Every Action Form must have a corresponding validation form.
If a Struts Action Form Mapping specifies a form, it must have a validation form defined under the Struts Validator. If an action form mapping does not have a validation form defined, it might be vulnerable to a number of attacks that rely on unchecked input.
Unchecked input is the root cause of some of today's worst and most common software security problems. Cross-site scripting, SQL injection, and process control vulnerabilities all stem from incomplete or absent input validation. Although J2EE applications are not generally susceptible to memory corruption attacks, if a J2EE application interfaces with native code that does not perform array bounds checking, an attacker may be able to use an input validation mistake in the J2EE application to launch a buffer overflow attack.
An action or a form may perform validation in other ways, but the Struts Validator provides an excellent way to verify that all input receives at least a basic level of checking. Without this approach, it is difficult, and often impossible, to establish with a high level of confidence that all input is validated.
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A1 Unvalidated Input
[2] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A6 Security Misconfiguration
[3] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3510 CAT I
[4] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 108
[5] Standards Mapping - Payment Card Industry Data Security Standard Version 1.2 - (PCI 1.2) Requirement 6.3.1.1
[6] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.1
[7] Standards Mapping - FIPS200 - (FISMA) SI
[8] T. Husted et al. Struts in Action: Building Web Applications with the Leading Java Framework Manning Publications
[9] The Struts project The Apache Foundation