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CVE Record

CVE-2021-23401: Open Redirect

This affects all versions of package Flask-User. When using the make_safe_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 or \\\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

CVE-2021-23401 is an open redirect issue in Flask-User. In affected deployments, a crafted URL can make an application send a user to an attacker-controlled site. Business impact is mainly phishing, credential capture, and trust abuse rather than direct server compromise.

Executive priority

Treat as moderate priority unless Flask-User is internet-facing in authentication flows. Prioritize validation where redirects affect login or account workflows, because the most credible harm is phishing through a trusted domain.

Technical view

Flask-User's make_safe_url validation can be bypassed using multiple leading slash or backslash characters. The source states exploitation depends on using a non-Werkzeug WSGI server, or changing Werkzeug's default autocorrect_location_header behavior to false. CVSS is 5.4 medium with required user interaction.

Likely exposure

Exposure is most likely in Python Flask applications using Flask-User redirect handling, especially login, logout, registration, or next-URL flows. Risk is narrower when Werkzeug defaults are unchanged.

Exploitation context

The bundle does not report active exploitation, and KEV status is false. An attacker would need to convince a user to follow a crafted application link that reaches vulnerable redirect logic in a susceptible WSGI configuration.

Researcher notes

Evidence is limited to public CVE, Snyk, and Flask-User repository references. No CWE is listed in the bundle. No patch version is provided here, so remediation should be tied to current vendor or package-maintainer guidance.

Mitigation direction

  • Inventory Flask applications using Flask-User and redirect parameters.
  • Check Flask-User and Snyk guidance for maintained fixes or migration advice.
  • Keep Werkzeug default location header autocorrection unless vendor guidance says otherwise.
  • Avoid non-Werkzeug WSGI behavior that permits unsafe redirect interpretation.
  • Prefer strict relative-path or allowlisted redirect destinations in application logic.

Validation and detection

  • Search code for Flask-User usage and make_safe_url-dependent redirect flows.
  • Confirm the deployed WSGI server and Werkzeug autocorrect_location_header setting.
  • Review login, logout, registration, and next-URL redirect handling.
  • Test redirect validation safely in staging without external user traffic.
  • Verify monitoring for suspicious redirects to untrusted domains.
Prepared
Confidence
high
Sources
5

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-2021-23401 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
4Source 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-2021-23401Attack 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-User0Listed
Weakness

CWE details

No CWE listed

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