Summary

Problem

Struts 2 allows define action mapping base on wildcards, like in example below:

<action name="*" class="example.ExampleSupport">
    <result>/example/{1}.jsp</result>
</action>

If a request doesn't match any other defined action, it will be matched by * and requested action name will be used to load JSP file base on the name of action. And as value of {1} is threaten as an OGNL expression, thus allow to execute arbitrary Java code on server side. This vulnerability is combination of two problems:

• requested action name isn't escaped or checked agains whitelist
• double evaluation of an OGNL expression in TextParseUtil.translateVariables when combination of $ and % open chars is used.

Proof of concept

Wildcard matching

1. Run struts2-blank app
2. Open the following url, resulting in dynamic action name resolution based on passed value of #foo

   http://localhost:8080/example/%24%7B%23foo%3D%27Menu%27%2C%23foo%7D

   http://localhost:8080/example/${#foo='Menu',#foo}

As you can notice, action name is resolved based on user input and you can put any arbitrary code to perform attack.

Double evaluation of an expression

1. Open example.xml present in the Struts Blank App and change result of HelloWorld action to one below:

   <result type="httpheader"> <param name="headers.foobar">${message}</param> </result>

2. Open HelloWorld.java and change execute() method as below:

   public String execute() throws Exception { return SUCCESS; }

3. Run struts2-blank app
4. Open the following url (you must have a tool to check response headers)

   http://localhost:8080/example/HelloWorld.action?message=%24{%25{1%2B2}}

   http://localhost:8080/example/HelloWorld.action?message=${%{1+2}}

5. Check value of foobar header, it should be 3

As you can notice, passed value of message parameter was used to set value of foobar header and the value was double evaluated - first time when ${message} was evaluated, secondly when parsed value (${%{1+2}}) was evaluated again.

Solution

With the new version actions' names whitelisting was introduced and by default is set to accept actions that match the following regex:

[a-z]*[A-Z]*[0-9]*[.\-_!/]*

user can change the definition by setting up a new constant in struts.xml as below:

<constant name="struts.allowed.action.names" value="[a-zA-Z]*" />

Double evaluation of passed expression was removed from OgnlTextParser which is used by TextParseUtil.translateVariables.