MyBB 1.8.6 Post Attachment cross site scripting

Summaryinfo

A vulnerability, which was classified as problematic, has been found in MyBB 1.8.6. This affects an unknown function of the component Post Attachment Handler. The manipulation leads to cross site scripting. The attack may be initiated remotely. In addition, an exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in MyBB 1.8.6 (Content Management System). It has been rated as problematic. This issue affects an unknown functionality of the component Post Attachment Handler. The manipulation with an unknown input leads to a cross site scripting vulnerability. Using CWE to declare the problem leads to CWE-80. The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special characters such as "<", ">", and "&" that could be interpreted as web-scripting elements when they are sent to a downstream component that processes web pages. Impacted is integrity.

The weakness was presented 11/10/2016 by Tim Coen with Curesec Research Team as MyBB 1.8.6: XSS as not defined mailinglist post (Full-Disclosure). It is possible to read the advisory at seclists.org. The public release was coordinated in cooperation with the project team. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. It demands that the victim is doing some kind of user interaction. Technical details are unknown but a public exploit is available. The attack technique deployed by this issue is T1059.007 according to MITRE ATT&CK. The advisory points out:

Attachments are uploaded to a public directory, and their extension is changed to .attach. Files with extension .attach that contain HTML code are interpreted as HTML files by some default server configurations (for example Apache). Additionally, the directory where the files are uploaded to does not prevent directory listing via an index.html file as all the other directories of MyBB do. Because of this, an attacker can find the name of the file and send it to a victim. Once the victim visits the link, the JavaScript code in the file would execute.

A public exploit has been developed by Tim Coen and been published immediately after the advisory. The exploit is available at seclists.org. It is declared as proof-of-concept. The vulnerability was handled as a non-public zero-day exploit for at least 279 days. During that time the estimated underground price was around $5k-$25k.

Upgrading to version 1.8.7 eliminates this vulnerability. The upgrade is hosted for download at resources.mybb.com. A possible mitigation has been published before and not just after the disclosure of the vulnerability.

See VDB-93560, VDB-93561, VDB-93562 and VDB-93564 for similar entries. Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Content Management System

Name

• MyBB

Version

• 1.8.6

License

• open-source

Website

• Product: https://mybb.com/

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