There is a bug with Apple's first version of iOS  7 where an attacker can bypass a device's lockscreen to gain access to email, photo, Twitter, and other data. The attack requires little technical knowledge and can be done in less than a minute. The bug has been successfully verified on a variety of iPhone and iPad devices running iOS 7.

Update: Apple released iOS 7.0.2 to correct this issue. Unfortunately, another bug was discovered which allows anyone to make calls on an iPhone using just a few swipes. It is still suggested to update to 7.0.2 while Apple is working on a fix for this latest issue. If for some reason the update cannot be applied, the following mitigation is highly suggested.

To mitigate this issue until Apple releases a fix, disable the Control Center from appearing on the lockscreen. This can be done in Settings > Control Center > Access on Lock Screen > and swipe to the let until the option is greyed out. Once Apple has released a fix and the device is updated to the latest version of iOS, this setting can be enabled again.

IOS Control Center

More information about Control Center is available on Apple's support site.