It is very rare today to find a business application that has not exposed its interface via SOAP/XML. XML is the building block that enables business or consumer applications to exchange data in a standard structured format.  The exchange of XML data typically takes place through an SOAP/XML interface based on the Web Services standard or through the REST-based standard.  These flexible standards that richly describe interface functions of an application also introduce a host of XML and Web Services security vulnerabilities.  This article is a quick guide to most common XML and Web Services security vulnerabilities and the two basic security models they follow.

XML and Web Services Security can be categorized into Trust and Threat Models.  The Threat Model helps identify both inbound and outbound threats and provides means of re-mediating such threats.  Trust Models ensure that message privacy and integrity are retained while ensuring appropriate authentication and authorization decisions are made before letting messages traverse a corporate network.

Threats: Three major threats are Denial-of-service attacks (DoS), Viruses, and SQL injections:

• DoS attacks prevent a user, or an organization, from accessing services or resources that they would normally be able to access.  Although this type of attack can cost time and money, usually there is no information loss involved. In the XML world a malformed XML document can cause a DoS attack.  For example, a malformed XML payload can come in the form of a deeply nested XML document that causes the back end application parser to go into a tail spin.
• A virus is a program, or a programming code, that replicates itself. Viruses are often found in email attachments, and downloaded files. They may erase data or damage the hard drive.  When a virus duplicates itself by resending itself as an attachment to an email or as a component of a network message, it is called a worm. There are three classes of viruses: file infections, system or boot-record infections, and macro viruses.  Viruses can use Web Services to enter corporate domains by going undetected through SOAP with attachments (MIME or MTOM).  Since such attachments are Base-64 encoded or maybe encrypted, traditional Anti-virus engines cannot match signatures to detect them.
  
• SQL injections are used to gain access to a database or retrieve information from a database.  This access is unauthorized and programs and applications are at risk of being attacked.  It is easy to defend programs and applications from SQL injections by using simple coding practices or by looking for malicious string patterns used for SQL injections. SQL injection attack in the XML world comes in the form of an inbound XML payload containing SQL injection commands.  If not detected correctly, these commands can reach the back end database and cause information to be stolen.

Trust: Three major categories of trust are privacy, integrity, and identity:

• When it comes to privacy, encryption protects personal information by encoding information.  This has to be done so that only the person or computer, with the private key can decode the information.
• Integrity insures that no one tampers with information.  Signatures and verification are both part of integrity. Signatures are strings of letters and numbers that represent a signature.  The message is scrambled with mathematical formulas or algorithms.  A key is needed to validate the signature.  Verification simply validates a users indeed signed a message with his private key.
• Identity involves authentication, authorization, access control and tokens.  Authentication verifies that information comes from a trusted source.  One must know who created the information, as well as be sure that the information has not been modified since created.  Authentication works closely with encryption to ensure that there is a secure environment.  Authorization is simply controlling the access and rights to resources, including things such as services or files.  Access control restricts what a user can do various resources.  There are many types of tokens including SSL tokens, SAML tokens, and WS-Username tokens.  Properly handing such Tokens both at the protocol and message level is crucial for establishing trust between business entities.

The threat and privacy/integrity/identity issues addressed in both of these models mitigate the most common XML vulnerabilities.  Thus, both Threat and Trust models should form the fabric of an enterprise's XML Security design.  From an actual implementation perspective,  an XML Gateway whose core engine is based on both models mitigate these common XML vulnerabilities thus ensuring robust exchange of XML data between enterprises.