IntelliAV: Toward the Feasibility of Building Intelligent Anti-Malware on Android Devices

Abstract

Android is targeted the most by malware coders as the number of Android users is increasing. Although there are many Android anti-malware solutions available in the market, almost all of them are based on malware signatures, and more advanced solutions based on machine learning techniques are not deemed to be practical for the limited computational resources of mobile devices. In this paper we aim to show not only that the computational resources of consumer mobile devices allow deploying an efficient anti-malware solution based on machine learning techniques, but also that such a tool provides an effective defense against novel malware, for which signatures are not yet available. To this end, we first propose the extraction of a set of lightweight yet effective features from Android applications. Then, we embed these features in a vector space, and use a pre-trained machine learning model on the device for detecting malicious applications. We show that without resorting to any signatures, and relying only on a training phase involving a reasonable set of samples, the proposed systems outperforms many commercial anti-malware products, as well as providing slightly better performances than the most effective commercial products.

Full text

IntelliAV: Toward the Feasibility of Building
Intelligent Anti-malware on Android Devices
Mansour Ahmadi(B
), Angelo Sotgiu, and Giorgio Giacinto
University of Cagliari, Cagliari, Italy
mansour.ahmadi@diee.unica.it
Abstract. Android is targeted the most by malware coders as the num-
ber of Android users is increasing. Although there are many Android anti-
malware solutions available in the market, almost all of them are based
on malware signatures, and more advanced solutions based on machine
learning techniques are not deemed to be practical for the limited com-
putational resources of mobile devices. In this paper we aim to show not
only that the computational resources of consumer mobile devices allow
deploying an efficient anti-malware solution based on machine learning
techniques, but also that such a tool provides an effective defense against
novel malware, for which signatures are not yet available. To this end, we
first propose the extraction of a set of lightweight yet effective features
from Android applications. Then, we embed these features in a vector
space, and use a pre-trained machine learning model on the device for
detecting malicious applications. We show that without resorting to any
signatures, and relying only on a training phase involving a reasonable
set of samples, the proposed system outperforms many commercial anti-
malware products, as well as providing slightly better performances than
the most effective commercial products.
Keywords: Android ·Malware detection ·Machine learning ·
On-device ·TensorFlow ·Mobile security ·Classification
1 Introduction
Nowadays, mobile devices are ubiquitous tools for everyday life. Among them,
Android devices dominated the global smartphone market, with nearly 90% of
the market share in the second quarter of 2016 [25]. The majority of the secu-
rity issues affecting Android systems can be attributed to third party applica-
tions (app) rather than to the Android OS itself. Based on F-secure reports on
mobile threats [35], researchers found 277 new malware families, among which
275 specifically targeting Android devices. Also other recent reports clearly show
that the malware infection rate of Android mobile devices is soaring. In partic-
ular, a report from McAfee [30] reported a significant growth of mobile malware
in the wild. We believe that this huge amount of mobile malware needs to be
timely detected, possibly by smart tools running on the device, because it has
c
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Published by Springer International Publishing AG 2017. All Rights Reserved
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DOI: 10.1007/978-3-319-66808-6 10

138 M. Ahmadi et al.
been shown that malware can bypass offline security checks, and live in the wild
for a while. As a matter of fact, to the best of our knowledge, even the most
recent versions of Android anti-malware products are still not intelligent enough
to catch most of the novel malware.
The success of machine learning approaches for malware detection and clas-
sification [5,8,26,36,41], as well as the advance in machine learning software for
the execution in mobile environments, motivated us to empower Android devices
with a machine-learning anti-malware engine. Although modern mobile devices
come to the market with a huge amount of computational power, the develop-
ment of any Android anti-malware product should consider its efficiency on the
device to avoid battery drain, in particular when machine learning techniques
are employed, as they are known to be computational demanding. On the other
hand, we observe that an intelligent Android anti-malware product doesn’t need
to be unnecessarily complex, as it has been shown that Android malware exe-
cutes simpler tasks than the desktop counterparts [7]. All the aforementioned
reasons motivate the proposal for a machine learning solution to be deployed on
mobile devices to detect potential malicious software.
1.1 On-Device Advanced Security
Although there are many offline systems proposed for mobile malware detection,
mostly based on machine learning approaches (see Sect.5), there are many rea-
sons for a user to have an intelligent security tool capable of identifying potential
malware on the device.
(i) The Google Play store is not totally free of malware. There has been many
reports that have shown that malware could pass the Google security checks, and
remain accessible to users for sometime on the Play store until someone flags it
as inappropriate. For instance, the Check Point security firm reported a zero-day
mobile ransomware found in Google Play in January 2017, which was dubbed as
aCharger application, and was downloaded by more than a million users [31].
Another report from the same vendor cites the case of new variants of the famous
Android malware family HummingBad [33]. We vet these samples in Sect. 3.2.
(ii) Third-party app stores are popular among mobile users, because they
usually offer applications at great discounts. Moreover, the Google Play store has
restricted access in some countries, so people have to download their required
applications from third-party app stores. Nevertheless, security checks on the
third-party stores are not as effective as those available on the Google Play
store. Therefore, third-party markets are a good source of propagation for mobile
malware. Many malware samples have been found on these stores during the
past years, that were downloaded by millions of users. In addition, quite often
users can be dodged by fake tempting titles like free games when browsing the
web, so that applications are downloaded and installed directly on devices from
untrusted websites. Another source of infection is phishing SMS messages that
contain links to malicious applications. Recent reports by Lookout and Google
[24,27] show how a targeted attack malware, namely Pegasus, which is suspected

IntelliAV: Intelligent Anti-malware on Android Devices 139
to infect devices via a phishing attack, could remain undetected for a few years.
We vet these samples in Sect. 3.2.
(iii) One of the main concerns for any ‘computing’ device in the industry,
is to make sure that the device a user buys is free of malware. Mobile devices
make no exception, and securing the ‘supply chain’ is paramount difficult, for the
number of people and companies involved in the supply chain of the components.
There is a recent report that shows how some malware were added to Android
devices somewhere along the supply chain, before the user received the phone
[32]. We vet these samples in Sect. 3.2.
(iv) To the best of our knowledge, almost all of the Android anti-malware
products are mostly signature-based, which lets both malware variants of known
families, and zero-day threats to devices. There are claims by a few Android anti-
malware vendors that they use machine learning approaches, even if no detail is
available on the mechanisms that are actually implemented on the device. We
analyze this issue in more details in Sect. 3.2.
All of the above observations show that an anti-malware solution based on
machine-learning approaches, either completely, or as a complement to signa-
tures, can reduce the vulnerability of Android devices against novel malware.
1.2 Contribution
Accordingly, in this paper we introduce IntelliAV1, which is a practical intel-
ligent anti-malware solution for Android devices based on the open-source and
multi-platform TensorFlow library. It is worth to mention that this paper does
not aim to propose yet another learning-based system for Android malware
detection, but by leveraging on the existing literature, and on previous works by
the authors, we would like to test the feasibility of having an on-device intelligent
anti-malware tool to tackle the deficiencies of existing Android anti-malware
products, mainly based on pattern matching techniques. To the best of our
knowledge, the performances of learning-based malware detection systems for
Android have been only tested off-device, i.e., with computational power and
memory space well beyond the capabilities of mobile devices. More specifically,
the two main contributions of IntelliAV are as follows:
(i) We propose a machine-learning model based on lightweight and effective
features extracted on a substantial set of applications. The model is carefully
constructed to be both effective and efficient by wisely selecting the features,
the model, and by tuning the parameters as well as being precisely validated
to be practical for the capabilities of Android devices.
(ii) We show how the proposed model can be embedded in the IntelliAV appli-
cation, and easily deployed on Android devices to detect new and unseen
malware. Performance of IntelliAV has been evaluated by cross-validation,
and achieved 92% detection rate that is comparable to other off-device
learning-based Android malware detection relying on a relatively small set of
1http://www.intelliav.com.

140 M. Ahmadi et al.
features. Moreover, IntelliAV has been tested on a set of unseen malware,
and achieved 72% detection rate that is higher than the top 5 commercial
Android anti-malware products.
The rest of the paper is organized as follows: First, we reveal the detail of
IntelliAV by motivating the choice of features and the procedure followed to
construct the model (Sect. 2). We then present the experimental setup and results
(Sect. 3). After that, we briefly mention the limitations of IntelliAV (Sect. 4)and
review the related works on Android malware detection (Sect.5). Finally, we
conclude our paper discussing future directions of IntelliAV (Sect. 6).
2 System Design
The architecture of the proposed IntelliAV system is depicted in Fig. 1, and its
design consists of two main phases, namely offline training the model, and then
its operation on the device to detect potential malware samples. As a first phase,
a classification model is built offline, by resorting to a conventional computing
environment. It is not necessary to perform the training phase on the device,
because it has to be performed on a substantial set of samples whenever needed
to take into account the evolution of malware. The number of times the model
needs to be updated should be quite small, as reports showed that just the
4% of the total number of Android malware is actually new malware [10]. To
perform the training phase we gathered a relatively large number of applications
(Sect. 3.1). Then, a carefully selected set of characteristics (features) is extracted
from the applications to learn a discriminant function allowing the distinction
between malicious and benign behaviors (Sect. 2.1). Next, the extracted features
are passed to the model construction step in which a classification function is
learnt by associating each feature to the type of applications it has been extracted
Feature
Extraction
Meta-Data
Dex Code
Training Apps
Model Construction
TensorFlow
Model Selection Feature Selection
Validation Parameter Tuning
Feature
Extraction
Meta-Data
Dex Code
Risk Score App
Prediction
TensorFlow
Shared Objects
Library (C++)
On Workstation On Mobile Device
Feature Vector
Feature
Vector
Optimized
Model
Fig. 1. Overview of IntelliAV.

IntelliAV: Intelligent Anti-malware on Android Devices 141
from, i.e., malware or goodware (Sect. 2.2). Finally, as the second phase, the
model is embedded in the IntelliAV Android application that will provide a
risk score for each application on the device (Sect. 2.3).
2.1 Feature Extraction
The feature extraction step is the core phase for any learning-based system.
Various kinds of features have been proposed for Android malware detection by
the security community, such as permissions, APIs, API dependencies, Intents,
statistical features, etc. (see Sect. 5for a detailed discussion on the issue of fea-
ture extraction for Android malware detection). However, some sets of features
related to basic Android behaviors, like permissions, APIs, and Intents, usually
allow achieving reasonable detection results, with the aim to alert for the pres-
ence of probably harmful applications [8,36]. Extracting this set of features is
also feasible on mobile devices because they do not need deep static analysis, thus
requiring a limited computational effort. Therefore, with the aim of extracting a
set of efficient and effective features for IntelliAV, we resorted to the following
four sets of features: permissions, Intent Filters, statistical features based on the
‘manifest’ of Android applications, and the APIs, which are extracted from the
dex code. Therefore, to construct the feature vector, we considered all the permis-
sions and intent-filters that are used by the samples included in the training set.
In addition, four statistical features from application’s components such as the
total number of activities, services, broadcast receivers, and content providers are
added to the feature vector as they can reveal the amount of abilities each appli-
cation has. For instance, the number of activities in many malware categories
is usually fewer than the number of activities available in benign applications,
except for the case of malware that is built by repackaging benign applications.
Moreover, we manually selected a set of 179 APIs as features and included in
the feature vector. The selected APIs are those that reveal some particular char-
acteristics of application that are known to be peculiar to either goodware or
malware. For instance, the invoke API from the java.lang.reflect.Method
class shows whether an application uses reflection or not. Note that permissions
and APIs are coded as binary features, which means that their value is either
one or zero depending on the feature being or not present in the application. By
contrast, intent-filters are integer-valued features, as they represent the number
of times an intent-filters is declared in the manifest. Considering this count for
intent-filter features makes them more meaningful rather than simply considering
their presence or not in the application. Similarly, the application’s components
are represented as integer valued features, as we count the number of compo-
nents for each different type (e.g., activities, services, etc.). On the other hand, if
we considered the number of permissions, we would have ended up with useless
information, as each permission needs to be declared just once in the manifest.
The same reasoning motivates the use of binary feature to represent API usage.
The main reason is that although it is possible to get the count of the usage of an
API in an application, the procedure would increase the processing time without
producing more useful information, so that we ignored it. In total, the feature

142 M. Ahmadi et al.
Table 1. Features used in IntelliAV.
Category Number of features Type
Meta-data
Permissions 322 Binary
Intent Filters 503 Count
Statistical 4Count
Dex code
APIs 171 Binary
vector contains 3955 features. To avoid overfitting, and make IntelliAV faster
on the mobile device, we decided to reduce the number of feature by selecting the
1000 meaningful features thorough a feature selection procedure (see Sect. 2.2).
The final set consists of 322 features related to permissions, 503 features related
to Intent filters, 4 statistical features from components (e.g., count of activities),
and 171 features related to API usage (see Table 1).
2.2 Model Construction
To discriminate malware from benign applications, we need to rely on binary
classification algorithms. Over the past years, a large number of classification
techniques have been proposed by the scientific community, and the choice of
the most appropriate classifier for a given task is often guided by previous expe-
rience in different domains, as well as by trial-and-error procedures. However,
among all of the existing classifiers, Random Forest classifier [14]haveshown
high performances in a variety of tasks [19]. Random Forests algorithm is an
ensemble learning method in which a number of decision trees are constructed
at training time by randomly selecting the features used by each decision tree,
and it outputs the class of an instance at testing time based on the collective
decision of the ensemble. As far as the Random forest model is an ensemble clas-
sifier, it often achieves better results than a single classifier. The main reason
of achieving good results by Random Forests is that ensemble methods reduce
the variance in performances of a number of decision trees, which in turn are
complex models with low bias. So, the final model exhibits low bias, and low
variance, which makes the model more robust against both the underfitting and
overfitting problems [13].
To be able to train our model offline, as well as to test it on Android devices,
we built IntelliAV on top of TensorFlow [2]. More specifically, we employ an
implementation of Random Forests in TensorFlow, called TensorForest [16]. Ten-
sorFlow is an open source library for machine learning, which was released by
Google in November 2015. To the best of our knowledge, IntelliAV is the first
anti-malware tool that has proposed employing TensorFlow. The TensorFlow
model is highly portable as it supports the vast majority of platforms such as
Linux, Mac OS, Windows, and mobile computing platforms including Android

IntelliAV: Intelligent Anti-malware on Android Devices 143
and iOS. TensorFlow computations are expressed as data flow graphs. Nodes in
the graph represent mathematical operations, while the graph edges represent
the multidimensional data arrays (tensors) communicating between them.
As mentioned in the previous subsection, to simplify the learning task and
reduce the risk of the so-called overfitting problem, i.e., to avoid that the model
fits the training set but exhibits a low generalization capability with respect to
novel unknown samples, we exploited feature selection that reduced the feature
set size by removing irrelevant and noisy features. In particular, as done in [4],
we computed the so-called mean decrease impurity score for each feature, and
retained those features which have been assigned the highest scores. Note that
the mean decrease impurity technique is often referred to as the Gini impurity,
or information gain criterion.
2.3 On-Device Testing
As we mentioned before, TensorFlow eases the task of using machine learning
models on mobile devices. So, we embedded in IntelliAV the trained model
obtained according to the procedure described in Sect. 2.2. The size of the Ten-
sorFlow models depends on the complexity of the model. For instance, if the
number of trees in TensorForest increases, consequently the size of the model
increases as well. The size of IntelliAV model that we obtained according to
the above procedure and that we transferred to the device, is about 14.1 MB.
Having said that, when it is embedded into the apk, the model is compressed and
the total size of the model becomes just 3.3 MB. Whenever an application needs
to be tested, first, IntelliAV extracts the features from the application on the
device, then it loads the model, and finally it feeds the model by the extracted
features to get the application’s risk score. The model provides a likelihood value
between 0 and 1, denoting the degree of maliciousness of the application, that we
scale to a percentage that we called risk score, to make it more understandable
for the end user. We empirically provide the following guideline for interpreting
the risk score. If the risk score is lower than 40%, the risk is low and we suggest
to consider the application as being benign. If the risk score is between 40% and
50%, then the application should be removed if the user isn’t sure about the
trustworthiness of the application. Finally, the application has to be removed if
the risk score is higher than 50%. These thresholds have been set after testing
the system on a set containing different applications. We deployed IntelliAV so
that two main abilities are provided, as shown in Fig. 2.IntelliAV can scan all of
the installed applications on the device, and verify their risk scores (Quick Scan).
In addition, when a user downloads an apk, it can be analyzed by IntelliAV
before installation to check the related risk score, and take the appropriate deci-
sion (Custom Scan). To access the contents of an application’s package on the
external storage, IntelliAV needs the READ EXTERNAL STORAGE permission. To
access the contents of the packages of installed applications, IntelliAV needs
to read base.apk in a sub-directory with a name corresponding to the pack-
age name, which is located in /data/app/ directory. As far as the permission of
base.apk file is -rw-r--r--, which means every user can read the content of

144 M. Ahmadi et al.
(a) Scan installed applications (b) Scan an APK
Fig. 2. IntelliAV abilities.
this file, IntelliAV doesn’t need neither any permission, nor a rooted device to
evaluate the installed applications.
3 Experimental Analysis
In this section, we address the following research questions:
•Is IntelliAV able to detect new and unseen malware (Sect. 3.2)?
•Are the performances of IntelliAV comparable to the ones of popular mobile
anti-malware products, although IntelliAV is completely based on machine
learning techniques (Sect. 3.2)?
•Which is the overhead of IntelliAV on real devices (Sect. 3.3)?
Before addressing these questions, we discuss the data used, and the experimental
settings of our evaluation (Sect. 3.1).
3.1 Experimental Setup
To evaluate IntelliAV, we have collected 19,722 applications, divided into
10,058 benign and 9,664 malicious applications from VirusTotal [39]. When gath-
ering malicious applications, we considered their diversity, by including samples
belonging to different categories, such as Adware, Ransomware [6,28], GCM
malware [3], etc. All of the gathered samples have been first seen by VirusTotal

IntelliAV: Intelligent Anti-malware on Android Devices 145
between January 2011 and December 2016. The whole process of feature extrac-
tion and model construction was carried out on a laptop with a 2 GHz quad-core
processor and 8 GB of memory. Two metrics that are used for evaluating the
performance of our approach are the False Positive Rate (FPR) and the True
Positive Rate (TPR). FPR is the percentage of goodware samples misclassi-
fied as badware, while TPR is the fraction of correctly-detected badware sam-
ples (also known as detection rate). A Receiver-Operating-Characteristic (ROC)
curve reports TPR against FPR for all possible model’s decision thresholds.
3.2 Results
To better understand the effectiveness of IntelliAV, we evaluate it in following
scenarios.
Cross Validation. One might fit a model on the training set very well, so that
the model will perfectly classify all of the samples that are used during the train-
ing phase. However, this might not provide the model with the generalization
capability, and that’s why we evaluated the model by a cross-validation pro-
cedure to find the best tuned parameters to be used for constructing the final
model as a trade-off between correct detection and generalization capability.
Consequently, we evaluated IntelliAV on the set of applications described in
Sect. 3.1 through a 5-fold cross validation, to provide statistically-sound results.
In this validation technique, samples are divided into 5 groups, called folds, with
almost equal sizes. The prediction model is built using 4 folds, and then it is
tested on the final remaining fold. The procedure is repeated 5 times on different
folds to be sure that each data point is evaluated exactly once. We repeated the
procedure by running the Random Forest algorithm multiple times to obtain
the most appropriate parameters. The ROC of the best fitted model is shown
in Fig. 3. The values of FPR and TPR are respectively 4.2% and 92.5% which
is quite acceptable although the set of considered features is relatively small,
namely 1000 features.
Evaluation on the Training Set. To verify the effectiveness of the tuned
parameters based on the cross-validation procedure explained in Sect. 3.2,we
tested the model on all the samples used for training. Table2shows the results
on the training set. It shows that IntelliAV misclassified just a few training
samples. This shows how the model is carefully fitted on the training set, so
that is able to correctly classify almost all of the training samples with very high
accuracy, while it avoids being overfitted, and thus can detect unseen malware
with a high accuracy as well (see the following).
Evaluation on New Malware. We then tested the system on a set made up of
2311 malware samples, and 2898 benign applications, that have been first seen by
VirusTotal between January and March of 2017. We considered an application

146 M. Ahmadi et al.
0.0 0.05 0.1 0.15 0.2 0.25 0.3
0.7
0.75
0.8
0.85
0.9
0.95
1.0
False Positive Rate
True Positive Rate
Fig. 3. ROC curve of TensorForest (5-fold cross validation). FPR and TPR are respec-
tively 4.2% and 92.5%.
Table 2. Training on the set of samples explained in Sect. 3.1 and testing on the same
set. GT refers to the Ground-truth of samples.
Train Te s t
#Samples GT (#Samples) Classified as
Malicious Benign
19,722 Malicious (9,664) 9,640 (TPR = 99.75%) 24
Benign (10,058) 7 (FPR = 0.07%) 10,051
as being malicious when it was labeled as malware by at least 5 of the tools used
by VirusTotal. This set of test samples contains randomly selected applications
that were newer than the samples in the training set, and thus they were not
part of the training set.
Test results are shown in Table3. The detection rate on the test set is 71.96%,
which is quite good if compared with the performances of other Android anti-
malware solutions that are available in the market, as shown in Sect. 3.2.More-
over, the false positive rate is around 7.52%, which is acceptable if we consider
that an individual user typically installs a few dozen applications, and thus it
might receive a false alert from time to time. This casual alert allows the user
that the application has some characteristics similar to badware, and so it can
be used only if the source is trusted. It is also worth noting that our classification
of false positives is related to the classification provided by VirusTotal at the
time of writing. It is not unlikely that some of these applications might turn out
to be classified as malware by other anti-malware tools in the near future, as we
have already noticed during the experiments. However, due to the small time
frame, we haven’t the possibility to collect enough examples to provide reliable
statistics, as the samples used for the test phase are quite recent. We expect in
a future work to show how many applications were correctly predicted as being

IntelliAV: Intelligent Anti-malware on Android Devices 147
Table 3. Training on the set of samples described in Sect. 3.1, and testing on new
samples in 2017. GT refers to the Ground-truth of samples.
Train Te s t
#Samples GT (#Samples) Classified as
Malicious Benign
19,722 Malicious (2311) 1,663 (TPR = 71.96%) 648
Benign (2898) 218 (FPR = 7.52%) 2,680
malicious before their signatures were created. However, our experience suggests
that even if the application is benign but labeled as being potentially risky by
IntelliAV, then the user might look for less risky alternatives applications in
Google Play [37]. In fact, we believe that it is better that people is aware of some
applications that might be potentially harmful, even if it turns out not to be so,
rather than missing some real threats.
Challenging Modern AV Vendors. Based on the recent reports by Virusto-
tal [39], there is an increase in the number of anti-malware developers that resort
to machine learning approaches for malware detection. However, the main focus
of these products appears to be on desktop malware, especially Windows PE
malware. Based on the available public information, there are just a few evi-
dences of two anti-malware developers that use machine learning approaches for
Android malware detection, namely Symantec [18] and TrustLook [38]. Their
products are installed by more than 10 million users. While it is not clear to us
how these products use machine learning, we considered them as two candidates
for comparison with IntelliAV. To provide a sound comparison, in addition
to the Symantec and Trustlook products, we selected three other Android anti-
malware products, i.e., AVG, Avast, and Qihoo 360, that are the most popular
among Android users as they have been installed more than 100 million times.2
We compared the performances of IntelliAV on the test dataset (see Sect. 3.2)
with the ones attained by these five popular Android anti-malware As shown in
Fig. 4,IntelliAV performs slightly better than two of the products used for com-
parison, while it outperforms the other three. As we gathered the label assigned
by anti-malware products to the test samples at most two months after they are
first seen in VirusTotal, the comparison could be more interesting if we had the
label given to samples at the time they are first seen in the wild. As an additional
check, we performed a comparison in detection performance by considering a set
of very recent malware reported by four vendors, namely Check Point, Fortinet,
Lookout, and Google (see Table4). The good performances of IntelliAV com-
pared to the ones of other products, shows that the selected lightweight features
and training procedure allows attaining very good performances, especially if we
consider that 21 of the considered samples were first seen before 2017, so it is
2http://www.androidrank.org/.

148 M. Ahmadi et al.
IntelliAV AV1 AV2 AV3 AV4 AV5
0
500
1,000
1,500
2,000
1,663
1,580
1,575
756
731
180
Total test malware
Fig. 4. Comparison between the detection rate of IntelliAV with top five Android anti-
malware. We didn’t put the name of vendors as we don’t aim to rank other anti-malware
products.
expected that they can be detected by anti-malware tools either by signatures,
or by the generalization capability provided by their machine learning engines. If
we have a close look at the two misclassified samples by IntelliAV (Table 4), we
can see that the associated risk scores are quite close to the decision threshold
that we set at training time. The main reasons for the misclassification of these
two samples can be related to the use of the runtime.exec API to run some
shell commands, and to the presence of native-code that is used to hide some of
their malicious behaviors.
3.3 IntelliAV Overhead on Device
To better understand the efficiency of IntelliAV, we show the time consump-
tion for feature extraction as well as classification of some medium/large-sized
applications on three devices with different technical specifications. The three
mobile devices used for the reported experiments are a Samsung Galaxy S6 Edge
(released in April, 2015), a Huawei P8 Lite (released in May, 2015), and an LG
D280 L65 (released in June, 2014), which respectively have 3 GB, 2 GB, and 1 GB
of RAM. In addition, we computed the time required on the Android Emulator
that is dispatched along with Android Studio. The time is simply computed by
specifying a timer before starting the feature extraction procedure, that stops
when the features from both the manifest and the dex code are extracted. For
classification, the reported time refers to the interval between the feature vector
is passed to the model, to the production of the risk score. The time required to
load the model is negligible, and so we are not reporting it for the sake of clarity.
AsshowninTable5, the time required to analyze even large applications is less
than 10 s, which makes IntelliAV practical and reasonable as the number of

IntelliAV: Intelligent Anti-malware on Android Devices 149
Table 4. Point to point comparison of IntelliAV and three anti-malware vendors on
some recent and well-known malware reported by Check Point, Fortinet, Lookout, and
Google from January to April of 2017. These samples were evaluated on an Android
emulator. The time column refers to the required time for performing both feature
extraction and classification on the emulator.
installed applications on each device is not too large. The classification part is
performed in native code, that provides a fast execution. As expected, it can be
noted that the largest fraction of the time required by IntelliAV is spent for
feature extraction, especially for the extraction of the API features. This figure
is even worse in the case an application is made up of multiple dex files, because
the extraction of API features is much slower. For example, the Uber app is
made up of 10 dex files, so that searching for a specific API requires much more
time compared to applications having just one dex file.

150 M. Ahmadi et al.
Table 5. Overhead of IntelliAV on different devices for very large applications. F.E.
refers to feature extraction time and C. refers to classification time. The number in
parenthesis shows the RAM size of the device.
App APK
size
(MB)
Galaxy S6 Edge
Marshmallow
(3 GB)
Huawei P8 Lite
Lollipop (2 GB)
LG D280 L65
KitKat (1 GB)
Emulator
Marshmallow
(1.5 GB)
F.E. (s) C. (s) F.E. (s) C. (s) F.E. (s) C. (s) F.E. (s) C. (s)
Google Trips 8.19 0.67 0.003 0.82 0.005 3.86 0.012 0.43 0.001
LinkedIn Pulse 12.9 1.28 0.003 1.14 0.005 4.40 0.012 0.55 0.001
Stack Exchan ge 8.15 1.27 0.004 1.27 0.006 5.13 0.014 0.60 0.001
Telegram 12.41 1.36 0.005 1.74 0.007 5.52 0.016 0.69 0.002
WhatsApp 27.97 2.29 0.006 3.22 0.008 12.91 0.018 1.10 0.002
SoundCloud 33.14 2.67 0.006 2.84 0.008 11.83 0.018 1.14 0.002
Spotify 34.65 2.51 0.006 3.03 0.008 13.67 0.018 1.22 0.002
Twitter 31.77 4.53 0.004 5.95 0.006 24.46 0.016 2.26 0.002
LinkedIn 40.39 4.67 0.004 4.69 0.006 16.73 0.016 2.40 0.001
Airbnb 54.34 8.24 0.006 8.79 0.008 35.71 0.018 4.23 0.002
Messenger 59.43 5.85 0.011 7.94 0.013 19.13 0.028 3.35 0.004
Uber 37.26 6.66 0.004 7.64 0.006 43.88 0.016 4.29 0.002
Average 30.05 3.50 0.005 4.08 0.007 16.43 0.016 1.86 0.002
4 Limitations
As far as IntelliAV is based on static analysis, it inherits some of the well-
known limitations of static analysis approaches. For instance, we didn’t address
reflection and dynamic code loading techniques that are used to hide the mali-
cious code. Moreover, in the proposed implementation, IntelliAV doesn’t han-
dle those malware samples that use JavaScript to perform an attack. However,
the most common evasion techniques are based on obfuscation of names, and
the use of downloaders that download the malicious payload at run-time. The
reported test results show that IntelliAV is robust against these common obfus-
cation techniques as it doesn’t rely on features extracted from strings or names
of classes or methods. In addition, as far as IntelliAV runs on the device, it can
track all downloaded and installed apps, scanning them on the fly. Consequently,
it can be more robust compared to off-device systems. In addition, we are aware
that the system can be a victim of evasion techniques against the learning app-
roach, such as mimicry attacks that let an attacker inject some data to the app
so that its features resembles the ones of benign apps [12]. Consequently, more
methodological and experimental analysis will be needed to make a quantita-
tive evaluation of the robustness of IntelliAV in an adversarial environment,
to provide the system with the required hardening. Nonetheless, we believe that
the good performances of the proposed system is a good starting point for fur-
ther development. Moreover, employing the multiple classifier system approach,
considering a larger number of semantic features, as well as performing a fine

IntelliAV: Intelligent Anti-malware on Android Devices 151
grained classifier parameter tuning, can provide a degree of robustness against
adversarial attacks against the machine learning engine.
5 Related Works
At present, a large number of papers addressed the topic of detecting Android
malware by proposing different systems. The proposed approaches can be divided
into two main categories, namely offline malware detection, and on-device mal-
ware detection. While a complete overview is outside of the scope of this paper,
and we suggest the interested reader to resort to one of the good survey that
have been recently published (e.g., the recent taxonomy proposed in [34]), we
provide here some of the more closely related papers that rely on static analy-
sis technique. We omit reviewing the malware classification systems based on
dynamic analysis [5,15,17] as they have their own benefits and pitfalls. More-
over, as we are dealing with an on-device tool, it is not officially possible that a
process access system calls of other process without root privileges, which makes
the dynamic analysis approaches almost impractical on the end user device.
Offline Malware Detection. Usually, offline testing has no hard computa-
tional constraints, thanks to the availability of computational power compared
to the one available on mobile devices, thus allowing for sophisticated applica-
tion analysis. Hence, a number of approaches have been proposed to construct
complex models capable of detecting malware with a very high accuracy. Some
of the prominent approaches that focus on building a model and offline testing of
Android applications by static analysis techniques are briefly summarized. Mud-
Flow [11], AppAudit [40], and DroidSIFT [42] rely on information flow analysis
[9], while DroidMiner [41], and MaMaDroid [29] use API sequences to detect mal-
ware. The use of complex features such as information flows and API sequences,
makes these approach more difficult to be carried out on the device. Lighter
approaches have been proposed, such as Drebin [8], DroidAPIMiner [1], and
DroidSieve [36] that make use of meta-data as well as syntactic features, and
that allow for their porting to on-device applications.
On-Device Malware Detection. Based on the best of our knowledge, there
are a few approaches in the research community that used machine learning for
on-device malware detection, and none of them is publicly available for perfor-
mance comparison. One that of the most cited research works on this topic is
Drebin, and while the paper shows some screenshots of the UI, the application
itself is not available. Among the commercial Android anti-malware tools, two
of them claim to use machine learning techniques, as evaluated and reported in
Sect. 3.2, but the real use of machine learning by these tools is blurred. Finally,
Qualcomm recently announced the development of a machine learning tool for
on-device mobile phone security, but the details of the system, as well as its
performances are not available [26].

152 M. Ahmadi et al.
As an overall comparison with the previous approaches, we believe that
IntelliAV provides a first practical example of an on-device anti-malware solu-
tion for Android systems, completely based on machine learning techniques, that
can move a step toward having an advanced security tool on mobile devices.
6 Conclusions and Future Work
In this paper, we introduced a practical learning-based anti-malware tool for
Android systems on top of TensorFlow, in which both the efficiency and the
effectiveness are considered. We showed that through the careful selection of a
set of lightweight features, and a solid training phase comprising both a robust
classification model, and a representative set of training samples, an efficient
and effective tool can be deployed on Android mobile device. Our tool will be
freely available so that it can help the end user to provide easy protection on
the device, as well as allowing researchers to better explore the idea of having
intelligent security systems on mobile devices. As a future plan, we aim to address
the limitations of IntelliAV, to improve its robustness against attacks on the
machine learning engine, while keeping the efficiency intact.
Acknowledgement. We appreciate VirusTotal’s collaboration for providing us the
access to a large set of Android applications.
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