Web Service Security Cheat Sheet

= Introduction =

This article is focused on providing guidance to securing web services and preventing web services related attacks. Please notice that due to the difference of implementation between different frameworks, this cheat sheet is kept at high level.

Transport Confidentiality
Transport confidentiality protects against eavesdropping and man-in-the-middle attacks against web service communications to\from the server.

Rule - All communication with and between web services containing sensitive features, an authenticated session, or transfer of sensitive data must be encrypted using well configured TLS. For more information see Transport Layer Protection Cheat Sheet.

Server Authentication
Transport level authentication verifies the identity of the user or the system trying to connect to the service. Usually, transport authentication is a functional of the container of the web service.

Rule - Basic Authentication has to be conducted using HTTP over SSL

Rule - Client Certificate Authentication using HTTP over SSL to be used if the client and server need to authenticate each other

Transport Encoding
SOAP encoding styles are meant to move data between software objects into XML format and back again.

Rule - Enforce the same encoding style between the client and the server.

Consumer Authentication
Rule - The Message Authentication over SSL mechanism attaches a cryptographically secured identity or authentication token with the message and use SSL for confidentiality protection.

Message Integrity
Encryption does guarantee confidentiality but it does not guarantee integrity as the message can be changed en route. In addition, encryption does not ensure the identity of the sender.

Rule - Use XML signatures to ensure message integrity using the sender's private key. Signature can be validated by the recipient using the sender’s digital certificate.

Message Confidentiality
Data elements meant to be kept confidential must be encrypted using a strong encryption cipher with an adequate key length to deter brute forcing.

Rule - SOAP Messages must be encrypted using a strong encryption cipher.

Authorization
Web services need to authorize web service clients the same way web applications authorize users. A web service needs to make sure a web service client is authorized to: perform a certain action (coarse-grained); access the requested data (fine-grained).

Rule - A web service should authorize its clients whether they have access to the method in question. This can be done using one of the following methods:


 * Having clients authorize to the web service using username and password
 * Having clients authorize to the web service using client certificates

Schema Validation
Schema validation enforces constraints, syntax and semantics defined by the schema.

Rule - Web services must validate SOAP payloads against the web service schema.

Content Validation
Rule - Like any web application, web services need to validate input before consuming it. Content validation include:


 * Validation against malformed XML entities
 * Validation against XML Bomb attacks
 * Validating inputs using a strong white list
 * Validating against external entity attacks

Output Encoding
Web services need to ensure that output sent to clients is encoded to be consumed as data and not as scripts. This gets pretty important when web service clients use the output to render HTML pages either directly or indirectly using AJAX objects.

Rule - All the rules of output encoding applies as per XSS (Cross Site Scripting) Prevention Cheat Sheet.

Virus Protection
SOAP provides the ability to attach files and document to SOAP messages. This gives the opportunity for hackers to attach viruses and malware to these SOAP messages.

Rule - Ensure Virus Scanning technology is regularly updated with the latest virus definitions / rules.

Message Size
Web services like web applications could be a target for DOS attacks by automatically sending the web services thousands of large size SOAP messages. This either cripples the application making it unable to respond to legitimate messages or it could take it down entirely.

Rule - SOAP Messages size should be limited to an appropriate size limit. Larger size limit (or no limit at all) increases the chances of a successful DOS attack.

Message Throughput
Throughput represents the number of Web Service requests served during a specific amount of time. Obsviously this depends on many factors including the hardware, the containser of the Web Service ... etc.

Rule - Configuration should be optimized for maximum message throughput to avoid running into DoS-like situations.

Endpoint Security Profile
Rule - Web Services must be compliant with Web Services-Interoperability (WS-I) Basic Profile at minimum.

Audit Logging
Rule - Security related activities such as successful and unsuccessful login attempts must be logged into a protected environment..

XML Denial of Service Protection
XML Denial of Service is probably the most serious attack against Web Services. So the Web Service must provide the following validation:

Rule - Validation against recursive payloads

Rule - Validation against oversized payloads

Rule - Protection against XML entity expansion

Administration
Rule - Ensure access to administration and management functions within the Web Service Application is limited to Web Service administrators.

= Authors and Primary Editors =

Gunnar Peterson Sherif Koussa