Apple's release note states that, "An attacker with a privileged network position may capture or modify data in sessions protected by SSL/ TLS. Other attacks involving X.509 certificate validation may also be possible. This issue is addressed through improved validation of X.509 certificate chains." It means that, iOS 4.3.5 fixes a flaw in digital certificate validation that could let hackers with a "privileged network position" to crack an encrypted session and steal your personal data without your knowledge, preventing a man-in-the-middle attack.
As usual, you can download and update the iOS 4.3.5 by connecting your iDevices to the iTunes. The total size of update is about 666 MB! This huge file size is due to the fact that Apple is downloading the entire iOS update to fix a small patch. However, Apple has fixed this in their next major update, iOS 5, which will be able to push only the patches, rather than the entire iOS.
iOS 4.3.5 update supports iPhone 4 (GSM model), iPhone 3GS, iPad 2, iPad, iPod touch (4th generation) and iPod touch (3rd generation).