I'm not going to demonize the amount of information the CA wants; obviously it has to ask a wide variety of questions to cover the needs of all its members. However, some of these questions do get rather personal, and they're not always required. Here's some information you might opt to leave out:
1. Your Social Security Number
I've noticed that it's not uncommon for people to ask about my SSN for various things—for example, my community college, which wanted to use it in place of a Student ID number. However, the truth is that the only people who need to know this number are:
1) Government agencies (such as the DMV)
2) Your employer
3) A school your applying to only if you're applying for NEED-BASED aid.
You should never give out your SSN unless you absolutely have to. And if you either have no interest in financial aid or are applying for merit-based aid only, you don't have to. Besides, do you really want your most critical (and most exploitable) ID number floating around admissions offices?
2. Your Demographics (Race, Religion, etc.)
I have yet to see a college application that doesn't ask questions about race/ethnicity, religious preference, or something similar. These are for statistical purposes, and they usually don't affect your admissions decision. However, not all of us want to give out such personal information. Even though it isn't necessarily sensitive information, you might feel uncomfortable with the idea that this college officials are so interested in getting this information from you (regardless of whether you are considered "majority" or "minority").
If you are uncomfortable with giving demographic information for any reason, know that you don't have to answer ANY question in the "Demographics" section. And you shouldn't feel bad about it, either. I am a firm believer in opting out of giving personal information whenever possible.
3. Certain Family Questions
The CA has a page where they ask you a lot of questions about your family. Again, this is extremely personal information. Again, it's not that you're ashamed of your family, just as there's no reason to be ashamed of your race/religion. It's just that you might find it a bit peculiar that a bunch of people you've never met are so interested in finding out your mother's middle name. (Or maybe it's just me. But that's why I'll make a good security specialist.)
Keep an eye out for the "Required" fields on this page, because some questions are optional. It's important to note that, while this wasn't the case last year, the "sibling" section is now mandatory. I have no idea why.
4. Unnecessary Contact Information
Yes, they need to know your name, your email, and your phone. They also do need your home address, even though it's an online system, so that the colleges you're applying to can contact you. (Though you can opt out of this in your account settings.) But only give them as much as they need—for example, only one phone number (whichever one you use most often. Only give them another number if you really think both are necessary for colleges to stay in contact with you.
(This isn't relevant, but I would like to say that I'm very amused by the fact that the CA no longer asks for your AIM address. That always cracked me up.)
BONUS: Stuff the CA Does Need to Know
Here's some info you don't want to leave out.
- Some form of contact information. They ask for email and phone, but they'll probably only use your email.
- The "Geography" section. This used to be listed under "Demographics", but has been separated because it's all required and more relevant. A college needs to know whether or not you are a citizen of the country in which the institution is located. In addition, if you are from a country that speaks a language other than the primary one spoken at said institution, the CA wants to be sure that you're good enough at the institution's language to learn successfully.
- Most family information. I don't know why they want it, but most of it's required.
- Self-reported grades and test scores. You might not know what your class rank is, but you can calculate your GPA, and if you took any standardized tests you probably know your scores by now. Colleges will accept all of this information as unofficial reports they can look at while they wait for your official scores and transcripts. (Just be sure you've got accurate information—you don't want to appear unreliable.)
- Anything that makes your application more cohesive. You might find that leaving out certain pieces of information, even if they aren't required, leaves some parts of your application unclear. (They may not be required, but they might help to elaborate more on your answers to required questions.) If this is the case, and you feel comfortable answering those questions, then go ahead and do so.
In the end, the chances that the CA is trying to scam you/is going to be ambushed by a wild gang of hackers with a preference for college admission websites is pretty miniscule. But it never hurts to take a few extra steps to protect yourself—it could really save you a lot of grief in the long run.