While the update includes several refinements--including better cellular network switching on the new iPad and improved HDR photography reliability--it also patches three serious vulnerabilities that could be exploited by attackers to potentially execute arbitrary code on a device.
The updated version of iOS will work on iPhone 3GS, iPhone 4, iPhone 4S, iPod Touch (3rd generation and later), as well as the iPad, iPad 2, and new iPad.
The first vulnerability patched in iOS 5.1.1 involves Safari. It would allow a malicious website "to spoof the address in the location bar," so users could be surfing a fake site that appeared to be real, according to an overview of the security issues from Apple. The company said it's addressed the issue "through improved URL handling," and that the bug isn't present in Mac OS X versions of Safari.
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The other two vulnerabilities are in the WebKit open source Web browser engine. The first WebKit bug, which was discovered by the Google Chrome Security Team, would allow an attacker to create a malicious website that launches attacks that unexpectedly terminate applications or even execute arbitrary code.
Anyone with a jailbroken device can also update to iOS 5.1.1, provided their device uses an A4 chip. "That excludes newer devices such as the iPhone 4S, the iPad 2, and the iPad which came after the iPad 2," said Paul Ducklin, head of technology for Sophos in the Asia Pacific region, in a blog post. "As with iOS 5.1, it's a tethered jailbreak. That means you need to connect your device to your computer and use the jailbreaking tool when you reboot."
Apple has been furiously updating its operating systems recently, most notably to patch OS X users against the Flashback malware, or any other malicious code that might like to target the underlying Java vulnerability. But not all of the patches have been going smoothly.
For example, the latest OS X Lion security update (10.7.3) has a bug that stores as plaintext the passwords for the optional Apple FileVault system that encrypts Mac user's home folders. The bug appears to affect only users who upgraded from OS X Lion to Snow Leopard and who retained FileVault instead of upgrading to FileVault 2, which encrypts the entire hard disc.
News of the bug, which appears to date from February, first appeared Friday when David Emery, a consultant at DIE Consulting, detailed the flaw to a cryptography email list. "Someone, for some unknown reason, turned on a debug switch (DEBUGLOG) in the current released version of MacOS Lion 10.7.3 that causes the authorization host process's HomeDirMounter DIHLFVMount to log in *PLAIN TEXT* in a system-wide logfile readable by anyone with root or admin access the login password of the user of an encrypted home directory tree ('legacy Filevault'). The log in question is kept by default for several weeks," he wrote.
But these plaintext passwords can persist much longer if they end up being saved to external media as part of automated Apple OS X Time Machine backups. "Vulnerable users who do not encrypt their Time Machine backups risk replicating this log file to their backups, which could mean long-term storage of their unencrypted password," said Chester Wisniewski, a senior security advisor at Sophos Canada, in a blog post.
All 10.7.3 users should change their FileVault passwords and be sure to not reuse their passwords elsewhere, he said. Furthermore, even after Apple issues a fix, any FileVault users on OS X Lion should assume that their previous password remains compromised, since it may continue to be stored in plaintext form in Time Machine.
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