Recently, I came across some code of a web application that, on brief inspection, was vulnerable to XSS and SQL injection attacks : the SQL queries and the HTML output were not properly escaped, the input variables were not sanitized. After a bit more reviewing I made a list of measures and notified the developer who quickly fixed the issues.
I was a bit surprised to come across code that was very insecure, which took the author only a few hours to drastically improve with a few simple changes. I started wondering why the code wasn't of better quality in the first place? Did the developer not know about vulnerabilities like SQL injection and how to prevent them? Was it time pressure that kept him from writing safer code?
Anyway, there are a few guidelines to write better and safer code.
Use frameworks and librariesAbout every language has a framework for web applications (Drupal, Symfony (PHP), Spring (Java), Django (Python), ...) that has tools and libraries for creating forms, sanitizing input variables, properly escaping HTML output, handling cookies, check authorization and do user and privileges management, database-object abstraction (so you don't have to write your own SQL queries) and much more.
Those frameworks and libraries are used by a lot of applications and developers, so they are tested much more than code you write yourself, so bugs are found more quickly.
It is also important to regularly update the libraries and frameworks you use, to have the latest bugs and vulnerabilities fixed.
Code reviewMore people see more than one. Have your code reviewed by a coworker and use automated tools to check your code for vulnerabilities. Most IDEs have code checking tools, or you can implement them in a Continuous Integration (CI) environment like Jenkins, Travis CI, Circle CI, ... to check your code during every build.
A lot of online code checking tools exist that can check your code every time you push your code to your version control system.
There is no silver bullet here, but a combination manual code review and automated checks will help to spot vulnerabilities sooner.
Test your codeCode reviewing tools can't spot every bug, so testing your code is important as well. You will need automated unit tests, integration tests, ... so you can test your code during every build in you CI environment.
Writing good tests is an art and takes time, but more tests means less possible bugs remaining in your code.
Coding styleWhile not directly a measure against vulnerabilities, using a coding style that is common for the programming language you are using, makes your code more readable both for you, the reviewer and future maintainers of your code. Better readability makes it easier to spot bugs, maintain code and avoid new bugs.
I guess there are many more ways to improve code quality and reduce vulnerabilities. Feel free to leave a comment with your ideas.