CVE-2021-23393: Flask Unchained Open Redirect Vulnerability

MODERATE
Advisory Identifier: GHSA-pjc4-3w99-j7v4
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CVSS 5.4
CWE-601
Published
Advisory Summary

CVE-2021-23393 is a MODERATE (CVSS 5.4) open redirect vulnerability (CWE-601: URL Redirection to Untrusted Site ('Open Redirect')) affecting Flask Unchained libraries in the PyPI ecosystem. The issue involves open redirect in flask-unchained. Remediation is available by upgrading affected packages to version 0.9.0.

CVE-2021-23393 at a Glance

CVE Identifier CVE-2021-23393
GHSA Identifier GHSA-pjc4-3w99-j7v4
Severity MODERATE
CVSS v3.1 Base Score 5.4
CWE Classification CWE-601: URL Redirection to Untrusted Site ('Open Redirect')
Affected Product Flask Unchained
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-2021-23393

To remediate this vulnerability, upgrade all affected Flask Unchained 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-Unchained fixed: 0.9.0 
pip install Flask-Unchained>=0.9.0
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-Unchained

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-23393, inspect your installed packages using pip:

pip show Flask-Unchained

Analyze the terminal output to locate any version of Flask Unchained in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.9.0), your application is affected.

Note: The pip show Flask-Unchained 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 Unchained, 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 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.

CVE-2021-23393 Affected Flask Unchained Versions

1 component(s)
Fixed Versions:
0.9.0
Product / Package Ecosystem Affected Versions Fixed Version
Flask-Unchained
pypi/Flask-Unchained
PyPI
< 0.9.0
0.9.0

References

NVD — CVE-2021-23393 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-23393
Security Fix Commit FIX COMMIT
https://github.com/briancappello/flask-unchained/commit/2bfeedf1bc31df851cab8c66df0c432b10406aad
Security Fix Commit FIX COMMIT
https://github.com/briancappello/flask-unchained/commit/71e36b28166f9ffbe0a991f51127f0984f7e6a40
GitHub Advisory / Wiki ADVISORY
https://github.com/advisories/GHSA-pjc4-3w99-j7v4
Flask Unchained Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/flask-unchained/PYSEC-2021-96.yaml
snyk.io/vuln/snyk-python-fl... WEB
https://snyk.io/vuln/SNYK-PYTHON-FLASKUNCHAINED-1293189

Frequently Asked Questions

What is CVE-2021-23393?
CVE-2021-23393 is a security vulnerability in Flask Unchained involving Open redirect in Flask-Unchained under CWE-601: URL Redirection to Untrusted Site ('Open Redirect').
Which Flask Unchained versions are affected?
Affected versions include releases prior to 0.9.0 across affected packages: Flask-Unchained.
What version fixes CVE-2021-23393?
Upgrading to 0.9.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is Flask-Unchained affected?
Yes, Flask-Unchained is affected in releases prior to 0.9.0. Upgrade your dependency to 0.9.0 or later.
What is the CVSS score of CVE-2021-23393?
CVE-2021-23393 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-2021-23393?
CVE-2021-23393 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-2021-23393?
Run the dependency tree command: 'pip show Flask-Unchained'. 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-pjc4-3w99-j7v4",
  "modified" : "2024-09-20T20:01:50Z",
  "published" : "2021-06-15T15:56:59Z",
  "aliases" : [ "CVE-2021-23393" ],
  "summary" : "Open redirect in Flask-Unchained",
  "details" : "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.",
  "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-Unchained"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.9.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-23393"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/briancappello/flask-unchained/commit/2bfeedf1bc31df851cab8c66df0c432b10406aad"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/briancappello/flask-unchained/commit/71e36b28166f9ffbe0a991f51127f0984f7e6a40"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-pjc4-3w99-j7v4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/flask-unchained/PYSEC-2021-96.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://snyk.io/vuln/SNYK-PYTHON-FLASKUNCHAINED-1293189"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-601" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-06-14T19:36:47Z",
    "nvd_published_at" : null
  }
}