Arbitrary Command Execution

Affecting org.apache.struts:struts2-core artifact, versions [,2.3.34) || [2.4,2.5.13)

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Overview

Apache Struts2 is a popular open-source framework for developing web applications in the Java programming language.

The REST Plugin in affected versions use a XStreamHandler with an instance of XStream for deserialization without any type filtering. By design, there are few limits to the type of objects XStream can handle. This flexibility comes at a price. The XML generated or consumed by XStream includes all information required to build Java objects of almost any type. The provided XML data is used by XStream to unmarshal Java objects. An attacker could use this flaw to execute arbitrary code or conduct further attacks.

A working exploit is publicly available and is actively exploited in the wild.

You can read more about this vulnerability on our blog.

Details

Serialization is a process of converting an object into a sequence of bytes which can be persisted to a disk or database or can be sent through streams. The reverse process of creating object from sequence of bytes is called deserialization. Serialization is commonly used for communication (sharing objects between multiple hosts) and persistence (store the object state in a file or a database). It is an integral part of popular protocols like Remote Method Invocation (RMI), Java Management Extension (JMX), Java Messaging System (JMS), Action Message Format (AMF), Java Server Faces (JSF) ViewState, etc.

Deserialization of untrusted data (CWE-502), is when the application deserializes untrusted data without sufficiently verifying that the resulting data will be valid, letting the attacker control the state or the flow of the execution.

Java deserialization issues have been known for years. However, interest in the issue intensified greatly in 2015, when classes that could be abused to achieve remote code execution were found in a popular library (Apache Commons Collection). These classes were used in zero-days affecting IBM WebSphere, Oracle WebLogic and many other products.

An attacker just needs to identify a piece of software that has both a vulnerable class on its path, and performs deserialization on untrusted data. Then all they need to do is send the payload into the deserializer, getting the command executed.

Developers put too much trust in Java Object Serialization. Some even de-serialize objects pre-authentication. When deserializing an Object in Java you typically cast it to an expected type, and therefore Java's strict type system will ensure you only get valid object trees. Unfortunately, by the time the type checking happens, platform code has already created and executed significant logic. So, before the final type is checked a lot of code is executed from the readObject() methods of various objects, all of which is out of the developer's control. By combining the readObject() methods of various classes which are available on the classpath of the vulnerable application an attacker can execute functions (including calling Runtime.exec() to execute local OS commands).

  • Apache Blog

Remediation

Developers are strongly advised to upgrade their Apache Struts components to version 2.3.34, 2.5.13 or higher.

It is possible that some REST actions stop working because of applied default restrictions on available classes. In this case please investigate the new interfaces that were introduced to allow class restrictions per action, those interfaces are:

  • org.apache.struts2.rest.handler.AllowedClasses
  • org.apache.struts2.rest.handler.AllowedClassNames
  • org.apache.struts2.rest.handler.XStreamPermissionProvider

If for some reason upgrading is not an option, consider the following workarounds:

  1. Disable handling XML pages and requests to such pages

    <constant name="struts.action.extension" value="xhtml,,json" />
  2. Override getContentType in XStreamHandler

    public class MyXStreamHandler extends XStreamHandler { 
    public String getContentType() {
      return "not-existing-content-type-@;/&%$#@";
    }
    }
  3. Register the handler by overriding the one provided by the framework in your struts.xml

    <bean type="org.apache.struts2.rest.handler.ContentTypeHandler" name="myXStreamHandmer" class="com.company.MyXStreamHandler"/>
    <constant name="struts.rest.handlerOverride.xml" value="myXStreamHandler"/>

References

CVSS Score

8.1
high severity
  • Attack Vector
    Network
  • Attack Complexity
    High
  • Privileges Required
    None
  • User Interaction
    None
  • Scope
    Unchanged
  • Confidentiality
    High
  • Integrity
    High
  • Availability
    High
CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:R
Credit
LGTM Security Team
CVE
CVE-2017-9805
CWE
CWE-20 CWE-502
Snyk ID
SNYK-JAVA-ORGAPACHESTRUTS-31495
Disclosed
05 Sep, 2017
Published
06 Sep, 2017