WS-Security: Is It Enough to Secure Your SOAP Web Services?

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What Is WS-Security?

Web Services Security, also known as WS Security, is an extension to the SOAP specification, which specifies how to secure SOAP web services from external attacks. WS Security offers a set of API security measures that can help ensure security for SOAP-based messages, through the implementation of several principles that help achieve confidentiality, authentication and integrity.

Important aspects of the WS security standard include:

  • SOAP web services operate independently of specific hardware and software implementations. This means that WS Security protocols must be flexible enough to accommodate new security mechanisms as well as provide alternative mechanisms when a certain approach does not work. 
  • SOAP-based messages traverse multiple intermediaries. Security protocols must be able to identify fake nodes as well as prevent data interpretation at any nodes. 

Related content: Read our guide to SOAP security

In this article:

  • The WS-Security Standard
  • WS Security Threats and Countermeasures
  • Web Service Security Best Practices
    • Ensure Transport Confidentiality
    • Maintain Message Integrity
    • Maintain Message Confidentiality
    • Prevent XML DoS Attacks
    • Ensure Availability
    • Test SOAP APIs with Dynamic Application Security Testing (DAST)

The WS-Security Standard

Here are the three main mechanisms described by the WS Security standard:

  • Signing SOAP messages – how to do this in a way that ensures both integrity and non-repudiation.
  • Encrypting SOAP messages – the standard explains how you can encrypt SOAP messages in a manner that ensures confidentiality.
  • Attaching security tokens – the standard recommends attaching security tokens in a way that helps ascertain the identity of the sender.

The WS Security specification considers a wide range of encryption algorithms and signature formats, as well as multiple trust domains. The standard is also open to a variety of security token models, including user ID/Password credentials, Kerberos tickets, X.509 certificates, SAML assertions, and custom-defined tokens. You can find information about semantics and token formats in the associated profile documents.

Can WS Security work as a complete security solution?

The WS-Security standard defines security measures that are incorporated within the header of a SOAP message. This means WS Security measures work in the application layer. By themselves, these mechanisms do not provide a complete security solution and WS Security alone does not offer a complete security guarantee for Web services. 

The WS Security specific serves as a building block, which you should use alongside other Web service extensions as well as with higher-level application-specific protocols. Ideally, your security should accommodate a variety of security technologies and security models. 

Note that trust bootstrapping key management, federation and agreement on the technical details (such as ciphers, algorithms, and formats) is outside the scope of WS Security. When you implement and use the WS Security framework and syntax, you are responsible to ensure that your result is not vulnerable. 

WS Security Threats and Countermeasures

To protect web services, you can use the HTTPS protocol, which helps establish secure communication between client and server over the web. To achieve secure communication, the HTTPS protocol uses the Secure Sockets Layer (SSL) protocol, which ensures that both client and server have a digital certificate that validates their identity. 

Here are several steps that occur during HTTPS communication between clients and servers:

  1. A client uses the client SSL certificate to send a request to a certain server. 
  2. The server receives the client certificate. 
  3. The server makes a note in the cache system. This ensures the server knows that a response to this request should go back only to this specific client.
  4. In order to authenticate itself to the client, the server sends its own certificate. This validation occurs to ensure the client achieves communication with the correct server.
  5. All future communication between the client and server is encrypted. If threat actors attempt to breach security and obtain this data, the encryption can prevent them from making use of the data.

The above security is effective in some cases, but does not offer complete protection for web services. For example, when a client talks to multiple servers, or when a client talks to a web server and a database simultaneously. In these scenarios, not all information can be transmitted through the HTTPS protocol.

SOAP and WS Security

WS Security specifications recommend applying several security measures to the SOAP security protocol. These measures should be defined within the SOAP header element, which can contain the following information:

  • If a message in the SOAP body is signed with any security key, then that key can be defined in the header element.
  • If any element in the SOAP body is encrypted, then the header should contain all necessary encryptions keys. This ensures that the message is decrypted once it reaches its destination.

Here is how the above SOAP authentication techniques can help contribute to the security of multiple server environments:

  • Only the host server can decrypt the SOAP body—by encrypting the SOAP body, you ensure that it can only be decrypted by the web server hosting your web service. 
  • A database server cannot read the message—if a message is transferred to a database server via an HTTP request, the database cannot decrypt it. This is because it does not have the mechanisms needed to decrypt the message.
  • Only a SOAP protocol message can be decrypted—a request can be decrypted only when it reaches the web server in the form of a SOAP protocol. The server can then decipher the message and send the relevant response back to the client.

Web Service Security Best Practices

In addition to using the WS Security standard, you can ensure your web services are secure by following these best practices.

Related content: For even more best practices, read our guide to general API security best practices

Ensure Transport Confidentiality

Transport confidentiality can help you protect against attacks like Man-in-the-Middle (MITM) and eavesdropping attacks trying to intercept, delete or modify communications made to and from your server. You should always assume that all communication with and between web services contain sensitive data.

Data transfer of any kind, particularly sensitive or regulated information, as well as authenticated sessions, must always be encrypted with properly-configured Transport Layer Security (TLS) protocols. The protocol offers additional benefits, such as protection against replay attacks and server authentication. 

Maintain Message Integrity

TLS can help you maintain the integrity of data at rest. This means you should implement TLS even if the content of a message is already encrypted. This is because TLS also protects the integrity of the transmitted data.

Public keys can protect the confidentiality of data but do not protect its integrity. This is because a public key may be accessed by others. Additionally, public-key encryption cannot maintain the identity of a sender.

To maintain XML data integrity, you need to use XML digital signatures. This process uses the private key of the sender for encryption. It enables recipients to validate the signature by using the public key of a sender. 

Maintain Message Confidentiality

You must always encrypt data with a strong encryption key. Ideally, the length of the key should be adequate to prevent brute force attacks. Here are several aspects to consider when implementing message confidentiality:

  • Identify messages containing sensitive or regulated data—always use strong encryption keys to protect sensitive data. You can implement both message encryption and transport encryption when the data is in transit. 
  • Identify sensitive data that must remain encrypted when at rest—always use strong encryption processes to protect this data. Transport encryption is not sufficient in this case. 

Prevent XML DoS Attacks

XML Denial-of-Service (DoS) attacks are highly common in web services and are arguably the most dangerous attacks that occur within web service environments. Here are two methods that can help you achieve XML DoS prevention:

  • Validate—always validate against XML entity expansion, oversized payloads and recursive payloads. You should also validate against overlong element names, especially in SOAP-based web services.
  • Test—develop test cases that can help you simulate and determine whether the XML parser or schema validator is capable of defending against XML DoS attacks. 

Ensure Availability

When attacks occur, a web service might need more resources than are available. This can lead to a state of instability and may result in denial of service. When configuring your resource usage, applying the following limits to ensure availability during attacks:

  • The number of CPU cycles—to ensure stability, you should set this limit according to the expected service rate. 
  • The amount of memory—to prevent system crashes, set a limit on the amount of memory the web service is allowed to use. 
  • The number of simultaneous operations—to ensure stability, set a limit to the number of processes, open files and network connections allowed to work simultaneously.

It is essential to prioritize security and ensure continuous protection for your web services. However, security should not come at the expense of availability. When implemented correctly, these WS security measures should help you maintain availability when attacks occur. When systems fail, you should be able to resume normal operations quickly and efficiently.

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