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.
No patches as this time
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.
No.
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.
github.com/Flask-Middleware/flask-security
github.com/Flask-Middleware/flask-security/commit/e39bb04615050448c1b8ba4caa7dacc0edd3e405
github.com/Flask-Middleware/flask-security/issues/486
github.com/Flask-Middleware/flask-security/pull/489
github.com/Flask-Middleware/flask-security/security/advisories/GHSA-6qmf-fj6m-686c
nvd.nist.gov/vuln/detail/CVE-2021-32618