CVE-2021-32618

Open Redirect in Flask-Security-Too

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.

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.

Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.

CVE-2021-32618 has a CVSS score of 3.1 (Low). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.1.0); upgrading removes the vulnerable code path.

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1. What is CVE-2021-32618? CVE-2021-32618 is a low-severity open redirect vulnerability in Flask-Security-Too (pip), affecting versions < 4.1.0. It is fixed in 4.1.0. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
2. How severe is CVE-2021-32618? CVE-2021-32618 has a CVSS score of 3.1 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of Flask-Security-Too are affected by CVE-2021-32618? Flask-Security-Too (pip) versions < 4.1.0 is affected.
4. Is there a fix for CVE-2021-32618? Yes. CVE-2021-32618 is fixed in 4.1.0. Upgrade to this version or later.
5. Is CVE-2021-32618 exploitable, and should I be worried? Whether CVE-2021-32618 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-32618 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-32618? Upgrade Flask-Security-Too to 4.1.0 or later.

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