Testing for XML Structural (OWASP-WS-003)

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XML Structural Attacks.

Attackers can create XML documents which are structured in such a way as to create a denial of service attack on the receiving server by tying up memory and CPU resources. This occurs via overloading the XML parser which is very CPU intensive in any case.

For example, elements which contain large numbers of attributes can cause problems with parsers. This category of attack also includes XML documents which are not well-formed XML (e.g. with overlapping elements,or with open tags that have no matching close tags). DOM based parsing can be vulnerable to DoS due to the fact that the complete message is loaded into memory 9as opposed to SAX parsing) oversized attachments can cause an issue with DOM architectures.

Web Services weakness: You have to parse XML via SAX or DOM before one validates the structure and content of the message.

Examples:

A web service utilising DOM based parsing can be "upset" by including a very large payload in the XML message which the parser would be obliged to parse:

VERY LARGE & UNEXPECTED PAYLOAD:

<Envelope> <Header>

   <wsse:Security>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>
     <Hehehe>I am a Large String (1MB)</Hehehe>…
    <Signature>…</Signature>
   </wsse:Security>
 </Header>
 <Body>
   <BuyCopy><ISBN>0098666891726</ISBN></BuyCopy>
 </Body></Envelope>