Testing for WS HTTP GET parameters/REST attacks (OWASP-WS-005)

Brief Summary
Many XML applications are invoked by passing them parameters using HTTP GET queries. These are sometimes known as “REST-style" Web Services (REST = Representational State Transfer). These Web Services can be attacked by passing malicious content on the HTTP GET string (e.g., extra long parameters (2048 chars), SQL statements/injection (or OS Injection parameters).

Description of the Issue
Given that REST Web services are in effect HTTP-In -> WS-Out, attack patterns are very similar to regular HTTP attack vectors, discussed throughout the guide. For example, in the following HTTP request with query string "/?viewDetail=detail-10293", the HTTP GET parameter is "detail-10293".

Black Box Testing and example
Say we had a Web Service which accepts the following HTTP GET query string:

https://www.ws.com/accountinfo?accountnumber=12039475&userId=asi9485jfuhe92

The response would be similar to:

  €100 Bank of Bannana account info 

Testing the data validation on this REST web service is similar to generic application testing:

Try vectors such as:

https://www.ws.com/accountinfo?accountnumber=12039475'  exec master..xp_cmdshell 'net user Vxr pass /Add&userId=asi9485jfuhe92

Grey Box Testing and example
Upon the reception of a HTTP request the code should do the following:

Check:
 * 1) Maximum and minimum length
 * 2) Validate payload
 * 3) If possible implement the following data validation stratigies: "exact match", "known good" and "known bad", in this order.
 * 4) Validate parameter names and existence.