[jamiesonbecker]

Exactly.

In other words, some fraction of the requests are responded to dynamically and then the result is cached. That dynamic nature can be exploited. Site search engines, etc, are also often (but not always) dynamic, server-generated results that have a greater likelihood of exploits via XSS, CSRF, SQL Injection, etc. Login forms almost always require server interaction and are great targets.

(I say "almost" because REST interactions might be stateless and thus login forms really just serve to generate an access token and verify that it's working; this is how Userify works, for instance. It's still theoretically more exploitable than pure static files, but it raises the bar quite a bit.)