Just saw this for the first time today: http://cwe.mitre.org/top25/
I'd always relied on this: https://www.owasp.org/index.php/Category:OWASP_Top_Ten_Project
Both are really good lists of security vulnerabilities.
I once had to listen to a DBA tell me that "we don't know what we don't know" as a way of saying that there was no way to be sure that a web app was "secure". That comment lead the project manager to go through the classic "risk exposure" exercise (and hours of discussion) to determine that security mattered. We defined the risks, the costs and the probability of occurrence so that we could document all kinds of potential exposures or something.
Instead of hand-wringing, these kinds of simple lists of the common vulnerabilities provides actionable steps for design, code, test and audit of operations. Further, they guide selection, configuration and operation of web server technology to assure that the vulnerabilities are addressed.