S2-001

Summary

Remote code exploit on form validation error

Problem

The 'altSyntax' feature of WebWork 2.1+ and Struts 2 allows OGNL expressions to be inserted into text strings and is processed recursively. This allows a malicious user to submit a string, usually through an HTML text field, containing an OGNL expression that will then be executed by the server if the form validation has failed. For example, say we had this form that required the 'phoneNumber' field to not be blank:

<s:form action="editUser">
  <s:textfield name="name" />
  <s:textfield name="phoneNumber" />
</s:form>

The user could leave the 'phoneNumber' field blank to trigger the validation error, then populate the 'name' field with %{1+1}. When the form is re-displayed to the user, the value of the 'name' field will be '2'. The reason is the value field is, by default, processed as %{name}, and since OGNL expressions are evaluated recursively, it is evaluated as if the expression was %{%{1+1}}.

The OGNL parsing code is actually in XWork and not in WebWork 2 or Struts 2.

Solution

The fixed version of XWork changes the OGNL parsing so that it is not recursive. Therefore, in the example above, the result will be the expected %{1+1}. You can either obtain the latest version of WebWork 2 or Struts 2, which contains the fixed XWork library, or download the fixed XWork library directly. Alternatively, you can obtain the patch and apply it to the XWork source code yourself.