CVE-2021-23401: Flask User Open Redirect Vulnerability

MODERATE
Advisory Identifier: GHSA-4298-89hc-6rfv
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CVSS 5.4
CWE-601
Published
Advisory Summary

CVE-2021-23401 is a MODERATE (CVSS 5.4) open redirect vulnerability (CWE-601: URL Redirection to Untrusted Site ('Open Redirect')) affecting Flask User libraries in the PyPI ecosystem. The issue involves open redirect in flask-user. Remediation is available in the latest vendor maintenance releases.

CVE-2021-23401 at a Glance

CVE Identifier CVE-2021-23401
GHSA Identifier GHSA-4298-89hc-6rfv
Severity MODERATE
CVSS v3.1 Base Score 5.4
CWE Classification CWE-601: URL Redirection to Untrusted Site ('Open Redirect')
Affected Product Flask User
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.4

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2021-23401

To remediate this vulnerability, upgrade all affected Flask User dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Flask-User fixed: Latest secure release 
pip install Flask-User>=Latest secure release
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show Flask-User

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-23401, inspect your installed packages using pip:

pip show Flask-User

Analyze the terminal output to locate any version of Flask User in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than the patched release), your application is affected.

Note: The pip show Flask-User command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-601: URL Redirection to Untrusted Site ('Open Redirect'), where the web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect. In Flask User, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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`.

CVE-2021-23401 Affected Flask User Versions

1 component(s)
Product / Package Ecosystem Affected Versions Fixed Version
Flask-User
pypi/Flask-User
PyPI
≤ 1.0.2.2

References

NVD — CVE-2021-23401 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-23401
GitHub Advisory / Wiki ADVISORY
https://github.com/advisories/GHSA-4298-89hc-6rfv
Flask User Source Repository REPOSITORY
https://github.com/lingthio/Flask-User
Flask User Source Repository REPOSITORY
https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.py
Flask User Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/flask-user/PYSEC-2021-337.yaml
snyk.io/vuln/snyk-python-fl... WEB
https://snyk.io/vuln/SNYK-PYTHON-FLASKUSER-1293188

Frequently Asked Questions

What is CVE-2021-23401?
CVE-2021-23401 is a security vulnerability in Flask User involving Open Redirect in Flask-User under CWE-601: URL Redirection to Untrusted Site ('Open Redirect').
Which Flask User versions are affected?
Affected versions include releases prior to current release patches across affected packages: Flask-User.
What version fixes CVE-2021-23401?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
Is Flask-User affected?
Yes, Flask-User is affected in releases prior to Latest secure release. Upgrade your dependency to Latest secure release or later.
What is the CVSS score of CVE-2021-23401?
CVE-2021-23401 has a CVSS v3.1 base score of 5.4 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N.
What CWE is associated with CVE-2021-23401?
CVE-2021-23401 is classified under CWE-601: URL Redirection to Untrusted Site ('Open Redirect') (The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.).
How can I check PyPI dependencies for CVE-2021-23401?
Run the dependency tree command: 'pip show Flask-User'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4298-89hc-6rfv",
  "modified" : "2024-09-20T17:59:20Z",
  "published" : "2021-08-09T20:44:32Z",
  "aliases" : [ "CVE-2021-23401" ],
  "summary" : "Open Redirect in Flask-User",
  "details" : "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`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "Flask-User"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.0.2.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-23401"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-4298-89hc-6rfv"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lingthio/Flask-User"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lingthio/Flask-User/blob/master/flask_user/user_manager__utils.py"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/flask-user/PYSEC-2021-337.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://snyk.io/vuln/SNYK-PYTHON-FLASKUSER-1293188"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-601" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-07-06T15:45:36Z",
    "nvd_published_at" : null
  }
}