CVE-2021-32618Public exploitNetwork attack vector

User redirection via browser URL parsing mismatch

Published May 17, 2021 · Updated Aug 3, 2024

Open redirect in Flask-Security-Too 4.0.1 and earlier allows remote attackers to redirect users via a crafted next parameter. The validate_redirect_url function trusts Python urlsplit results that classify browser-normalized backslash, slash, whitespace, and control-character forms as relative URLs. Exploitation requires a user to follow an attacker-supplied link and an application to disable Werkzeug's default Location-header correction, enabling redirection to an attacker-chosen site.

CVSS severity2.1
Low
EPSS probability2.84%
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-Security-Too 4.0.1 and earlier allows remote attackers to redirect users via a crafted next parameter. The validate_redirect_url function trusts Python urlsplit results that classify browser-normalized backslash, slash, whitespace, and control-character forms as relative URLs. Exploitation requires a user to follow an attacker-supplied link and an application to disable Werkzeug's default Location-header correction, enabling redirection to an attacker-chosen site.

The record

CVE
CVE-2021-32618
Published
May 17, 2021
Updated
Aug 3, 2024
Vendor
Flask-Middleware
Product
Flask-Security
Classifications
CWE-601, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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=<= 4.0.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Flask-Middleware · Flask-Securityversion=<= 4.0.1

Published CVSS scores

6.1NISTCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
5.8NISTAV:N/AC:M/Au:N/C:P/I:P/A:N
2.1GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/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.

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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