  By offering sophisticated services and centralizing a huge volume of personal
data, modern smartphones changed the way we socialize, entertain and work. To
this aim, they rely upon complex hardware/software frameworks leading to a
number of vulnerabilities, attacks and hazards to profile individuals or gather
sensitive information. However, the majority of works evaluating the security
degree of smartphones neglects steganography, which can be mainly used to: i)
exfiltrate confidential data via camouflage methods, and ii) conceal valuable
or personal information into innocent looking carriers.
  Therefore, this paper surveys the state of the art of steganographic
techniques for smartphones, with emphasis on methods developed over the period
2005 to the second quarter of 2014. The different approaches are grouped
according to the portion of the device used to hide information, leading to
three different covert channels, i.e., local, object and network. Also, it
reviews the relevant approaches used to detect and mitigate steganographic
attacks or threats. Lastly, it showcases the most popular software applications
to embed secret data into carriers, as well as possible future directions.
