Open redirect in Flask-Unchained

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.3 · 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 · score source

EPSS

• CVE-2021-23393: 0.72% estimated exploitation activity in the next 30 days; percentile 53.83 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

This affects the package Flask-Unchained before 0.9.0. When using the the validateredirecturl 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 'autocorrectlocationheader=False.

Published by the source

15 Jun 2021, 15:56 UTC

Last modified by the source

20 Sep 2024, 20:25 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-pjc4-3w99-j7v4

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-pjc4-3w99-j7v4

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# Patch Next advisory brief: CVE-2021-23393

Advisory: Open redirect in Flask-Unchained
IDs: CVE-2021-23393, GHSA-pjc4-3w99-j7v4
Record: https://patchnext.com/advisories/cve-2021-23393 (revision 3, source checked 26 Oct 2026, 13:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: flask-unchained 0.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI flask-unchained: affected < 0.9.0; fixed in 0.9.0

## Suggested inspection (read-only)
- `python -m pip show flask-unchained`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on flask-unchained.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-pjc4-3w99-j7v4 (OSV record GHSA-pjc4-3w99-j7v4 lists affected ranges: PyPI flask-unchained >=0 <0.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-pjc4-3w99-j7v4 (GitHub-reviewed advisory GHSA-pjc4-3w99-j7v4 lists affected ranges: PyPI flask-unchained >=0 <0.9.0)

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