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Friday, November 21, 2014

Some artifacts in the /data/system/ directory

A few nice artifacts


All blog posts to date
In a previous post, I demonstrated how to image an Android device and then I made two different posts on how to examine the image.  You can see by examining an image that your device is divided into partitions.

Android devices are partitioned, and the following partitions should be in every image:

  • data - the partition with user-related data, which may also include a directory representing an SD card
  • system - pre-loaded apps, libraries, settings, images, and more
  • boot - the Android system kernel
  • recovery - the Android recovery kernel

And other devices may have all kinds of other partitions.  Try imaging a Galaxy S4 and see how many partitions FTK Imager and Autopsy recognize.

As you may have reasoned, the data partition is where an investigator will be examining the most.  This partition contains data about the user.  Within the data partition will be a few directories of note:

  • data - data related to installed apps, including the user's text message history, web browsing history, call logs, contacts, Facebook messages, calendar events, etc.
  • app - apps which the user installed.  This directory will contain the actual apk files which the user downloaded or sideloaded and installed
  • media - older devices may not have this directory, but newer Android devices will contain the media directory, which represents an SD card.  This directory will contain photos, unless there is an external SD card in the device, and may contain all kinds of user files.  This directory also includes files the user downloaded using a web browser.

As you also may have reasoned, the data directory is where an investigator will be spending a lot of time.

This post focuses on another directory within the data partition.  This directory is system, which contains more information about user behavior.  This directory contains useful logs that the user is unlikely aware of yet can say a good amount about the user.  I will detail just a few artifacts.  There are far more artifacts than these, but I will detail some useful ones and can field questions about others.

To get to these artifacts, you'll either need to have an image of the device, or you will  need root access.  Non-root users cannot access these files through a shell.


List of installed apps
Check out the file /data/system/packages.xml.  (Note:  The device I used here runs Lollipop, or a newer version of Android.  The packages.xml may contain different data for older versions of the operating system, like Gingerbread and older.)  This file contains a list of all apps installed, plus some extra information about each app.  Here is the entry for ES File Explorer in my /data/system/packages.xml file.

<package name="com.estrongs.android.pop" codePath="/data/app/com.estrongs.android.pop-2.apk" nativeLibraryPath="/data/app-lib/com.estrongs.android.pop-2" flags="4767300" ft="146b6659890" it="14346b1705b" ut="146b665d076" version="212" userId="10109" installer="com.android.vending">
    <sigs count="1">
        <cert index="77" key="3082(bunch of hex ...)733f" />
    </sigs>
    <perms>
        <item name="android.permission.READ_EXTERNAL_STORAGE" />
        <item name="com.android.launcher.permission.INSTALL_SHORTCUT" />
        <item name="android.permission.CHANGE_WIFI_MULTICAST_STATE" />
        <item name="android.permission.SET_WALLPAPER" />
        <item name="android.permission.WRITE_EXTERNAL_STORAGE" />
        <item name="android.permission.ACCESS_WIFI_STATE" />
        <item name="com.android.launcher.permission.UNINSTALL_SHORTCUT" />
        <item name="android.permission.READ_PHONE_STATE" />
        <item name="android.permission.ACCESS_SUPERUSER" />
        <item name="android.permission.BLUETOOTH" />
        <item name="android.permission.INTERNET" />
        <item name="android.permission.WRITE_SETTINGS" />
        <item name="android.permission.CHANGE_WIFI_STATE" />
        <item name="android.permission.VIBRATE" />
        <item name="android.permission.BLUETOOTH_ADMIN" />
        <item name="android.permission.WAKE_LOCK" />
        <item name="android.permission.ACCESS_NETWORK_STATE" />
    </perms>
    <signing-keyset identifier="3" />
    <signing-keyset identifier="5" />
    <signing-keyset identifier="4" />
    <signing-keyset identifier="2" />
    <signing-keyset identifier="1" />
    <signing-keyset identifier="6" />
</package>

There will be such an entry for every installed app.  I'll go over what some of these entries mean.

  • package name="com.estrongs.android.pop" - This is the package name of the app.  Here is quick documentation on what the package name is. 
  • codePath="/data/app/com.estrongs.android.pop-2.apk" - This is the path to the APK, or the application install file.  If you need to investigate an app, or reverse engineer the app, here is the file you should be examining.
  • nativeLibraryPath="/data/app-lib/com.estrongs.android.pop-2" - This is the path to a directory containing native libraries which the app uses.  In my phone, the directory /data/app-lib/com.estrongs.android.pop-2 contains two native library files.  If you are interested in reversing native executables, you can reverse these files and examine.
  • <item name="android.permission.READ_EXTERNAL_STORAGE" (and a bunch more) /> - There are a bunch of entries of Android permissions.  This app contains 17 permissions, ranging from reading and writing to external storage to Internet access to using the vibrate function.  If you ever browse through the packages.xml file and find an app with an extraordinary amount of permissions or some permissions that just seem odd, like a game that has the permission to send and receive SMS, then you might want to take a close look.

The packages.xml file is a useful file to see what all files the user has installed on the device.  This file also lists associated permissions with each app which can be a useful hint to malicious apps, and each app entry also includes a path to the actual APK file so you can reverse the app if you need.


Log of last usage of an app
Next, look at the file usagestats/usage-history.xml.  This file contains log entries with the last time a user used an app.  Here is the entry in my phone for the Chrome app.  Note: I imaged my phone in late October 2014.

<pkg name="com.android.chrome">
    <comp name="com.google.android.apps.chrome.ChromeTabbedActivity" lrt="1414545913713" />
    <comp name="com.google.android.apps.chrome.bookmark.ManageBookmarkActivity" lrt="1398440159237" />
    <comp name="com.google.android.apps.chrome.ManageBookmarkActivity" lrt="1391453561436" />
    <comp name="com.google.android.apps.chrome.Main" lrt="1414545745091" />
</pkg>


What you see here are four different activities within the app. Activities, in Android lingo, are basically different screens allowing for user activity.  Each activity above also contains a timestamp in Epoch time of the last time the activity ran.  Here's a handy writeup on Epoch time, in case you are unfamiliar, and here is a nifty Epoch converter.

Based on the log, here is the last time I used each of these activities before I imaged the phone:

  • com.google.android.apps.chrome.ChromeTabbedActivity: Wed, 29 Oct 2014 01:25:13 GMT
  • com.google.android.apps.chrome.bookmark.ManageBookmarkActivity: Fri, 25 Apr 2014 15:35:59 GMT
  • com.google.android.apps.chrome.ManageBookmarkActivity: Mon, 03 Feb 2014 18:52:41 GMT
  • com.google.android.apps.chrome.Main: Wed, 29 Oct 2014 01:22:25 GMT

Apparently I do not use bookmarks very often!  Note, these timestamps are all in GMT.  You'll need to convert this timestamp to the local timezone.

The usage-history.xml file is a useful file.  It will not let the investigator know the complete history for an app, but it will indicate the last time each activity was used.  If a user indicates that he/she has never used an app yet the usage-history.xml file indicates that the app was used yesterday, you may want to investigate some.

Database of accounts on the device
Finally, open the file system/users/0/accounts.db in a SQLite browser.  (I intend to at some point do a post on SQLite databases but have not yet.  If you're not sure how to open a SQLite database file, contact me and I'll help you out.)  Here's what the "accounts" table of my accounts.db file looks like in a SQLite browser (with personal information blacked out):


This database file includes a table called "accounts", which is a list of accounts associated with the device.  The three accounts seen above are a Google account, a Facebook account, and a LinkedIn account.

Each entry has three columns of data: name, type, and password.  Name is the username associated with the account, and in all three cases above the username is an email address.  The type is the account provider.  You can see above that my accounts are clearly Google, Facebook, and LinkedIn.  And the password contains a hashed version of the password.  The actual password in plaintext is not included.

This file is a useful list to see what services a user frequently uses.  I can't say I am on LinkedIn all that frequently, but I use Google and Facebook frequently.


Other artifacts
There are all kinds of other useful artifacts - battery stats, process states, network states, the wallpaper image, and some more.  If you are an Android enthusiast, I highly enjoy exploring the /data/system directory further.  You may find some more useful artifacts.

Finally, do you have some insights into useful artifacts in /data/system?  If so, comment below.  I'd be happy to field questions and I also am always eager to learn more.

Summary
  • The /data/system directory includes useful logs about the user and user behavior
  • The file /data/system/packages.xml contains a list of installed apps including the APK path and a list of permissions
  • The file /data/system/usagestats/usage-history.xml contains logs of the last time a user used an app
  • The file /data/system/users/0/accounts.db contains a list of accounts and associated usernames and service providers
Questions, comments, suggestions, or experiences?  Leave a comment below, or send me an email.

Thursday, November 6, 2014

Using Autopsy to examine an Android image

A solid, open source tool


All blog posts to date
Autopsy is an open source digital forensics tool by Basis Technologies.  This is a powerful free tool with many of the same capabilities as the expensive tools (FTK, EnCase).  Some people in the digital forensics community will debate until they are blue in the face over whether open source forensics software is better or if paid software is better.  This is a debate from which I will spare my readers, but I'll say this: Autopsy is a fantastic tool.

I've had all kinds of success with Autopsy before.  There have been several times where FTK Imager did not properly load an image.  Errors included not recognizing the image as an image or missing partitions.  In all cases where FTK Imager has made these sorts of mistakes, Autopsy has come through for me.

And on top of the above statement, I was using an old version of Autopsy which did not include specific Android functionality.  I was using a version of Autopsy which was reading a disk image as a disk image, not as specifically an Android image.  Autopsy's file system engine does an incredible job at identifying partitions and file systems.  This has been a tool which I have used with all kinds of success.

In this post, I will load an image of my personal Nexus 5 into Autopsy and will show some of the useful functionality for investigations.  I created the image using the same method in my post on live imaging an Android device.

Getting started
Download and install the newest version of Autopsy from this link.  (Note: the downloads are for Windows.  You can download the source for Autopsy and compile it for Linux.  I have not done this yet but intend to soon.)

Once the software is installed, open Autopsy and create a new case.  Fill in the basic info.  The entry for "Base Directory" is where you intend to store data related to cases.  This directory is not necessarily where you store an image you intend to examine and analyze, but it stores information and analyses about the image.  Be advised, this directory can get filled up quickly.  My phone is 32 gigabytes, and my base directory now contains 7 gigabytes of data.



Next, add your Data Source, or your image.



Autopsy has several "ingest modules" built in for analysis.  These ingest modules identify files and extract known data as records, such as emails or time-based data.  You can select or deselect whatever modules you want.  The more ingest modules you select, the more time and disk space the analysis will take, but you also may find more insight about the image with more modules.  Do be sure to select the "Android Analyzer" module when analyzing an Android image.




You can also optionally give a case number or an investigator name.  Yes, you are an investigator, so take credit.

Once the case is created, you can see the main Autopsy interface.



At this point, analysis will be ongoing.  The ingest modules each pass through the image to find relevant data.  In the bottom right corner there is a status bar which you can click on to see analysis status.  In the below shot, there are three different ingest modules working simultaneously.




Depending upon how big the image is, how many files are in the image, how many modules you select, how much disk storage space you have, how fast your computer's processor is, and how much RAM you have, analysis may take a while.  I'm running Autopsy on a Windows netbook and analyzing an image of a 32 gigabyte phone took around an hour.  You can browse around the image and do some investigation before the ingest modules are done, but you will be viewing incomplete results.  For example, there is a great tool for timeline analysis which I will show later in this post.  If you try to do a timeline analysis before the modules are complete, there will be evidence missing from the timeline.

You also can always wait for the analysis to complete before getting started.

Android Analyzer module
I indicated above to enable the Android Analyzer module.  This module will identify files containing contact data and communications records.  I said above that ingest modules will extract records and present them to the investigator.  The below screenshot indicates that Autopsy identified Call Logs, Contacts, and more.  I can tell you that the Android Analyzer ingest module is to credit for these finds.  You can click each of these and see what data was collected.





Android by default stores your text messages in a SQLite database in the file /data/data/com.android.providers.telephony/databases/mmssms.db, and you can load this file into a SQLite database viewer to see the SMS.(Note:  one of these days I intend to do a post on viewing SQLite database files.  The long and short of it is Android apps, including SMS and phone dialer and contacts, use SQLite databases to store data.  The apps present data in their own ways, such as SMS conversations, but you can always view the raw data stored as it is stored in a SQLite database viewer.)

Below is how Autopsy presents SMS.



(Black boxes inserted for privacy.)

Convenient?

Here is the call log ...




... and here is the contacts list.



Browsing the image
Autopsy allows you to browse through the image.  The below screenshot shows all of the partitions which Autopsy identified.  You can see the userdata partition, which will store most of the data about the user.



And then you can browse through the individual partitions.




You can view individual files as text or hex.  You can also see extracted strings and metadata about the file.  And picture files load as pictures.

Timeline
One of Autopsy's best features is the timeline.  Autopsy will find events associated with a date and time, such as text messages or call logs or any other time-based events, and make a timeline of events.  As an investigator, I always like to create a timeline of events which a digital device has recorded because all of these events ultimately tell the story about a person using the device.

To create a timeline, go to Tools -> Timeline.  (Wait for all ingest modules to finish first.)  Then wait for a bit, and when the timeline is ready it opens in a new window.




The timeline clearly indicates a lot of activity in 2013-2014.  But you may also see a weird anomaly around 1970.  Do not worry about those or the odd 2008 files as those are Linux and Android artifacts, respectively, and they deal with "Unix time" or "epoch time."  For a quick explanation on how Linux keeps time, check out this Wikipedia page.

You can zoom in to see detailed events.  The following is my phone events from October 10-23 2014.



The bar colors represent different events as seen in the legend on the timeline.




You can choose to view "Details" instead of "Counts" which allows you to see what events occurred.



And then you can also zoom in for more details.  I see that there is an SMS event, so I chose to see details of the event.  (SMS message blacked out for privacy.)




The timeline is just an incredibly useful tool.  And it is a tool that the more you use it, the more uses you find with it.

More features
Autopsy has many more features which I'll let you explore.  But just to list a few that I've used before:

  • Plugins
    • You can download plugins to act as further ingest modules or even develop your own
  • Extract files
    • You can extract files to analyze them with other tools, such as a hex editor of choice or an advanced media file analysis tool
  • Report
    • Like with other forensic tools, you can tag files of interest and generate a report highlighting important files and other findings
  • Known hashes
    • If you have a list of hashes of known files you are interested in finding, you can load this hash set into Autopsy and it will let you know if it found these files
  • Carving
    • Autopsy includes Scalpel for data carving

Summary
  • Autopsy is an awesome tool.  This point deserves an individual bullet
  • You can browse an image like in FTK Imager but I've had cases where FTK Imager fails to load an image properly and Autopsy has correctly loaded the image
  • Ingest modules process through evidence and extract useful records
  • The Android Analyzer ingest module can extract contacts, SMS, Calls, and more
  • Autopsy's timeline tool is incredibly useful in investigations
Questions, comments, suggestions, or experiences?  Open source vs paid forensic software debate? Leave a comment below, or send me an email.

Monday, October 20, 2014

Some hidden artifacts in a physical image

Always get a physical image


All blog posts to date
Introduction
Picking a Toolkit
Imaging an Android Device
Live imaging an Android device
Examining the image
Reverse Engineering an Android App File
The differences between a physical image and a logical extraction
Some hidden artifacts in a physical image
Using Autopsy to examine an Android image
Some artifacts in the /data/system/ directory
Viewing SQLite Databases
Facebook for Android Artifacts
Why not load ClockworkMod or TWRP to image a device?
Identifying your Userdata Partition
Some non-root methods to learn about a device
Interpreting data from apps
Dirty cow
Waze for Android forensics
Fun with Apktool
A quick note on imaging newer Android devices
Using Windows to Live Image an Android device
Imaging and examining an Android car stereo

Introduction
In my previous post, I discussed the differences between a physical image and the results of a basic Android forensics tool.  This post is a dive into some artifacts of a physical image which a logical extraction tool will not find.  Note:  this is a basic dive, not a deep dive.  The results should not be too surprising, but this post demonstrates some good reasons to obtain a physical image and browse the image in depth.

Initialization
For ease sake, I used an Android emulator instead of a physical device.  If anybody would like, I can redo the process on a physical phone.  Just let me know if that is important to you and I'll get on it.

I loaded the following PNG picture to the phone's /sdcard partition at the root.


Pirate Android!!

And then I sent a text message.  (This is an emulator, so the message doesn't actually go anywhere, but the emulator will store data just the same way regardless.)



I deleted the Pirate Android image, and then I deleted the text message.






Extractions
I performed some basic extractions.  First, I ran the Open Source Edition of viaForensics' viaExtract tool, which is included free with Santoku.  viaExtract is a basic logical extraction tool that can extract SMS.

Then I used the Android shell to browse through the Android emulator to see what files exist and do not exist.

And finally, I emulated a physical image of the device.  What I mean by an emulated physical image is I copied the files which represent the emulator's storage and viewed them with a hex editor.  On Linux, these files are stored at /home/<username>/.android/avd/

The above extractions represent a logical extraction, logical browsing, and a physical extraction.

Results
The viaExtract tool did not detect the deleted text message.  viaExtract uses device APIs to extract data and stores the extracted data in CSV files.  The below image is the CSV file containing discovered SMS messages, which in this case is none.  (And for what it is worth, I have run this tool on emulators before and I can confirm that viaExtract can successfully extract SMS from an Android emulator, so the results from viaExtract do not represent an error.)



When browsing through the emulator's SD card, the Pirate Android file is gone, predictably, as I deleted the file previously.



The physical image contained some more interesting results.  First, I used a hex editor to explore the userdata partition and found in slack space the deleted SMS.


The deleted message clearly contains the deleted text ("You will never find this message!" and the "recipient" ("678-9").

I also found a PNG image in the image of the SD card.



So I copied the beginning to the end of the PNG file I found in slack to a new file and opened the resulting file as a graphical file and ...


Pretty cool, huh?

Discussion
So what can explain all of these results, and what does it all mean?

viaExtract is a basic logical extraction tool and relies upon device APIs to extract data.  The device presents an API to extract SMS messages, but there is no API to extract deleted messages.  There is no way that any logical extraction tool will be able to leverage only device APIs to extract deleted SMS.

Note:  the tool from viaForensics is a basic logical extraction tool.  viaForensics has some true experts who can do the dive demonstrated in this post and way, way better.  The folks at viaForensics are definitely experts in mobile forensics.  I personally respect the company greatly.  I chose the viaExtract tool because it is a basic logical extraction tool and it is free with Santoku.

It should not be a surprise that a deleted file does not show up when shelling into the emulator.  As seen above, the Pirate Android image is not in the device according to the shell.  A device shell can only display what files the file system knows exist, and the file system knows that the Pirate Android image is deleted.

So why was I able to recover a deleted text message and a deleted picture?  The reason is simple: the deleted text message and the deleted picture were not overwritten or destroyed.  As long as data is not overwritten, it can be found in a physical image using a hex editor.

The big picture of this post is that you really want a physical image, and you also really need to examine the image using a hex editor.  It can be hard to find artifacts, but these artifacts are there. A digital forensics expert should be proficient with a hex editor.

There are some ways automate finding artifacts, such as file carving.  I recommend using a variety of automated tools, and I have a relevant example here.  I used scalpel, an open source file carving tool, and scalpel actually did not recover the Pirate Android image.  I would bet that if I tried several other file carving tools, at least one tool would have recovered the Pirate Android.  Not all tools work all the time, but the more tools you try, the better results you will get.  But of course, you can always use a hex editor and take a lot of time to find as many artifacts manually as you can.

Summary
  • Logical extraction tools rely upon device APIs which limits their effectiveness
  • Obtain a physical image of the device if you can
  • All kinds of artifacts can lie in the hex of an Android device
  • Use automated tools but do not rely on just one
Questions, comments, suggestions, or experiences?  Pirate Android fan mail?  Leave a comment below, or send me an email.

Thursday, September 25, 2014

The differences between a physical image and a logical extraction

There's a reason we want a physical image

All blog posts to date
Introduction
This post is a request from a reader.  Thanks for the request!  If you, the reader, ever have a topic you would like to see me dive into, message me.

The question was what data do you have when you obtain a physical image instead of a logical extraction.  Great question.  First, to define a couple of working terms here.  A physical image will be the image you would obtain when following this guide on a previous blog post or using a similar tool, such as a Cellebrite UFED Physical.  A logical extraction of data is a set of data extracted using a forensic app.  For this blog, I'll reference AFLogical by viaForensics, which is a free tool you can find here and you can follow instructions for using it here.

(Please note.  In no way am I trying to bash viaForensics here.  viaForensics is a great company and I admire their work.  I'm referencing this tool as a free logical extraction tool you can download and use while pointing out the weaknesses of using logical extractions.  The fact that the tool is free should be an indication that this tool is not their premiere tool.  They have far more powerful tools and their professional services are among the best in the industry.)

So with all of the above out of the way, here we go ...

Data obtained with a physical image

The answer is everything in storage on the device.  You get every file, every database, every picture, plus also all of the slack.  For a writeup on slack space, check out this page by viaForensics.  Simply with a physical image, you get everything in storage.

There is a good reason why we always want a physical image.  Examining a physical image takes specialty tools, and I go over the basics in this blog post.  If you want to look at data records, such as text messages, you do not have a simple file to examine with all of the records.  You need to find the file storing these records, which is most likely a database, and examine the database file.  The examination process is not straightforward, but you obtain the most data.

What you do not obtain is live running memory.  Sometimes live running memory can contain important data, including decrypted data if the data in storage is encrypted.  I do not intend to go over how to image live memory simply because it is a very complicated process which sometimes does not work.

Data obtained using a logical record extraction tool

A logical record extraction tool is an app which installs on the device.  As I discussed in my post on live imaging, the imaging process requires an exploit.  In that previous post, the exploit allows for root privileges.  Root access is required to image the device, and root access is also required to read files in the /data partition, which is where user records are stored.  A logical record extraction tool does not require root access.  A logical record extraction tool uses Android APIs to extract records from the device and save them to external storage.  These APIs allow a programmer to write an app to request certain records.  The APIs do not return the actual database files but they do return the records.  For a guide on this process, check out this programming guide on how to programatically read SMS from the inbox.  Look specifically at this code snippet (from the website, I cleaned it up some to make it more readable):
if (cursor != null)
{
  try
  {
    count = cursor.getCount();
    if (count > 0)
    {
      cursor.moveToFirst();

      long messageId = cursor.getLong(0);
      long threadId = cursor.getLong(1);
      String address = cursor.getString(2);
      long contactId = cursor.getLong(3);
      String contactId_string = String.valueOf(contactId);
      long timestamp = cursor.getLong(4);

      String body = cursor.getString(5);

      if (!unreadOnly)
      {
        count = 0;
      }

      SmsMmsMessage smsMessage = new SmsMmsMessage(
      context, address, contactId_string, body, timestamp,
      threadId, count, messageId, SmsMmsMessage.MESSAGE_TYPE_SMS);

      return smsMessage;
    }
  }
}


This source code has permissions to read the SMS database.  The program goes through the database row by row and extracts the message ID, thread ID, address, contact ID, and timestamp.  All of this data goes into an “SmsMmsMessage” object.  A programmer can use this object to save the message ID, thread ID,  address, contact ID, and timestamp to a file, which effectively means all SMS records are retrieved and exported.

Here is the problem.  The APIs will give you a certain set of data.  There may be more data associated with these records which the APIs do not return.  The above code, for example, does not return any location related data associated with the message or any metadata associated with the contact or the phone number.  These extra data records will be in the database file which you can read if you obtain a physical image of the device.

The APIs also will not return any deleted records.  When an SMS message is deleted, the database file no longer retains the message.  However, if you have a physical image, you may be able to find the deleted message in slack space.  The APIs only return what records they are programmed to return; they cannot return records floating in slack space.

The logical record extraction process is incapable of extracting files in the /data partition.  You need root access to extract the actual files.  The APIs only return the records, not the files.

Also, there may not be APIs available to return data from third party apps, ranging from Facebook to third party messaging apps to web browsing apps.  If there is not an API, the data can not be retrieved using a logical record extraction app.  With a physical image, you can examine the database files associated with these apps and examine the database files.

Conclusion

In summation, you want a physical image.  The logical extraction tool is a good tool to use if you need a quick look at text messages or call logs, and it also is a good tool to use if you are unable for whatever reason to obtain a physical image of the device.  If you are doing a detailed examination of the device, you will need a physical image.

The logical extraction tools have their purposes.  I am not here to denigrate those tools by any means.  I am here to point out their limitations.

Thank you to one of my readers for suggesting this post.  If you, the reader, have a good topic you would like to see a full post on, shoot me a message and I'd be glad to oblige.

Questions, comments, suggestions, or experiences?  Requests for posts?  Leave a comment below, or send me an email.

Reverse Engineering an Android App File

It is okay to be frustrated


All blog posts to date
Introduction
The Android operating system has all kinds of great apps. I use Netflix, YouTube, Facebook, and the Chrome browser all the time. The development environment for writing Android apps is easy and free, so it attracts some great developers and all kinds of innovation.

The problem is, great developers and all kinds of innovation are not all that the development environment attracts. It depends on which report you go with (this one, this one, this one, or many other excellent reports by trusted security firms), but every security researcher who looks at mobile malware agrees on one thing: the Android operating system is the number one mobile operating system for malware. Malware may be spyware which steals personal data, ransomware which “locks” the device until the user forks over money to some hacker, or a particularly annoying variant which uses up expensive services like premium text messages or large volumes of data and forces the user to pay exorbitant fees to their service provider.

So how do you know if an app is malware? There are malware scanners out there which work with varying effectiveness. (By the way, I totally suggest if you use Android you download a virus scanner, just in case. I personally use Lookout because one of the nice features is a find-my-phone feature, which sometimes is quite handy in the morning when I can't hardly find anything. If only there were a find-my-keys app …)

If you desire, you can reverse engineer an app install file to its source code to determine if there is any malware present. Obviously pouring over code requires programming experience, or at least programming knowledge. If you have it, you may enjoy this exercise.

Introduction to Android app install files

Android app install files are packaged with the extension .apk. If you have downloaded an app, the .apk file is on your phone in the directory /data/app. You can retrieve it in a few ways:

  • if you've imaged the phone, retrieve it from the image using FTK imager
  • if you are root, you can copy the file from /data/app to /sdcard
  • you can install a file manager, like Astro File Manager or ES File Manager, and use the built in app management to backup the app to your /sdcard directory. I personally use ES File Manager for this functionality.

System apps, like Gmail, Browser, Calendar, and other default apps, are in the system partition at /system/app. The easiest way to retrieve those, in my opinion, is to use adb. You can use adb shell to navigate to /system/app and find the name of the file you wish. Exit the adb shell and return to your computer and type the following:
adb -d pull /system/app/<filename>.apk
This command pulls the file to your working directory.

APK files are just zip files. Once the APK file is on your computer, you can rename the file to include a .zip extension and navigate around.

Within the APK file is a file classes.dex. This is the actual app binary. If you navigated through the system/app directory, you may have noticed a bunch of files with the .odex extension. These are the classes.dex file from the associated APK file optimized for the version of the Android OS. If you've ever done a factory reset or installed an update, you know upon the first boot that you have to wait for a while as you see a screen indicating that all of your apps are being optimized. The app optimization process results in creating these .odex files.

Also within is a directory called res. This is the resources directory, including images and other files used by the app.

Another directory in APK files is META-INF. Within here is the digital signature of the app. When the app is compiled, it is digitally signed for authenticity.

When a developer writes an Android app, there is a file called the Android Manifest. Now you'll see there there is a file called manifest.xml, but if you load it in a text editor you won't be able to read much. The manifest includes details about the app, including intents called, broadcast signals sent, and permissions called. The permissions are very important. For example, if you have a simple app, such as a simple game, but the app has the permission to record audio or send text messages, something could be fishy here. Of course, it is possible that the game can take voice commands and send your high score to your friends to brag, so you never know. If there are odd permissions in an app, that should raise some red flags.

There's a ton more you can learn about Android apps than this. What is important to know for now is how to retrieve an app, what the classes.dex file is, and what the manifest is.

Reverse engineering the manifest

So you have an app file. The first thing I like to do is retrieve the manifest. Copy the app to a working directory in your Linux machine and navigate there in a shell. Type the following:
aapt l -a filename.apk > manifest.txt

aapt is a debug tool included with adb.  If adb is not included in your system path, neither will aapt most likely. The command above translates the unreadable manifest.xml file in the APK file to a human readable format and outputs it to the manifest.txt file. Note: what you get out of this is NOT the original manifest. You will need the original source code to retrieve the manifest as it was prior to compiling.

Open the manifest.txt file in a text editor. Look for your permissions. You can do a text search for permissions. You'll see entries along the lines of the following:
    E: uses-permission (line=1238)      A: android:name(0x01010003)="android.permission.BATTERY_STATS" (Raw: "android.permission.BATTERY_STATS")    E: uses-permission (line=1239)      A: android:name(0x01010003)="android.permission.ACCESS_NETWORK_STATE" (Raw: "android.permission.ACCESS_NETWORK_STATE")    E: uses-permission (line=1240)      A: android:name(0x01010003)="android.permission.ACCESS_WIFI_STATE" (Raw: "android.permission.ACCESS_WIFI_STATE")
If you see any suspicious permissions, take note.

Reverse engineering the classes.dex file to source

To reverse engineer the classes.dex file and read it, you'll need a couple of programs which are both installed in Santoku. If you are using Santoku Linux, you're good. Otherwise, download and install dex2jar and JD GUI. Dex2jar is a tool which converts an Android classes.dex file to a Java JAR archive file, and JD GUI allows you to read the JAR file as Java source. Install links are here and here. Install these both.

In the terminal, type the following:
d2j-dex2jar filename.apk
jd-gui filename-dex2jar.jar
The first line creates your jar file, and the second opens the jar file in JD GUI.

In JD GUI, you'll see how the app source is organized. If you have no java experience, you'll probably be lost navigating around, but if you have java experience you'll figure this out quickly. Regardless of your java experience, reverse engineering app source is a royal pain.

Now let's say you see something like this:

or this:

The screenshots are from JD GUI and the Netflix app. Anyone who knows much about programming knows that a, b, c, d, e, and such make terrible class and variable names. Variables and classes should be descriptive. What happened here is the Netflix developers use code obfuscation. Before the app is compiled, a tool goes through the source and renames variables and classes to useless names like a, b, c, and such. They do this as a service to you, just in case you didn't think reverse engineering was already frustrating. When you see obfuscation like this, often your best indication of what is going on are functions you cannot rename (like getCacheDir and getAbsolutePath), and strings. The code obfuscation does not change the functionality of the app, but if the obfuscation changes the text of strings, then functionality is altered.

Strategy

So what strategies do I suggest in reverse engineering source? Honestly I do not suggest a strategy. I suggest figuring things out and finding what works well for you. It can be frustrating but extremely insightful. I've reverse engineered apps before and whenever you find something suspicious, you find a thread that you keep pulling until you might actually find something malicious. Remember to correlate what you find with the manifest. And as always, if you need help, please comment or reach out to me.

Questions, comments, suggestions, or experiences?  Frustrations related to reverse engineering efforts?  Leave a comment below, or send me an email.

Tuesday, August 26, 2014

Examining the image

See what's underneath the hood

All blog posts to date
Introduction
At this point, you have an image of the device. I hope you've patted yourself on the back by now because you have done more in the field of Android forensics than most people ever will.

(If you don't know what a file system is, go ahead and check out this link.)

There are some good free Windows tools for examining an image.  Any good forensic tool will allow an examiner to browse around an image file and will not alter the image file in any way.  This post will detail FTK Imager by AccessData.  So copy your image over to your Windows environment and install FTK Imager.  You can find it on this page.

(Note:  If you would prefer to work on Linux, you can run FTK Imager Lite in Wine.  If you have Windows, I suggest running it natively in Windows instead of in Linux using Wine.)

Open FTK Imager, go to File → add evidence item → image, and open the image. You'll see that the image has opened in Imager, and you'll see all of your partitions. Here's what I see:


And zoomed in on the left side looks like this:


At minimum, you will see the partitions boot, recovery, system, and userdata. Depending on your device, you could see all kinds of other ones.   My phone is a Nexus 5, which as you can see above has a lot of partitions.

Expand the “userdata” partition as seen in the following image:


The image above indicates that “userdata” contains an unnamed ext4 file system. Ext4 is a Linux file system, and FTK Imager can read this file system perfectly.

Navigate around the userdata partition. (If by chance you have encrypted userdata, you may or may not be able to make heads or tails of this partition. If you encrypt your userdata and you don't see a file system and want help decrypting, contact me.) You'll see directories at the root of the partition, some of which are quite important. You'll see /data, /app, and you might see /media. /data stores all of the data associated with your installed apps, /app stores all apps you have installed, and if there is a /media, then that is an “internal sd card”, or a directory which acts like an SD card.


Browse around the directory data and find the directory com.android.providers.telephony. This directory stores data associated with your text messages. Within is a directory called databases and file called smsmms.db. That is a database file which stores all of your text messages. Pretty cool, huh? In a future post, I'll show how to open database files. You can export files by right-clicking on a file as seen below.


Navigate around the system partition and go the directory app. This directory stores default installed apps. Just seeing filenames, you'll probably see some familiar names.


With FTK imager, you can navigate around an image. You can also extract files so you can interact with them in another tool. You can also view the hex of an image. Though it is difficult to make sense of hex, it is important to look at files and even device images in hex.


For example, you may have deleted a photograph you took with your camera and cannot recover it (or so you think.) You may be able to find the photograph in the hex. It takes an experienced examiner, or a curious tech mind, to do this. It also helps to have some good forensic tools at your disposal.

Looking at the hex of files allows you to understand the file at a deeper level.  A photograph file opens by default in an image viewer, but the image viewer will not display geolocation data or data related to the camera which took the photograph if it is embedded in the file.  Viewing the hex of the file may reveal this kind of data.

Previously I mentioned that you can export a file to your computer.  Go ahead and export a photograph if you can find one.  You may find some in the userdata partition at /media/0/DCIM, which is your camera directory (assuming your userdata partition acts like an SD card, and most modern phones work this way.) Pick out a photograph you've taken and export it to a location on your computer.  If you've not installed a Hex editor, go ahead and install a hex editor.  I personally use HxD Hex Editor, though there are many other wonderful ones.

Open the photograph you extracted in a hex editor.  You'll notice the first few bytes of the image look something like this:


All JPG files begin with this header.

Quick forensics lesson: file headers and footers. The way we traditionally identify a file type is by the extension. We see a .jpg file, it's a picture. We see a .docx file, it's a word document. (It's actually a ZIP file. Seriously, try it out. Rename a .docx file to .zip and open it up.) However, that is not how files actually work. When a .jpg file is encoded and saved, the first few bytes, or the file header, are FF D8 FF as seen above.  There are equivalencies with other file types, like ZIP archives, PDF documents, and executables. If you have a nasty piece of malware and rename it with a .docx extension, it may pass under some basic file scanners, but good forensic tools will identify this renamed file as suspicious and indicate that you should check into it. Here is a good writeup on file carving, or putting together files based off of headers and footers

FTK Imager is a powerful, free tool which allows the user to examine a forensic image.  The image of your phone is a file which Windows, Microsoft Office, or any other program you frequently use could not possibly understand, but FTK Imager parses through it perfectly.  Android uses the ext4 filesystem, which is a Linux file system that Windows cannot understand, but FTK Imager can parse through it with ease.

FTK Imager, however, is limited.  It is not a full forensic tool; it is a tool for understanding filesystems.  FTK is AccessData's powerful forensic suite, and it is expensive.  It is a wonderful tool that I have used for years, but this is a blog about free tools.

A free alternative to FTK is Autopsy. I will not be covering Autopsy on this post, but I might do a rundown of it in the future.  It is a very powerful, free, open source tool with great support.  I've had some good luck with Autopsy on Android devices.

Summary
  • FTK Imager can allow the examiner to easily take a look at the image
  • No forensic tool will alter an image
  • Headers and footers, not file extensions, determine the file type
  • Viewing files at the hex level allows for a great understanding of the file
Questions, comments, suggestions, or experiences?  File system questions?  Leave a comment below, or send me an email.