Security considerations for web services

When you configure Web Services Security, you should make every effort to verify that the result is not vulnerable to a wide range of attack mechanisms. There are possible security concerns that arise when we are securing web services.

In WebSphere Application Server, when you enable integrity, confidentiality, and the associated tokens within a SOAP message, security is not guaranteed. This list of security concerns is not complete. We must conduct our own security analysis for the environment.

• Ensuring the message freshness

  Message freshness involves protecting resources from a replay attack in which a message is captured and resent. Digital signatures, by themselves, cannot prevent a replay attack because a signed message can be captured and resent. IBM recommends that you allow message recipients to detect message replay attacks when messages are exchanged through an open network. We can use the following elements, which are described in the Web Services Security specifications, for this purpose:

  Timestamp

  We can use the timestamp element to keep track of messages and to detect replays of previous messages. The WS-Security 2004 specification recommends that you cache time stamps for a given period of time. As a guideline, we can use five minutes as a minimum period of time to detect replays. Messages containing an expired timestamp are rejected.

  Nonce

  A nonce is a child element of the <UsernameToken> element in the UsernameToken profile. Because each nonce element has a unique value, recipients can detect replay attacks with relative ease.

  Important: Both the time stamp and nonce element must be signed. Otherwise, these elements can be altered easily and, therefore, cannot prevent replay attacks.

• Use XML digital signature and XML encryption properly to avoid a potential security hole

  The Web Services Security 2004 specification defines how to use XML digital signature and XML encryption in SOAP headers. Therefore, users must understand XML digital signature and XML encryption in the context of other security mechanisms and their possible threats to an entity. For XML digital signature, you must be aware of all of the security implications resulting from the use of digital signatures in general and XML digital signature in particular. When you build trust into an application based on a digital signature, you must incorporate other technologies such as certification trust validation based upon the Public Key Infrastructure (PKI). For XML encryption, the combination of digital signing and encryption over a common data item might introduce some cryptographic vulnerabilities. For example, when you encrypt digitally signed data, you might leave the digital signature in plain text and leave the message vulnerable to plain text guessing attacks. As a general practice, when data is encrypted, encrypt any digest or signature over the data. For more information, see http://www.w3.org/TR/xmlenc-core/#sec-Sign-with-Encrypt.

• Protecting the integrity of security tokens

  The possibility of a token substitution attack exists. In this scenario, a digital signature is verified with a key that is often derived from a security token and is included in a message. If the token is substituted, a recipient might accept the message based on the substituted key, which might not be what you expect. One possible solution to this problem is to sign the security token (or the unique identifying data from which the signing key is derived) together with the signed data. In some situations, the token that is issued by a trusted authority is signed. In this case, there might not be an integrity issue. However, because application semantics and the environment might change over time, the best practice is to prevent this attack. We must assess the risk assessment based upon the deployed environment.

• Verify the certificate to leverage the certificate path verification and the certificate revocation list

  IBM recommends that you verify that the authenticity or validity of the token identity used for digital signature is properly trusted. Especially for an X.509 token, this issue involves verifying the certificate path and using a certificate revocation list (CRL). In the Web Services Security implementation in WAS v6 and later, the certificate is verified by the <TokenConsumer> element. WebSphere Application Server provides a default implementation for the X.509 certificate that uses the Java CertPath library to verify and validate the certificate. In the implementation, there is no explicit concept of a CRL. Rather, proper root certificates and intermediate certificates are prepared in files only. For a sophisticated solution, you might develop our own TokenConsumer implementation that performs certificate and CRL verification using the online CRL database or the Online Certificate Status Protocol (OCSP).

• Protecting the username token with a password

  IBM recommends that we do not send a password in a username token to a downstream server without protection. We can use transport-level security such as SSL (for example, HTTPS) or use XML encryption within Web Services Security to protect the password. The preferred method of protection depends upon the environment. However, you might be able to send a password to a downstream server as plain text in some special environments where you are positive that you are not vulnerable to an attack.

Secure web services involves more work than just enabling XML digital signature and XML encryption. To properly secure a Web service, you must have knowledge about the PKI. The amount of security that we need depends upon the deployed environment and the usage patterns. However, there are some basic rules and best practices for securing web services. IBM recommends that you read some books on PKI and also read information on the Web Services Interoperability Organization (WS-I) Basic Security Profile (BSP).

Subtopics

• 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.

• Basic Security Profile compliance tips
  The Web Services Interoperability Organization (WS-I) Basic Security Profile (BSP) 1.0 promotes interoperability by providing clarifications and amplifications to a set of nonproprietary web services specifications. WebSphere Application Server Web Services Security provides configuration options to ensure that the BSP recommendations and security considerations can be enabled to ensure interoperability. The degree to which you follow these recommendations is then a measure of how well the application you are configuring complies with the Basic Security Profile (BSP).

• Distributed nonce cache
  In previous releases of WAS, the nonce was cached locally. WAS v6 and later use distributed nonce caching. The distributed nonce cache makes it possible to replicate nonce data among servers in a WebSphere Application Server cluster.

Related concepts

Overview of standards and programming models for web services message-level security

Related information:

Basic Security Profile Version 1.0