PatchTheCVECVE-2021-23393: This affects the package Flask-Unchained before 0.9.0
CVE-2021-23393 is a medium-severity flaw (CVSS 5.4) affecting flask unchained. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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.
Why this priority
- No elevated risk signals yet
A fixed release or distro patch exists.
Risk facts
| CVSS score | 5.4 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | Required |
| EPSS exploit chance | 0.7% (top 46%) |
| CISA exploited list | No |
| Weakness | CWE-601 |
| Published | 2021-06-11 |
| Last modified | 2026-06-17 |
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2021-23393?
flask unchained: upgrade to 0.9.0. Exact commands for each OS are on this page.
Is CVE-2021-23393 being exploited?
CVE-2021-23393 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2021-23393 affect?
CVE-2021-23393 affects flask unchained. Severity is medium (CVSS 5.4). 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.co
How urgent is CVE-2021-23393?
Priority 32/100. No elevated risk signals yet.