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CVE Record

CVE-2021-23393: Open Redirect

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.

MediumCVSS 5.4Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is an open redirect in Flask-Unchained. A malicious link could send a user through a trusted application to an external site, supporting phishing or token capture scenarios. The sources rate it medium and show no KEV-listed active exploitation.

Executive priority

Treat as a focused medium-priority remediation. Prioritize internet-facing login, SSO, password reset, and invitation flows because open redirects can make phishing more credible.

Technical view

Flask-Unchained before 0.9.0 could incorrectly validate redirect URLs when _validate_redirect_url is used. The bypass depends on multiple leading backslashes and is exploitable only with non-default WSGI behavior: an alternative WSGI server or Werkzeug with autocorrect_location_header disabled.

Likely exposure

Exposure is likely limited to Python applications using Flask-Unchained before 0.9.0, calling _validate_redirect_url for user-influenced redirects, and running under the specific WSGI conditions described in the CVE.

Exploitation context

The CVSS vector requires user interaction and no privileges. The bundle does not provide evidence of active exploitation, and KEV is false. Business risk is mainly phishing enablement and trust abuse, not direct server compromise.

Researcher notes

The important boundary condition is deployment-specific WSGI behavior. Do not assume every Flask-Unchained use is exploitable. The source bundle lists no CWE and does not provide operational exploit evidence beyond proof-of-concept maturity in CVSS.

Mitigation direction

  • Upgrade Flask-Unchained from versions before 0.9.0.
  • Keep Werkzeug redirect header autocorrection enabled unless vendor guidance says otherwise.
  • Review redirect endpoints for strict same-origin or approved-destination validation.
  • Check vendor guidance before relying on local validation changes.
  • Add regression tests for rejected external redirect destinations.

Validation and detection

  • Inventory applications for Flask-Unchained dependency versions before 0.9.0.
  • Search application code for _validate_redirect_url usage.
  • Confirm which WSGI server handles affected applications.
  • Verify Werkzeug autocorrect_location_header is not disabled.
  • Review redirect flows where users control next or return URLs.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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CVE-2021-23393 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.4 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
5.4CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C2.82.5Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

5.4Medium
CVSS 3.1 vector shape for CVE-2021-23393Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/aFlask-UnchainedunspecifiedListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.