Expression Engine 3.4.2: Code Reuse Attack5 Dec 2016 by Hendrik Buchwald
Expression Engine is a popular general purpose content management system that is used by thousands of individuals, organizations, and companies around the world. The open-source version has about 250,000 lines of code and is a medium-sized web application. In this post, we will examine a code reuse vulnerability that leads to remote code execution. This vulnerability type allows an attacker to partly control the applications logic and to chain existing code fragements.
Example 1: PHP Object Injection
The most serious issue reported is a PHP Object Injection vulnerability. As the name suggests, this vulnerability allows an attacker to inject objects of arbitrary class type. The issue resides in the administration control panel but it can be triggered through cross-site request forgery (CSRF). An attacker can create a malicious website and trick an administrator into visiting the page which then fires the attack against his administration panel in the background. The following code contains the PHP object injection vulnerability.
Here, the GET parameter
url is passed to the sensitive sink
unserialize() without any sanitization. As already explained in our Coppermine post, older installations of PHP (before 5.5.37, 5.6.x before 5.6.23, and 7.x before 7.0.8) are vulnerable to arbitrary code execution due to a security issue in the
unserialize() function itself.
But even with the latest and greatest PHP version, this object injection vulnerability poses security risks because it can be used to trigger other vulnerabilities in the code base that would not be reachable otherwise. An attacker can inject an object and control its class type and its properties by modifying the serialized string that is used to re-create the object. For example, the following string invokes an object of class
Link when deserialized:
Example 2: Chain for XSS to RCE
In order to exploit the PHP object injection vulnerability, an attacker can utilize an attack technique called property oriented programming. It allows the attacker to chain code fragments of existing classes in the code base and to jump from one method to another until a sensitive code is reached that triggers another vulnerability.
Our starting point is the magic method
__toString() that is automatically called when an object is type-casted into a string. When we inject an object of the class
Link, the object is concatenated into a template and the
__toString() method of
Link is automatically invoked. This is our initial gadget to takeover the control flow of the application. In the
__toString() method, the method
render() is called.
Since not only the class name, but also the properties of the injected object are within the control of an attacker, the property
filepicker can be chosen arbitrarily. This enables to jump to any method named
getUrl() in line 37. Placing an object of class
FilePicker into this property then calls the method
getUrl() of this class.
From here, we continue our journey through the
url property. Again, any object of any class can be injected into this property and defer the control flow to arbitrary
make() methods. An interesting
make() method is found in the class
At first sight, it is possible to execute arbitrary functions by injecting into the first argument of
call_user_func_array(). However, this is prevented in line 72 that ensures that the argument is a
Closure which cannot be injected through
unserialize() and, thus, line 74 cannot be reached.
Another call to
call_user_func_array() is found in line 80 though. It is not possible to directly execute PHP code here, but at least we can control the class name that is taken from the property
delegate. Based on this property, we can call arbitrary
make() methods again, but no other interesting
make() methods were found in the code base. Interestingly, when a faulty class name is injected, the error page of Expression Engine is shown and the name of the class is printed without any sanitization. This at least leads to a cross-site scripting vulnerability.
Cross-site scripting in Expression Engine can lead to PHP code evaluation though because it can be abused to enable the PHP parser for a template and then to write arbitrary code into it. For this purpose, an attacker can retrieve the secret token used for CSRF protection and then submit requests in the name of an administrator. You can see a video demonstration of this step in our FreePBX post.
|2016/09/14||First contact with vendor|
|2016/09/14||Vendor responds with fix|
|2016/09/20||Vendor releases fixed version|
The code of Expression Engine is well written and the architecture sophisticated. But as demonstrated in this post, one single vulnerability is enough to endanger your complete system. A simple code slip can set off an avalanche and lead to arbitrary code execution for attackers. The team at EllisLab takes security very seriously, promptly responded to our reporting, and published a fix only 6 days later. Kudos and thank you for the professional collaboration.
To prevent PHP object injection vulnerabilities, the deserialization of user-supplied data should be avoided in general. It is much safer to use similar functions such as
json_decode(). It is also possible to protect the input with a keyed-hash message authentication code (HMAC) in order to prevent unauthorized modifications.
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