Reviewing Web Services

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Reviewing Webservices and XML payloads

When reviewing webservices one should focus firstly on the generic security controls related to any application. Webservices also have some uniques controls should be looked at.


XMLL Schema : Input validation

Schemas are used to ensure that the XML payload received is within defined and expected limits. They can be specific to a list of known good values or simply define lenght and type. Some XML applications do not have a schema implemented which may mean input validation is perfromed downstream ot even not at all!!

Keywords:

Namespace: An XML namespace is a collection of XML elements and attributes identified by an Internationalised Resource Identifier (RI). 
In a single document, elements may exist with the same name that were created by different entities.
To distinguish between such different definitions with the same name an XML Schema allows the concept of namespaces -  think Java packages :)

The schema can specify a finite amount of parameters, the expected parameters in the XML payload alongside the expected types and values of the payload data:

The ProcessContents attribute indicates how XML from other namespaces should be validated. When the processContents attribute is set to lax or skip, input validation is not performed for wildcard attributes and parameters.

The value for this attribute may be

  • strict: There must be a declaration associated with the namespace and validate the XML.
  • lax There should attempt to validate the XML against its schema.
  • skip There is no attempt to validate the XML.
processContents=skip\lax\skip

Infinite occurances of an element or attribute

The unbounded value can be used un an XML schema to specify the there is no maximum occurance expected for a specific element.

maxOccurs= positive-Integer|unbounded

Given that any number of elements can be supplied for an unbounded element it is subject to attack via supplying the web service with vast amounts of elements and hence a resource exhaustion issue.


Weak namespace, Global elements, the <any> element & SAX XML processors

The <any> element can be used to make extensible documents, allowing documents to contain additional elements which are not declared in the main schema. The idea that an application can accept any number of parameters may be cause for alarm. This may lead to denial of availability or even in the case of a SAX XML parser legitimate values may be overwritten.

<xs:element name="cloud">
 <xs:complexType>
   <xs:sequence>
     <xs:element name="process" type="xs:string"/>
     <xs:element name="lastcall" type="xs:string"/>
     <xs:any minOccurs="0"/>
   </xs:sequence>
 </xs:complexType>
</xs:element>

The <any> element here premits additional parameters to be added in an arbitary manner.

A namespace of ##any in the <any> element means the schema allows elements beyond what is explicitly defined in the schema, thereby reducing control on expected elements for a given request.

<xs:any namespace='##any' />

A schema that does not define restrictive element namespaces permits arbitrary elements to be included in a valid document which may not be expected by the application. This may give rise to attacks such as XML Injection which consists of including tags which are not expected by the applicaiton.