CVE-2021-32618

The Python "Flask-Security-Too" package is used for adding security features to
your Flask application. It is an is an independently maintained version of
Flask-Security based on the 3.0.0 version of Flask-Security. All versions of
Flask-Security-Too allow 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 is used (which is
very common with Flask applications) as the WSGI layer, it 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`.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-32618
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-32618
https://github.com/Flask-Middleware/flask-security/issues/486
https://github.com/Flask-Middleware/flask-security/security/advisories/GHSA-6qmf-fj6m-686c