Released last Tuesday, iPhone OS 3.1.3 was a minor update, bringing a battery reporting fix, fixing an issue where third-party applications would not launch, and patching a number of security holes. In its first six days of release, an estimated 14 percent of users opted to upgrade, mobile ad group Medialets reported Monday.

As of Feb. 6, most iPhone users — 72.3 percent — were still running iPhone OS 3.1.2, while 3.1.1 has 0.8 percent of users, and 3.1.0 has 4.6 percent. About 13 percent of total users are running a version prior to 3.1.2.

The company also noted that it saw a 13 percent drop in installs of 3.1.2, meaning 87 percent of the upgrades to iPhone OS 3.1.3 came from users who were already running the latest version of the mobile operating system.

The statistics were obtained through developers’ applications the service tracks through the App Store.

Following the release of the latest iPhone OS upgrade, the Dev Team, which is the leading group of hackers that work to “jailbreak” the iPhone and allow users to run unauthorized code, released their latest hacking tool. Dubbed PwnageTool 3.1.5, the Mac OS X software allows users to upgrade to Apple’s latest release while preserving a jailbreak and carrier unlock.

Last summer, the first jailbreaking tool for the iPhone 3GS was released. Apple later quietly updated the iPhone 3GS to thwart potential hackers, but a workaround was discovered in November.

Last year, Apple posted a job listing to hire a new iPhone OS security manager to help lock down its mobile operating system. While jailbreaking an iPhone can allow unauthorized capabilities such as multitasking and custom wallpapers, it can also be used to pirate software from the App Store.