Nonce, a randomly generated token

Nonce is a randomly-generated, cryptographic token used to prevent replay attacks. Although nonce can be inserted anywhere in the SOAP message, it is typically inserted in the <UsernameToken> element.

Without nonce, when a UsernameToken is passed from one machine to another machine using a nonsecure transport, such as HTTP, the token might be intercepted and used in a replay attack. The same password might be reused when the user name token is transmitted between the client and the server, which leaves it vulnerable to attack. The user name token can be stolen even if we use XML digital signature and XML encryption. However, nonce alone, used in a non-secure transport, cannot adequately address the replay problem. Nonce is most useful when the SOAP message is transmitted via a communication channel that is secured, either at the transport level, or at the message level.

To help eliminate these replay attacks, the <wsse:Nonce> and <wsu:Created> elements are generated within the <wsse:UsernameToken> element and used to validate the message. The server checks the freshness of the message by verifying that the difference between the nonce creation time, which is specified by the <wsu:Created> element, and the current time falls within a specified time period. Also, the server checks a cache of used nonces to verify that the user name token in the received SOAP message has not been processed within the specified time period. These two features are used to lessen the chance that a user name token is used for a replay attack.

To add a nonce for the UsernameToken, we can specify it in the token generator for the user name token. When the token generator for the UsernameToken is specified, we can select the Add nonce option to include nonce in the user name token.

Related:

Web Services Security enhancements

Distributed nonce cache

Distributing nonce caching to servers in a cluster