There are quite a few web application fuzzing (finding coding errors and security loopholes) programs out there that you can download for free and run against your code to verify that there are no issues that must be fixed before deploying the application.
Also, check out Kali Linux...this Linux distro has all of the below mentioned tools already installed plus more! https://www.kali.org/
A few are below:
OWASP (Open Web Application Security Project) ZAP (Zed Attack Proxy)
This tool can be used to find vulnerabilities on web applications and provides a very nice GUI for users to interact with.
Download Link: https://www.owasp.org/index.php/OWASP_Zed_Attack_Proxy_Project
Nikto: Open source web application vulnerability assessment scanner.
This tool is a CLI (Command Line Interface) tool that can be used to find vulnerabilities in your web application code and is open source.
Download Link: https://cirt.net/Nikto2
Nessus Home: Very well known and trusted web/vulnerability assessment framework and scanning engine
Nessus is the standard when it comes to web app scanning and vulnerability scanning and assessment...however it is only free for personal use, if you're using it for commercial use, you must buy a commercial license.
Download Link: https://www.tenable.com/products/nessus-home