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🐍 PyPI
Not in CISA KEV
LOW severity

CVE-2021-32618 — flask-security-too

LOWFix: Flask-Middleware/flask-security@e39bb04

CVE-2021-32618 is a low-severity (CVSS 3.1) Open Redirect vulnerability in flask-security-too. A fix is available for flask-security-too — see the affected versions and patch details below.

Open Redirect in Flask-Security-Too

Also known asPYSEC-2021-123
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
2.8%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs86th percentile — riskier than 86% of all scored CVEsHighest risk
0.00%7.11%14.2%21.3%17.1%2.8%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-32618 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍flask-security-too

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Flask-Security allows redirects after many successful views (e.g. /login) by honoring the ?next query param. There is code in FS to validate that the url specified in the next parameter is either relative OR has the same netloc (network location) as the requesting URL.

This check utilizes Pythons urlsplit library. However many browsers are very lenient on the kind of URL they accept and 'fill in the blanks' when presented with a possibly incomplete URL. As a concrete example - setting http://login?next=\\\github.com will pass FS's relative URL check however many browsers will gladly convert this to http://github.com. Thus an attacker could send such a link to an unwitting user, using a legitimate site and have it redirect to whatever site they want.

This is considered a low severity due to the fact that if Werkzeug by default ALWAYS ensures that the Location header is absolute - thus making this attack vector mute. It is possible for application writers to modify this default behavior by setting the 'autocorrect_location_header=False` which would then open up their application to this attack.

Patches

No patches as this time

Workarounds

If using Werkzeug, make sure to use the default Location header setting. If you can't - then [email protected]_request and write your own validation of the Location header if it is set.

References

No.

For more information

If you have any questions or comments about this advisory follow: https://github.com/Flask-Middleware/flask-security/issues/486

Thanks to Claroty (2021-0141) and @snoopysecurity for providing details and proof of concept.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIflask-security-tooall versions4.1.0pip install --upgrade 'flask-security-too==4.1.0'

Affected Products

1 product · 1 configurations
Application
flask-securityflask-security_project
all

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flask-security-too, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update flask-security-too to 4.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-32618 is resolved across your whole dependency graph.

  3. Workarounds

    Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

How to detect CVE-2021-32618

A community-maintained Nuclei template exists for this CVE. You can scan for it directly:

nuclei -id cve-2021-32618 -u https://target
Template
Python Flask-Security - Open Redirect
Severity
medium
Impact
An attacker can craft a malicious URL that redirects users to a malicious website, leading to potential phishing attacks or the exploitation of other vulnerabilities.
Remediation
Upgrade to the latest version of Python Flask-Security library to fix the open redirect vulnerability.

Template by ProjectDiscovery nuclei-templates (0x_Akoko), MIT licensed. View the full template. Scan only systems you are authorised to test.

Frequently Asked Questions

### Impact Flask-Security allows redirects after many successful views (e.g. /login) by honoring the ?next query param. There is code in FS to validate that the url specified in the next parameter is either relative OR has the same netloc (network location) as the requesting URL. This check utilizes Pythons urlsplit library. However many browsers are very lenient on the kind of URL they accept and 'fill in the blanks' when presented with a possibly incomplete URL. As a concrete example - setting http://login?next=\\\github.com will pass FS's relative URL check however many browsers will glad
O3 Security · Impact-Aware SCA

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CVE-2021-32618: flask-security-too — Fixed in 4.1.0