Recently apple released iOS 4.3.4 to remove the jailbreakMe exploit. Just week after that apple has now release iOS 4.3.5 which is suppose to fix security vulnerabilities with certificate validation.
All Apple hardware that can run iOS 4 is supported. Verizon iPhone has a similar fix available, labeled iOS 4.2.10.
Users which are using non-jailbreak iPhone, iPad, iPod touch are recommended to download this update as soon as possible. You can grab the update by plugging your iOS device into iTunes and hitting the “Check for Update” button. It will automatically start downloading and installing updates.
However, If you have a jailbroken device, you are advised to stay as far away from this update as possible.
Direct Download Links for iOS 4.3.5 (Official from Apple)

How to fix various problems including Bluetooth on your iPhone, and how to reboot iPhone 7 and other iPhone devices
How to activate Universal Clipboard on iOS 10 and macOS Sierra supported Apple devices
Check your iPhone, iPad and iPod touch compatibility with iOS 10 and how to obtain the iOS 10.0.1 version
YouTube Live Stream Video for iPhone, iPad, iPod Touch, Coming Soon