CVE-2021-23401Public exploitNetwork attack vector

Open redirect via backslash URL parsing

Published Jul 5, 2021 · Updated Sep 16, 2024

Open redirect in Ling Thio Flask-User 1.0.2.2 and earlier allows remote attackers to redirect users through a crafted URL. The make_safe_url method clears the scheme and network location returned by urlsplit but does not reject paths beginning with repeated slashes or backslashes, which redirect handling can reinterpret as an external location. Exploitation requires a WSGI server other than Werkzeug or Werkzeug with location-header autocorrection disabled, plus user interaction with the crafted redirect.

CVSS severity5.4
Medium
EPSS probability1.11%
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 Ling Thio Flask-User 1.0.2.2 and earlier allows remote attackers to redirect users through a crafted URL. The make_safe_url method clears the scheme and network location returned by urlsplit but does not reject paths beginning with repeated slashes or backslashes, which redirect handling can reinterpret as an external location. Exploitation requires a WSGI server other than Werkzeug or Werkzeug with location-header autocorrection disabled, plus user interaction with the crafted redirect.

The record

CVE
CVE-2021-23401
Published
Jul 5, 2021
Updated
Sep 16, 2024
Vendor
Ling Thio
Product
Flask-User
Classifications
CWE-601, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 5, 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=0 <unspecified

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Ling Thio · Flask-Userversion=0 <unspecified

Published CVSS scores

5.4SnykCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
5.3GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A: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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitPublished

Weakness, pattern, technique

CWE-601URL Redirection to Untrusted Site ('Open Redirect')
T1204.001User Execution: Malicious Link

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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