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OWASP Top 10
The Open Web Application Security Project (OWASP) is a community dedicated to helping people and organizations with application security topics. If you'll be working as an AppSec expert, then OWASP should be your bible; they have plenty of help sections that will make your life much easier. Just follow their guidelines and tutorials at http://www.owasp.org.
The OWASP community defined the Top 10 vulnerabilities related to web applications. As for Mutillidae, it dedicated a menu to these vulnerabilities. On the left menu, you will see the OWASP items organized by year (the latest is the OWASP Top 10 for 2017; see the following screenshot). OWASP always keeps this list updated with the latest web vulnerabilities:
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I have dedicated a whole chapter to these vulnerabilities, later in this book. For the time being, try to get familiar with the menu items.