2. THREAT MODEL
We present three types of threats posed by third-party
smartphone applications and discuss the security measures
that are intended to detect and prevent them.
2.1 Threat Types
The mobile threat model includes three types of threats:
malware, grayware, and personal spyware. We distinguish
between the three based on their delivery method, legal-
ity, and notice to the user. This paper focuses specifically
on malware; personal spyware and grayware use different
attack vectors, have different motivations, and require dif-
ferent defense mechanisms.
Malware. Malware gains access to a device for the pur-
pose of stealing data, damaging the device, or annoying the
user, etc. The attacker defrauds the user into installing the
malicious application or gains unauthorized remote access
by taking advantage of a device vulnerability. Malware pro-
vides no legal notice to the affected user. This threat in-
cludes Trojans, worms, botnets, and viruses. Malware is
illegal in many countries, including the United States, and
the distribution of it may be punishable by jail time.
Personal Spyware. Spyware collects personal informa-
tion such as location or text message history over a period
of time. With personal spyware, the attacker has physical
access to the device and installs the software without the
user’s knowledge. Personal spyware sends the victim’s in-
formation to the person who installed the application onto
the victim’s device, rather than to the author of the applica-
tion. For example, a person might install personal spyware
onto a spouse’s phone. It is legal to sell personal spyware in
the U.S. because it does not defraud the purchaser (i.e., the
attacker). Personal spyware is honest about its purpose to
the person who purchases and installs the application. How-
ever, it may be illegal to install personal spyware on another
person’s smartphone without his or her authorization.
Grayware. Some legitimate applications collect user data
for the purpose of marketing or user profiling. Grayware
spies on users, but the companies that distribute grayware
do not aim to harm users. Pieces of grayware provide real
functionality and value to the users. The companies that
distribute grayware may disclose their collection habits in
their privacy policies, with varying degrees of clarity. Gray-
ware sits at the edge of legality; its behavior may be legal
or illegal depending on the jurisdiction of the complaint and
the wording of its privacy policy. Unlike malware or per-
sonal spyware, illegal grayware is punished with corporate
fines rather than personal sentences. Even when the activity
of grayware is legal, users may object to the data collection if
they discover it. Application markets may choose to remove
or allow grayware when detected on a case-by-case basis.
2.2 Security Measures
Smartphone operating system vendors use curated mar-
kets and/or application permissions to protect users. We
focus on iOS, Android, and Symbian 9.x.
1
1
We limit our consideration of Symbian malware to Symbian
9.x. Earlier versions of the Symbian OS do not have all of
the security measures discussed in this paper.
Markets. Smartphone users are encouraged to download
and purchase applications from centralized application mar-
kets. Apple, Google, and Nokia promote the use of central-
ized markets with decreasing strictness.
Apple iOS devices allow the user to install applications
only from the Apple App Store [4], and applications in the
App Store are reviewed by Apple for security. If iOS users
want to install applications from other sources, then they
must jailbreak their devices, which involves exploiting a vul-
nerability in iOS to gain superuser access. This process car-
ries the risk of rendering the phone inoperable, and it voids
the phone’s warranty. Apple’s review process is intended to
prevent malware from being distributed through the App
Store. Their review standards also disallow personal spy-
ware, but an attacker with physical access to the victim’s
device could jailbreak the phone without the victim’s knowl-
edge to install personal spyware. The App Store is known to
have included grayware [31, 24]; in some cases, the grayware
has been removed from the App Store.
Android also provides users with an official application
store, the Android Market [2]. Most Android phones allow
users to also install applications from unofficial markets, al-
though the user is warned that installing non-Market appli-
cations may expose the user to malware. Google does not
review applications prior to listing them in the Android Mar-
ket, although they may review some applications later. Per-
sonal spyware (e.g., GPS Spy Plus) and grayware are listed
in the Android Market [25]. The Android security team has
removed malware from the Android Market following user
complaints, and they are able to remotely uninstall known
malware from users’ devices [17].
Nokia runs Ovi [5], which is currently the official Symbian
application market. Like the Apple App Store, all applica-
tions are reviewed prior to being listed in Ovi. Symbian does
not prevent or discourage users from installing applications
from other sources. Several popular alternative markets are
available, and they lack review processes. However, Sym-
bian offers an application signing service that incorporates
security reviewing. Only Symbian Signed applications are
allowed to access dangerous privileges. All Symbian Signed
applications must undergo an automated security review.
Applications that use the most dangerous privileges are ad-
ditionally reviewed by humans, and some number of other
Symbian Signed applications are also human-reviewed. As
a consequence of the signing process, many applications in
third-party Symbian markets have undergone review.
Permissions. Smartphone operating systems may also pro-
tect users by requiring user consent before an application
can access sensitive information or dangerous capabilities.
User-approved permissions can alert users to the activities
of grayware or malware, although malware may seek to cir-
cumvent permission systems. Permissions do not prevent
the installation of personal spyware because the attacker
can grant all necessary permissions during installation.
Android informs users of an application’s desired permis-
sions during installation. The Android permission system
is extensive; user-approved install-time permissions control
access to the phone’s number, list of contacts, camera, Blue-
tooth, etc. The iOS permission system is much less compre-
hensive than the Android permission system, likely because
Apple primarily relies on the App Store review process for
security. iOS requires user approval only to access location
and send notifications, and permission is requested at run-