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CVE-2021-23401

Open Redirect

5.4 Medium

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
Low
Integrity
Low
Availability
None

Exploit likelihood

1.11%Up 0.3 points since Jul 6, 2021
90%50%10%1%

What each source reports

CVE List Score used 5.4

Open Redirect

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.

Record
CVE-2021-23401
CVSS 3.1
5.4 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C
Updated
2026-11-02
EUVDEUVD-2021-0089 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.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.

Record
EUVD-2021-0089
CVSS 3.1
5.4 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C
Updated
2026-11-03
GitHub AdvisoriesGHSA-4298-89hc-6rfv 5.3

Open Redirect in Flask-User

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 backslashes 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.

Record
GHSA-4298-89hc-6rfv
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
CVSS 3.1
5.4 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
Weakness
CWE-601
Updated
2026-11-02
NVD 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.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.

Record
CVE-2021-23401
CVSS 3.1
5.4 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
CVSS 2.0
5.8 MediumAV:N/AC:M/Au:N/C:P/I:P/A:N
Weakness
CWE-601
Updated
2026-11-02
OSVGHSA-4298-89hc-6rfv 5.3

Open Redirect in Flask-User

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 backslashes 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.

Record
GHSA-4298-89hc-6rfv
CVSS 4.0
5.3 MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
CVSS 3.1
5.4 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
Weakness
CWE-601
Updated
2026-11-02
OSVPYSEC-2021-337 No score

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.

Record
PYSEC-2021-337
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
flask-user
PyPI
 through 1.0.2.2 None known
flask-user
PyPI
 through 1.0.2.2, plus listed versions
0.30.3.10.3.20.3.30.3.4
Show 45 moreShow less
0.3.50.3.60.3.70.3.80.4.00.4.10.4.20.4.30.4.40.4.50.4.60.4.70.4.80.4.90.5.00.5.10.5.20.5.30.5.40.5.50.60.6.10.6.100.6.120.6.130.6.140.6.150.6.160.6.170.6.190.6.20.6.200.6.210.6.30.6.40.6.50.6.60.6.70.6.80.6.91.0.1.11.0.1.21.0.1.31.0.1.41.0.1.5
 None known
flask-user
PyPI
 All versions, plus listed versions
0.30.3.10.3.20.3.30.3.4
Show 45 moreShow less
0.3.50.3.60.3.70.3.80.4.00.4.10.4.20.4.30.4.40.4.50.4.60.4.70.4.80.4.90.5.00.5.10.5.20.5.30.5.40.5.50.60.6.10.6.100.6.120.6.130.6.140.6.150.6.160.6.170.6.190.6.20.6.200.6.210.6.30.6.40.6.50.6.60.6.70.6.80.6.91.0.1.11.0.1.21.0.1.31.0.1.41.0.1.5
 None known

History

History
RecordedChangedSources
facts
aliases, publishedEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-23401 is a medium vulnerability (CVSS 5.4) in Flask-user Project Flask-user, published July 5, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.11%, the 66th percentile. No source names a fixed version.

References

This page doesn't list everything

Left out here: 5 versions. The JSON record has them.

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