Does an iPhone have a firewall built in?

If you're on an unsecured network, a person would have to install SSH over the air to get access to the files on your phone. That currently isn't possible.

One thing that can be done is snooping of your network activity if you're on un-secure websites.

I would avoid accessing any email account over a public WiFi network that does not support using SSL for the incoming and outgoing mail server - and if not, don't use the account to download or send any sensitive information.

When using a public WiFi network - any besides a WiFi network at your home or at your office/place of business, one must assume the worst - that any data transferred to or from a computer or device is being captured and act accordingly. Using a firewall doesn't help in this area.

You're welcome.

SSL is the abbreviation for Secure Sockets Layer. All email account providers do not support this with their incoming and outgoing mail servers. Apple supports this for both with a .Mac account and many other email account providers do also - you need to check with your email account provider for an email that you access with the iPhone. Regardless, I wouldn't feel comfortable exchanging any sensitive information via email when connected to a public WiFi network but if you do, this should be done with an email account provider that supports using SSL for their incoming and outgoing mail servers to be on the safe side.

This link provides some basic information on SSL.

http://www.webopedia.com/TERM/S/SSL.html

This link provides some basic information on SSH.

http://www.webopedia.com/TERM/S/SSH.html

gdgmacguy is correct, there is no user-configurable firewall. However, the iPhone listens on remarkably few TCP ports -- one to be precise. I forget which one it is, it's up in the 60,000 range, and for me it disconnected immediately after completing the three-way handshake. I'm not sure what it's there for, but if it immediately kills the connection without sending or receiving data, I'm not sure how much of a potential threat it is.

Given that, and given the general robustness of the underlying OS (assuming it's still very darwin-ish), I'm not very worried about getting hacked simply by connecting to unknown wifi networks. SLAPSHOTW's and Allan Sampson's concerns are perfectly valid, however.

There's no built in firewall