This is about rouge apps that preset the environment that other mass used apps will need once activated. Those rouge apps can later hook to those resources, having unauthorized access due to the fact they initiated the creation of the place holder for those resources.
The iOS sandbox protection mechanism can't yet block this vulnerability.
This becomes very unsettling if you consider your iOS and OSx keychain password store can be exposed as well…
Read more on how this works and how to mitigate the risk. Basically avoid installing apps from