How to perform security tests on SOAP request that contains XML in a string attribute.

I have a web service that submits a soap request that contains an XML file in the request in a string element.  The actual request (i.e. data that would change in different requests) are in the embedded document.   And yes, the xml document is enclosed in CDATA.

Is there a way to have the security functionality perform the testing on the elements in the embedded XML document?

Thanks.