flask-user MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy flask-user || rpm -q flask-user || flask-user --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-23401?

flask-user: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-23401 being exploited?

CVE-2021-23401 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-23401 affect?

CVE-2021-23401 affects flask-user. Severity is medium (CVSS 5.4). 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

How urgent is CVE-2021-23401?

Priority 40/100. Public exploit reference

Software pages

Flask vulnerabilities

References

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