The main objective of this version is to prevent the hacking in Apple iOS devices which occurs through malicious pdf file. Another objective is to prevent the jailbreaking which occurs as a consequence of the previous effect.
In previous versions, the iOS device is easily vulnerable to attacks. It happens because of mishandling of fonts embedded in pdf file by iOS devices. It is quite common to download a pdf file through e-mail or web pages. Sometimes the downloaded file may be malicious and there is possibility that the file could inject malware into the iOS device — that gives a chance for the hackers to access the hardware of the iOS device.
iOS 4.3.4 has already been jail-breaked using RedsnOw and PwnageTool.
Read more: http://www.gizmocrazed.com/2011/07/apple-releases-ios-4-3-4-prevents-hacking-jailbreaking/#ixzz1SRzU7hdb