Overview
Performing validation in client side code, generally JavaScript, provides no protection for server-side code. An attacker can simply disable JavaScript, use telnet, or use a security testing proxy such as WebScarab to bypass the client side validation.
Consequences
- Unvalidated input corrupts business logic (XSS, injection, etc...)
Exposure period
Platform
- Languages: any
- Operating platforms: any
Required resources
Severity
Very high -- allows malicious input to be used in business logic.
Likelihood of exploit
Very likely
Avoidance and mitigation
Validation must be performed in the business layer.
Discussion
Client-side validation is widely used, but is not security relevant. The only advantage of client-side validation, from a security perspective, is that intrusion detection may be more effective. Because only pre-validated input should arrive at an application (assuming JavaScript is enabled), it can be assumed that any bad input represents an attack. The application can respond much more harshly if it knows that it is under attack. Some responses might be session invalidation, logging, or notification to an administrator.
