Apple recently updated iOS to version 4.0.2 in order to fix a security vulnerability discovered and exploited by web-based jailbreaking tool JailbreakMe. The security hole is found in Mobile Safari, and it allows malicious PDF files to take control of an unsuspecting victim’s device.
Unfortunately, first generation iPod Touch and iPhone units are still at risk, given that iOS 4.0.2 is not supported by those devices. As a matter of fact, those who downgraded their iPhone 3G to iOS 3.1.3 because of the performance issues presented by iOS 4 are also at risk. Ironically, the only existing way to protect the aforementioned devices against the PDF vulnerability is to jailbreak them.
You see, Jay Freeman, also known as @saurik, has made available a patch properly named “PDF Patch” via Cydia. The PDF Patch is said to fix the vulnerability on devices running iOS all the way back to version 2.x
via Engadget