Advisory ID: HTB23277
Product: Ultimate Member WordPress plugin
Vendor: Ultimate Member
Vulnerable Version(s): 1.3.28 and probably prior
Tested Version: 1.3.28
Advisory Publication:  October 29, 2015  [without technical details]
Vendor Notification: October 29, 2015 
Vendor Patch: October 31, 2015 
Public Disclosure: November 19, 2015 
Vulnerability Type: Cross-Site Scripting [CWE-79]
CVE Reference: CVE-2015-8354
Risk Level: Medium 
CVSSv3 Base Score: 6.1 [CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N]
Solution Status: Fixed by Vendor
Discovered and Provided: High-Tech Bridge Security Research Lab ( https://www.htbridge.com/advisory/ ) 

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Advisory Details:

High-Tech Bridge Security Research Lab discovered vulnerability in Ultimate Member WordPress plugin, intended for managing users’ profiles. The vulnerability can be used against website administrators to perform Cross-Site Scripting (XSS) attacks. 

Anonymous attacker might be able to steal administrator's cookies, credentials and browser history, modify web page content to perform phishing attacks, or even to perform drive-by-download attacks by injecting malware into website pages when the website administrator follows a specially crafted link with XSS exploit.

The vulnerability is caused by absence of filtration of input-data passed via the "_refer" HTTP GET parameter to "wp-admin/users.php" script, when "update" is set to value "confirm_delete". A remote unauthenticated attacker can trick a logged-in administrator to open a specially crafted link and execute arbitrary HTML and script code in browser in context of the vulnerable website.

A simple exploit below will display JS popup with "ImmuniWeb" word:

https://[host]/wp-admin/users.php?update=confirm_delete&_refer='"><script>alert('ImmuniWeb');</script>

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Solution:

Update to Ultimate Member 1.3.29

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References:

[1] High-Tech Bridge Advisory HTB23277 - https://www.htbridge.com/advisory/HTB23277 - Reflected XSS in Ultimate Member WordPress Plugin.
[2] Ultimate Member Wordpress plugin - https://ultimatemember.com/ - The easiest way to create powerful online communities and beautiful user profiles with WordPress.
[3] Common Vulnerabilities and Exposures (CVE) - https://cve.mitre.org/ - international in scope and free for public use, CVE® is a dictionary of publicly known information security vulnerabilities and exposures.
[4] Common Weakness Enumeration (CWE) - https://cwe.mitre.org - targeted to developers and security practitioners, CWE is a formal list of software weakness types.
[5] ImmuniWeb® SaaS - https://www.htbridge.com/immuniweb/ - hybrid of manual web application penetration test and cutting-edge vulnerability scanner available online via a Software-as-a-Service (SaaS) model.

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Disclaimer: The information provided in this Advisory is provided "as is" and without any warranty of any kind. Details of this Advisory may be updated in order to provide as accurate information as possible. The latest version of the Advisory is available on web page [1] in the References.