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Chapter 10 XML and Web Services Topics Why a standards-compliant XML parser Why a standard (off the shelf) XML parser Validation. External references Document queries Input validation for Web services WSDL potential problems/a bad example with blacklisting New opportunities for old errors JavaScript hijacking.. Introduction XML web services Service oriented architecture The promise: Software reuse Less software security headaches Why a standards-compliant XML parser XML is very hard to parse Recursive XML entities (&xxxx;) Better to use a standard, off the shelf XML parser; there are many... Validation Validating XML documents can be done against an XML schema or Document Type Definition (DTD) Slight preference for XML Schema because “Simple Object Access Protocol” (Soap) uses XML schema and SOAP used for man Web Services standards. Can be tricked, so be careful (page 354 has a good example of “XML injection”) External References Document queries may be malicious, so be careful about following them! Example Document Type Declaration: <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN” “http://www.w3.org/TR/xhtml1/DTD/strict.dtd”> One example redefined the XML parser (ref. P 359) Another example exported the boot sector. Recommendation: use XML Schema (easier, more powerful) Document Queries Looking around in XML documents is difficult. Solution: XPATH Problem: XPATH queries can be subject to XPATH injection if not validated correctly. (See example and cure on pp 363-365 Web Services ● ● ● Web services are the main reason firewalls are not too useful Proponents often equate security features with secure features. Many chances for problems. Input validation for Web Services WS frameworks make it easy to “push a button” and get a web service. Case in point: Class xxx.java is copied into webapp directory as <webapp>/axis/xxx.jws Now xxx is available via SOAP, with all its vulnerabilities!!! WSDL potential problems/a bad example with blacklisting ● ● ● WSDL stands for Web Services Description Language: does for Web Services what HTML does for web pages: makes them “discoverable. Example: Direct Web Remoting (DWR) (see http://directwebremoting.org/dwr/index.html) has a facility that allows listing “excluded” methods: ● <exclude ...> ● <include ...> DWR is another easy way to include java code in a web services page. New opportunities for old errors ● Automatic Stack traces ??? ● Session management traps ● Ambiguous security requirements: ● Versatility vs security (XSS for example) JavaScript Hijacking ● ● AJAX is a well used platform for developing Web applications; it is very easy to use. Biggest problem is that browser requests are for small datapieces, leaving the door open for attackers to steal data (technology exists to mix data sources, similar to mash-ups). Confidential data can be retrieved by a malicious website. Very possible with XSS. Advice: ● ● Decline malicious requests Prevent direct execution of JavaScript responses. Declining Malicious Requests A Javascript hijacking attempt looks like an XSS. Suggest: Include a random cookie (can be a session or a dialog cookie) Check the HTTP referer header. Do not respond to GET requests. (Always used by <script> tag) (efficiency hit) Preventing direct response execution Program in such a way that responses include code that has to be removed in order to make it executable: for example, prefix it with: While(1); or convert the code into comments (enclose in /* */)
