CVE-2021-23393: Open Redirect

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-06-11 · updated 2026-06-17
Weakness: CWE-601

Description

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.

Affected products

Flask-Unchained: before 0.9.0 — fixed in 0.9.0

How to fix

Upgrade to a fixed version: Flask-Unchained 0.9.0. Check the vendor advisory for mitigations.

References

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