Testing for XML Structural (OWASP-WS-003)

Brief Summary
XML needs to be well-formed to function properly. XML which is not well-formed shall fail when parsed by the XML parser on the server side. A parser needs to run thorough the entire XML message in a serial manner in order to assess the XML well-formedness.

An XML parser is also very CPU labour intensive. Some attack vectors exploit this weakness by sending very large or malformed XML messages.

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, as we mentioned, is very CPU intensive.

Description of the Issue
This section discusses the types of attack vectors one could send to a web service in an attempt to assess its reaction to malformed or maliciously crafted messages.

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 (as opposed to SAX parsing). For example, 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.

Black Box Testing and example
Examples:

Malformed structure: The XML message must be well formed in order to be successfully parsed. Malformed SOAP messages may cause unhandled exceptions to occur;

 OWASP EOIN I am Malformed  Don’t forget me this weekend!

A web service utilizing 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:

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

Binary attachments:

Web Services can also have a binary attachment such as a Blob or exe. Web service attachments are encoded in base64 format since the trend is that DIME (Direct Internet Message Encapsulation) seems to be a dead-end solution.

By attaching a very large base64 string to the message, an attacker may consume parser resources to the point of affecting availability. Additional attacks may include the injection of a infected binary file into the base64 binary stream. Inadequate parsing of such an attachment may exhaust resources:

UNEXPECTED LARGE BLOB: <Envelope> <Header> <wsse:Security> jgiGldkooJSSKFM%LFM$MFKF)$KRFWF$FRFkflfkfkkorepoLPKOMkjiujhy:llki-123-01ke123-      04QWS03994k£R$Trfe£elfdk4r-45kgk3lg"£!04040lf;lfFCVr$V$BB^^N&*<M&NNB%...........10MB      <Signature>…</Signature>    </wsse:Security>  </Header>  <Body>    <BuyCopy><ISBN>0098666891726</ISBN></BuyCopy>  </Body> </Envelope>

Grey Box Testing and example
If one has access to the schema of the web service it should be examined. One should assess that all the parameters are being data validated. Restrictions on appropriate values should be implemeneted in accordance to data validation best practice.

enumeration: Defines a list of acceptable values

fractionDigits: Specifies the maximum number of decimal places allowed. Must be equal to or greater than zero

length: Specifies the exact number of characters or list items allowed. Must be equal to or greater than zero

maxExclusive: Specifies the upper bounds for numeric values (the value must be less than this value)

maxInclusive: Specifies the upper bounds for numeric values (the value must be less than or equal to this value)

maxLength: Specifies the maximum number of characters or list items allowed. Must be equal to or greater than zero

minExclusive: Specifies the lower bounds for numeric values (the value must be greater than this value)

minInclusive: Specifies the lower bounds for numeric values (the value must be greater than or equal to this value)

minLength: Specifies the minimum number of characters or list items allowed. Must be equal to or greater than zero

pattern: Defines the exact sequence of characters that are acceptable

totalDigits: Specifies the exact number of digits allowed. Must be greater than zero.

whiteSpace: Specifies how white space (line feeds, tabs, spaces, and carriage returns) is handled