Hey Andrew,
Black: DoS attacks - employ tinyget or similar to simulate DoS attacks, see what your app does.
Black: Canonicalization attacks. Mentioned a bit, may be special focus can be on a directory traversal attack in case of downloads.
White: Usage of cookies for the sensitive info? See cookies are not used for sensitive data and are not persisted locally over the intented interval.
Black: Sniff in the temp IE/XYZ folder for cookies.
Black: Again, use scripted tinyget or try manually to see if brute force password guess would work or if you app has smart delays/denials for a password guess attacks.
Black: Do any of the attacks and see if admin is notified automatically of the attack or it is only the attacker who knows about it Image may be NSFW.
Clik here to view.
.
"Make sure your security decisions do not rely on HTTP headers info" - http headers are used for ntml/kerberos authentication? May be just don't use them stupidly, don't invent or rely on referer, etc?
General: Employ a commercial black/white-box security scanner, can be expensive but can be hard to do security regression tests otherwise.
Black: DoS attacks - employ tinyget or similar to simulate DoS attacks, see what your app does.
Black: Canonicalization attacks. Mentioned a bit, may be special focus can be on a directory traversal attack in case of downloads.
White: Usage of cookies for the sensitive info? See cookies are not used for sensitive data and are not persisted locally over the intented interval.
Black: Sniff in the temp IE/XYZ folder for cookies.
Black: Again, use scripted tinyget or try manually to see if brute force password guess would work or if you app has smart delays/denials for a password guess attacks.
Black: Do any of the attacks and see if admin is notified automatically of the attack or it is only the attacker who knows about it Image may be NSFW.
Clik here to view.

"Make sure your security decisions do not rely on HTTP headers info" - http headers are used for ntml/kerberos authentication? May be just don't use them stupidly, don't invent or rely on referer, etc?
General: Employ a commercial black/white-box security scanner, can be expensive but can be hard to do security regression tests otherwise.