What should every programmer know about security? – Stack Overflow.
Security is one of those topics you can’t possibly escape if you write any sort of code. It’s also one of those things that’s really difficult to study until you get bit at some point along the way and really learn your lesson. Being proactive about security is really important to me and I do my best to read applicable material on a consistent basis, even if it feels repetitive. There are some great line items and links in this Stack Overflow answer on best practices in security.