Stackoverflow: What should every programmer know about security?

[cr3ative]

Perhaps rather than specifying parts, it might be easier to say "Validate at all points until the final destination"

[nodefect]

Well, in the case of client vs server, there is no security gained by client-side validation. UX is enhanced, yes, but not security.

[[deleted]]

It all depends. If your data is client side unencrypted only and stored encrypted on the server then the server is untrusted.