Unencrypted messages fail to protect confidentiality.
Encryption at the SOAP message level ensures true end-to-end confidentiality. SOAP messages can be sent over a number transport protocols such as HTTPS, HTTP, TCP, SMTP, UDP, etc. Message-level encryption ensures the confidentiality of the message regardless of the transport protocol. A common scenario among web services is to relay SOAP messages between services, so message-level encryption means message senders and receivers do not need to worry about all transport security between relay points.
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A10 Insecure Configuration Management
[2] Standards Mapping - OWASP Top 10 2007 - (OWASP 2007) A9 Insecure Communications
[3] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A9 Insufficient Transport Layer Protection
[4] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3250.1 CAT I, APP3250.2 CAT I, APP3250.3 CAT II, APP3250.4 CAT II
[5] Standards Mapping - FIPS200 - (FISMA) CM
[6] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 311
[7] Standards Mapping - SANS Top 25 2009 - (SANS 2009) Insecure Interaction - CWE ID 319
[8] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) Insufficient Authentication
[9] Standards Mapping - SANS Top 25 2010 - (SANS 2010) Porous Defenses - CWE ID 311
[10] Standards Mapping - Payment Card Industry Data Security Standard Version 1.2 - (PCI 1.2) Requirement 4.1, Requirement 6.3.1.4, Requirement 6.5.9
[11] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 4.1, Requirement 6.5.10
[12] Standards Mapping - Payment Card Industry Data Security Standard Version 2.0 - (PCI 2.0) Requirement 4.1, Requirement 6.5.4
[13] Web Services Handbook for WebSphere Application Server 6.1 IBM Redbooks