The earlier in the development process a vulnerability is found, the easier and cheaper it is to fix. For example, imagine a vulnerability is found during a penetration test in four or five month old code. It is very difficult for your developers, if they are still the same ones, to go back and understand the logic and the code that they have written five months before.
It can be even worse. A web developer introduces a vulnerability with his new code, but since no one is pointing it out to him he keeps on using similar code for the next few months. So over a few months, he can introduce a handful of vulnerabilities.
That’s why it is important to integrate automated web application security scanning in your continuous integration. Automated scanning does not replace the need for penetration tests, but it definitely helps to streamline the web development process, as Ferruh Mavituna well explained during episode 53 of Enterprise Security Weekly.