CVE-2021-32618 Information

Description

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`.

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Reference

https://github.com/Flask-Middleware/flask-security/issues/486 https://github.com/Flask-Middleware/flask-security/security/advisories/GHSA-6qmf-fj6m-686c 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`.

Attack Complexity

LOW

Privileges Required

NONE

User Interaction Required

NONE

Scope

REQUIRED

Confidentiality Impact

CHANGED

Integrity Impact

LOW

Availability Impact

LOW

Base Score

NONE

Base Severity

6.1