CVE-2021-23401 json
Open Redirect
published 2021-07-05 5 y ago modified 2026-06-17 4 mo ago also GHSA-4298-89hc-6rfv , PYSEC-2021-337 , EUVD-2021-0089
- cvss
- 5.4
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 1.1%
- EPSS 1.1% chance of exploitation in 30 days, percentile 66%
- public exploits
- 0
- none known
Summary
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.
Exploit prediction
1.11% chance of exploitation in the next 30 days, higher than 66% of all scored CVEs (FIRST EPSS, 2026-11-08).
Exploits and checks
- github.com https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.py referenced PoC, tagged "exploit" by a source, unverified
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 5.4 | 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 |
| CNA | 3.1 | 5.4 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N |
| NVD | 3.1 | 6.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N |
| NVD | 2.0 | 5.8 | AV:N/AC:M/Au:N/C:P/I:P/A:N |
| GitHub | 3.1 | 5.4 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | flask-user | <=1.0.2.2 all versions |
no fix listed |
Affected products
- n/a Flask-User
Classification
- Weakness
-
- CWE-601 URL Redirection to Untrusted Site
References
-
github.com
/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.py
web, exploit, third-party-advisory
-
snyk.io
/vuln/SNYK-PYTHON-FLASKUSER-1293188
web, third-party-advisory, advisory
-
github.com
/lingthio/Flask-User
package, product, third-party-advisory, web
-
nvd.nist.gov
/vuln/detail/CVE-2021-23401
advisory
-
github.com
/advisories/GHSA-4298-89hc-6rfv
advisory
-
github.com
/pypa/advisory-database/tree/main/vulns/flask-user/PYSEC-2021-337.yaml
web
Sources
- CVE List CVE-2021-23401
- GitHub Advisory GHSA-4298-89hc-6rfv
- NVD CVE-2021-23401
- OSV PYSEC-2021-337
Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.