CVE-2021-23393Network attack vector

Open redirect via backslash URL parsing mismatch

Published Jun 10, 2021 · Updated Sep 16, 2024

Open redirect in Flask Unchained before 0.9.0 allows remote attackers to redirect users to arbitrary sites through crafted URLs. The _validate_redirect_url function split and compared backslash-containing input without first normalizing separators, causing browsers to treat an accepted value as an external authority. Exploitation requires user interaction and either a non-Werkzeug WSGI server or Werkzeug with autocorrect_location_header disabled, and the bounded consequence is navigation to an attacker-controlled site.

CVSS severity5.4
Medium
EPSS probability0.72%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Open redirect in Flask Unchained before 0.9.0 allows remote attackers to redirect users to arbitrary sites through crafted URLs. The _validate_redirect_url function split and compared backslash-containing input without first normalizing separators, causing browsers to treat an accepted value as an external authority. Exploitation requires user interaction and either a non-Werkzeug WSGI server or Werkzeug with autocorrect_location_header disabled, and the bounded consequence is navigation to an attacker-controlled site.

The record

CVE
CVE-2021-23393
Published
Jun 10, 2021
Updated
Sep 16, 2024
Vendor
Brian Cappello
Product
Flask Unchained
Classifications
CWE-601
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <0.9.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Brian Cappello · Flask Unchainedversion=unspecified <0.9.0

Published CVSS scores

5.4Snyk CNACVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
5.3GitHub Advisory DatabaseCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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