A missing timestamp can leave a SOAP message open to replay attacks.
A Security timestamp indicates a message's freshness. If an attacker intercepts a message retransmits it at a later time, the receiver can reject the replay attack because the timestamp will indicate that the message is stale. Optionally, timestamps can include an expiration attribute which places a hard limit on how long security semantics are valid.
To prevent attackers from tampering with timestamps, timestamps should be signed. Without a signed timestamp, it might be possible to intercept a SOAP message, modify the timestamp, and send the message on without the receiver's knowledge. Under these circumstances, an attacker can potentially trick a recipient into accepting a malicious message.
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A10 Insecure Configuration Management
[2] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A6 Security Misconfiguration
[3] Standards Mapping - FIPS200 - (FISMA) CM
[4] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 254
[5] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) Insufficient Authentication
[6] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.10
[7] Web Services Handbook for WebSphere Application Server 6.1 IBM Redbooks