There is a flaw in lockdownd:
MOVW R0, #(aPrivateVarDbTi-0x4DB8A) ; "/private/var/db/timezone" MOVW R1, #0x1FF ; mode_t -> 0777 MOVT.W R0, #4 ADD R0, PC ; char * BLX _chmod
chmod("/private/var/db/timezone",0777) without any further checks and is executed on every launch. By setting a symbolic link on
/var/db/timezone though MobileBackup and pointing the symlink to any other file and crashing lockdownd by sending it a malformed property list (see Malformed PairRequest) to make it relaunch causes it to perform the actual permission change on any file.
This vulnerability is CVE-2013-0979 and Apple describes it in the iOS 6.1.3 security fixes like this:
Impact: A local user may be able to change permissions on arbitrary files
Description: When restoring from backup, lockdownd changed permissions on certain files even if the path to the file included a symbolic link. This issue was addressed by not changing permissions on any file with a symlink in its path.
Note: At WWJC 2013 pimskeks mentioned that the description in Apple's security fix is wrong. It says "when restoring from backup", although this is totally unrelated to the backup, as the only thing that needs to be done is to restart lockdownd. In evasi0n this was done with a restore (to set a symbolic link to the file), but this vulnerability here is not related to the backup/restore.