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[Technical Tear Down] Fiesta Exploit Kit – Java Exploit (CVE-2012-0507)

Today, we’re going to look at another exploit that is delivered by the Fiesta Exploit Kit. As usual, the purpose of this post is to provide a technical understanding on how the exploit work. This time it is a JAVA exploit.

This is the link to the fiesta_ek_java_2012_0507 (The .JAR file contained 4 .class files and a .txt file containing the parameters that was passed to the JAVA applet by the launching Javascript). Do note that these are MALICIOUS files, so please run them in a “safe” environment.

[Part 1: Overview]

This exploit uses the CVE-2012-0507 vulnerability. It is a type confusion vulnerability affecting the AtomicReferenceArray class. More information about this vulnerability can be found at these 2 articles (http://schierlm.users.sourceforge.net/TypeConfusion.html and http://weblog.ikvm.net/PermaLink.aspx?guid=cd48169a-9405-4f63-9087-798c4a1866d3 ). This exploit will result in a JAVA sandbox escape and allow the attacker to perform privileged actions.

Exploiting type confusion vulnerabilities are very interesting and very different from exploiting overflow/code execution vulnerabilities. In a sense, it is much easier to perform. To fully understand how it works, we must first discuss about the JAVA sandbox.

[Part 2: Java Applets and Sandboxing]

For the full explanation, refer to http://docs.oracle.com/javase/tutorial/deployment/applet/security.html

In summary, there are 2 types of JAVA applets, Sandbox applets and Privileged applets. As the name suggests, sandbox applets run in a sandbox and have limited access to certain operations. Privileged applets do not have such restrictions but they have to be signed by a recognized CA (Certificate Authority) and the user must give permission to the applet to perform higher privileged operations. Most applets we see are sandbox applets.

The picture below shows some of the restricted operations of sandbox applets.

00

Take special notes of point #1 (cannot access local filesystem and execute files), #2(cannot connect to third party servers), #4(cannot change security level) and #5 (cannot create classloaders).

[Part 3: The Malicious Applet]

Below is the de-obfuscated Javascript code that is responsible for loading the applet.

01

The Javascript first checks that the Java version is Java 6u30 or lower. If it is lower than that, it will fetch the applet from the malicious url and start running the code in the “tampi” class. 2 paramters “peso” and “lapp” are also passed along to the applet.

In the following source codes, I have de-obfuscated some of the strings and variable names so as to make your reading easier. Let’s look at the “init” method of the “tampi” class. The “init” method is first called when a Java applet is loaded.

02

The 2 params that was passed in will be stored into the javaparam_lapp and javaparam_peso variables. Take note of the javaparam_peso which contains a URL link to the malicious server.  I will come back to it later.

Although a sandbox applet cannot create a ClassLoader, it is still able to get a handle to the Applet’s ClassLoader seen on line 17. The “lapp” parameter is the serialized form of an Object[] array. It is decoded by the “foro” method then deserialized by the ObjectInputStream and cast back to its Object[] form on line 19 and 20. The diagram below shows the reconstructed Object[].

03

The Object[] array contains 2 Objects.

  • preyx[] array containing 1 element (null at this point)
  • AtomicReferenceArray (I shall refer to it as ARA) containing 1 element
    • This is actually the same preyx[] array above, only casted as a ARA object

Line 22 to 24 is a Java Reflection call to set the first element of the ARA object to the classloader that was referenced earlier (Line 25 comment line is the non-reflection way to call the same method).  This means a reference to the Applet’s ClassLoader has been assigned into the preyx[] array. This violates JAVA’s type safety and hence a Type Confusion vulnerability.

Line 27 then calls the “juri” method on the first element of the preyx[] array which contains the ClassLoader now. Let’s look at the “preyx” class and its parent class, “odesh

04

05

The preyx class is actually a subclass of the ClassLoader class (See Update #1 for more details).

The “juri” method reads in the “teemb.class” class file and creates a Class object from it with the “defineClass” method on line 16. Take special notice of the last parameter of the method call. This is the “privilege” level to be assigned to this class. An instance of this class is then created on line 17. Let’s look at the “tampi.rous()” method.

06

The commented block is the non-reflection way to call the same methods. This is a typical JAVA privilege escalation method. It creates a ProtectionDomain which has access to “AllPermissions”. By AllPermissions, I mean ALL permissions. This means the “teemb” class would have access to all operations including file I/O onto the local filesystem, network connections to third party servers and so on. The applet can now perform whatever a normal executable jar file can perform when it is run from the local filesystem.

Let’s look at the “teemb” class now.

07

The 3 methods “lewd”, “orly” and “callWriteMethod” are helper methods.

  • lewd – some form of decoding function based on xor
  • orly – reads everything from the inputstream into the input byte array and returns the total number of bytes read
  • callWriteMethod – calls the write method of the specified class

The constructor of the “teemb” class calls the “doPrivileged” method which then calls the “run” method below.

08

Line 53 assigns the malicious URLs in “javaparam_peso” into the arrayOfString variable. For each url in there, the for loop on line 59 is executed. In summary, the for loop perform these operations.

  • Creates the temp file path to store the malicious file
  • Opens a URL stream to the malicious url
  • Read the first 256 bytes from the stream (this is the xor key)
  • If key is read successfully, read in the rest of the stream
  • Decode the rest of the stream with the xor key and store it into the temp file path
  • Use the Runtime class to execute the downloaded file in the temp file path

In gist, this is a Download and Execute code. At this point, the attacker would be able to download any malicious file from the Internet and execute it on the victim’s machine. This defeats all the restriction that was placed on sandbox applets, effectively bypassing the sandbox.

[Update #1]

The purpose of this update is to better explain the role of the vulnerability in this exploit. The original article did not really explain this part clearly.

Earlier in the article, I mentioned that preyx is a subclass of ClassLoader. This means under normal circumstances, the applet would not be able to create a preyx instance (e.g. thru a “new preyx()” call) because sandbox applets are not allowed to create ClassLoaders. CVE-2012-0507 allowed the applet to “smuggle” a reference to a preyx object into the applet’s execution environment without actually creating one.

However, the next question is why is the preyx object important? The answer is privilege escalation. In order to execute malicious operations, a higher privilege level is needed other than the sandbox privilege level. The defineClass method allows the attacker to create a class with a specified “privilege” level (thru the last param of the function call). However, the defineClass is a protected method which means only subclasses of ClassLoader would be able to call it and that is where the preyx class comes in.

To understand this easier, put yourself in the shoes of the exploit writer. The thinking would go something like this.

  • Goal is to run malicious code
  • Need a way to load a class with elevated privilege levels (defineClass method)
  • To call the defineClass method, need a subclass of ClassLoader (preyx class)
  • Normal applet would not allow the creation of a preyx instance, need a way to get a preyx object into the applet environment (CVE-2012-0507 vulnerability)

[End Update #1]

I hope you have gained a better understanding of this exploit. Until next time!!

Glenn (@_graypanda)

[ Technical Tear Down: Fake WeChat Android App (Packed using Bangcle)]

[ Sample used in the analysis ]

MD5: 35C0A075CBC6135D957BD10769E3A620
SHA1: 554FD3D80B696F0677231A54F35B61B5774F2940

This is an Android malware found in the wild. You may be wondering what’s so special about this malware as there are thousands of Android malware out there. Well, this is one of the first few Android malware that has been packed. By being packed, it make reversing almost impossible for  static code analysis. As mentioned in my previous writeup, script from @timstrazz is available if bangcle is used to pack the apk.

As we at vxsecurity.sg always encourages people to get their hands dirty, we have attached the malicious sample here.
The password is “infected29A

[ Tools Used ]
Cebero Profiler is used to disassemble the apk file to analyse the smali code.

Dex2Jar and Java Decompiler are used to decompile the apk file to a jar file and subsequently to get the java code for analysis.

Android Emulator is used for the dynamic analysis portion.

[ Before debangcling the app ]

As explained in my previous writeup, if Bangcle is used to pack an apk, when you try to reverse the file, you will notice a certain pattern like additional library files and new Java class files (ACall.class, ApplicationWrapper.class, FirstApplication.class). As shown in the picture below, this particular .apk when we decompile using dex2jar or disassembled using Profiler also contains similar files.

banksteal-bangcled

One thing my team and i found out is whenever we see this class (com.secapk.wrapper.ApplicationWrapper) and it’s be instantiated by default whenever the process is started.
We can safely deduce that it could have been packed using bangcle. So I tried using the script from @timstrazz found  here to unpack it.

Success!! It could be unpacked 🙂

[ Debangcled APK ]

The resultant of the debangcling process is a dex file which can now be reversed.

debangcled-apk

There are some obvious clues which flags out as a sorethumb to the analyst analysing the apk. Like the package name is called “com.example.banksteal“!! Who names their application banksteal anyway? And furthermore if your package name contains words like “example”, you can’t upload your apk into Google Playstore as it is deem as a generic name. So there are 2 things you can conclude even without analysing anything. 1st: This .apk is obviously from some third party market or circulated elsewhere other than the official Google Playstore. 2nd: This .apk is highly suspicious due to the naming conventions used. You can’t be 100% sure that this is a malware based on the above mentioned 2 pointers. So let’s delve in deeper.

[Analysis]

The app requests for the following permissions:

app-permissions

Upon installing the app, you will get the following icon. This is the popular WeChat Android app, a Whatsapp like instant messaging app. The App is named as “Wēixìn” in Chinese which is the pinyin name for WeChat and means micro message.

bangcle-install

 

When the app is invoked, it will first ask you for device admin permission for lock your phone, it can be seen from the code snippet below taken from the Launcher class.

launcher-device-admin

 

bangcle-activate-lockscreen

and then for your credit card details as shown from the screenshot below when the app is executed on an emulator:

bangcle-creditcard

Next, more details will be asked like the type of bank from which the credit card was issued, card’s cvc number, expiry date, social security number, address etc

bangcle-socialsecurity-number

bangcle-mastercard

Let’s say, I chose the JCB card as my credit card and fill in all the details and click next:

banksteal-email-failed-to-send-out

I am running this in an emulator and it prompts that, “The email/message is not sent out!” I hope that my Chinese translation is accurate but then this raises another question of why a messaging app needs to send out an email. Let’s look into the source code for more evidence.

If you look at the BankData class, it summarises the data it harvests:

bangcle-bankdata

 

Under the Const class you will also get the email address of the receiver.

email

 

There are more clues in the MailUtil class where the host of the receiver is resolved.

email-host

In summary, this app requests for your credit card details and mail out these details.

Well ok, what’s wrong you may ask.

Just that in an actual WeChat app registration/installation no such details are required.

[Actual WeChat App]

Now let’s look at the actual WeChat app’s permissions when downloaded from the Google PlayStore.

real-wechat3

You may think, this looks more malicious as it requests for much more permissions as compared to the previous app!!! But hold on.

Let’s take a look at the registration portion:

real-wechat1

real-wechat2

And that’s it. No credit card nor any bank details is required during installation nor registration of the official WeChat app.

[Conclusion]

The analysed application is an Android malware masquerading as the WeChat android application and request user for their credit card details. Supposedly to give the user a surprise at the end of the month!! On the other end the original app even though it requests for much more permissions, it does not cause financial loss for the user. This reiterates the point that users must not install app from unknown sources or third party markets. Most importantly, whichever app from third party or even the official Google Playstore requests for your credit card details, you are highly recommended to uninstall the app unless you feel generous 🙂

Another unique feature of this app is the usage of a packer. In this case, it is using Bangcle and we already have an unpacker available which helps in our analysis. What if a more complicated packer is used? It will further impede the analysis process. Then we have to only rely on dynamic analysis and if the apk is advanced enough to have anti-emulator techniques etc, how do we overcome this ? I will have another writeup on the possible dynamic analysis paradigm in the near future. If you have ideas, do share with us too.

David Billa (@billa316)

[ Android Packers: Bangcle ]

In this writeup, let’s take a look at Android packers in general before focusing on one of the more popular packer, Bangcle.

[An Introduction to Android Packers]

There are many useful reasons packers are used in software engineering, like for code obfuscation, compression, protecting your work against reverse engineers etc. The more sinister usage are for AV bypassing, masquerading malware and making the job of whitehat reverse engineers harder.

The common packers used for packaging EXEs are UPX Packer, RLPack, NSPack etc. As till recent times, the bulk of computer usage has been limited to PCs thus these were commonly used.

With the increase in usage of Android devices, different types of malicious Android applications were beginning to proliferate. In recent times, Android malware packaged using packers are beginning to surface in the wild.

A more detailed analysis on Android packers can be found here.  In the next section, I will be focusing on one of the increasingly popular Android packer which is being used for Android malware.

[Bangcle: Android Packer]

bangcle

Bangcle is a mobile application security services provider company based in China. One of the services it offer is Android app protection as shown in the screenshot above.

A user just have to register and subsequently use the dashboard to upload your apk file and you will get the bangcled apk ready for use.

bangcle-appShield

I tested with a simple Hello World application. Before bangcling this is how the app’s structure will look like.

hello-world

 

After uploading the hello world apk to be bangcled the resultant apk will be as shown below:

hello-world-bangcledAfter the app has been bangcled you will have additional library files (libsecexe.so, libsecmain.so etc) and new Java class files (ACall.class, ApplicationWrapper.class, FirstApplication.class).

And the new bangcled application will also have additional permissions. Do take note that the actual hello world had no permissions initially.

hello-world-bangcled-new-permissions

The ACall.class file loads the additional libraries, FirstApplication.class file calls the super.onCreate function to initialize the activity and the ApplicationWrapper class subsequently loads the encrypted dex file which will be then be decrypted in the memory by the loaded libraries and finally executed. As it is being decrypted in memory, this makes static analysis of an Bangcled apk hard and tedious.

A more detailed writeup from Virus Bulletin can be found here.

[Reversing Bangcled APK]

In DEFCON 2014, TIM “DIFF” STRAZZERE (@timstrazz) presented and subsequently released the solution to debangcle a bangcled apk.

The full source can be found at his github acct here. Below is his shellscript portion.

To run his script (in a Linux environment), configure an Android enmulator (using AVD from the Android SDK) and install the bangcled apk in it using adb tools (adb install <bangcled-app.apk>).

Now execute his script after starting the app in the Android emulator. The dex will be decrypted in memory as discussed earlier. The script actually scans the memory in areas where an odex shouldn’t be to detect the magic bytes (“dey\n036”) for the optimized dex file and subsequently dumps out the specific memory portion as an dex file using baksmali. So the end product is an decrypted dex file which can now be analysed 🙂

[In Summary]

As the usage of Android increases, more variants and complex malwares are being now detected. As it was in PC malware ecosystem, packers are making inroads into Android now. Bangcle which is a China based Android packer created as a means to protect one’s apk is currently being misused for packing Android malwares making reversing hard.

However, bangcled apks can now be reversed for static analysis thanks to the great work by @timstrazz.

The next writeup will be on an technical teardown of a bangcled apk found in the wild recently.

David Billa (@billa316)

 

[ Technical Tear Down: UUPlay, ANDROID Malware ]

[ Sample used in the analysis ]
MD55: E7D6FEF2F1B23CF39A49771EB277E697
SHA1: F5735DC4D9502F99D3B62C49EF083B403D627124

We gotten this sample from Jacob’s old friend, “Amnesia”. UUPlay application portrays itself as an legitimate Google playstore application but it also doubles up as an malware in disguise, sending out victim(s)’ information from the device. You can check the VirusTotal detection rate here. https://www.virustotal.com/en/file/918ec0a543b6774c54564fe676e7bd47456b6a95facca42a2da4a995703129b8/analysis/

[ Tools Used ]
Cebero Profiler is used to disassemble the apk file to analyse the smali code.

Dex2Jar and Java Decompiler are used to decompile the apk file to a jar file and subsequently to get the java code for analysis.

Android Emulator and Burp Suite are used for the dynamic analysis portion.

[ Permissions ]
uuplay-permissions

From the permissions list above, you can notice that it also haves the capability to download and install applications. Users may think that since it is a Google PlayStore app , it needs such permissions. But in this case, it also have the READ,WRITE and SEND SMS permissions. These permissions can be abused to send premium SMSes to premium numbers resulting in additional costs for the victim(s).

[ Source Code Analysis ]
The .apk is most probably obfuscated using ProGuard. From the screenshot below, one can notice that after reversing the .apk file, it seems bloated with multiple alphabetical java class files like any typical .apk file that is protected by ProGuard.

uuplay-proguard

Thus it makes analyzing the source code harder. For more information on what ProGuard actually does, you can refer here.  In this analysis, dynamic analysis was primarily done to map out the behavior of the application.

[ Analysing Manifest File ]
From the manifest file, there are quite a lot of activities declared under the “com.google.hfapservice” package.

uuplay-google-hfapservice

For the list of Google Package names available for Android, you can refer to the list here. When checked against this list, it does not have any “com.google.hfapservice” package. It is a clear sign that these might potentially be malicious activities running in the background masquerading as Google services.

Other than these activities, from the manifest file, it can also be deduced that a total of 3 services are running.

uuplay-services

Two of the services are linked to the fake Google packages and the third service is supposedly a log service linked to the “com.uucun” package which is the main package name as noted from the manifest file.

Now let’s move on to the dynamic analysis portion.

[ Dynamic Analysis ]

Upon installing the uuplay.apk, the app’s icon does not appear under the device home or the application’s display. However, it can be found under the apps’ listings under Android system’s settings.

uuplay -appinfo

As you can notice, the icon is the same as the Google PlayStore’s icon.

The app can be started using Android Debug Bridge (adb) commands. From the manifest file, the main activity’s name can be deduced as RootActivity. The following command was used to invoke the main activity:

adb shell am start -n com.uucun4470.android.cms/com.uucun4470.android.cms.activity.RootActivity

After firing up the application, the UI is displayed as below.

uuplayUI

Next, we used Burp Suite to monitor the network traffic. For instructions on configuring Android emulator to work with Burp Suite you can refer here.

Upon installing the app, information like imei number, sim card type, os version, date timestamp, app version and airpush version (mobile ads) were double url encoded and was posted to h–p://cloud6.devopenserv.com as shown in the screenshot from Burp Suite below.

uuplay-burpsuite1Next it checks for updates for all the pre-installed apps from h–p://agoldcomm.plat96.com

uuplay-burpsuite-appupdate

If you search for any app, e.g Whatsapp, you can see that the searched information together with the phone information being posted out and subsequently the relevant apk file will be downloaded from h–p://apk.hiapk.com

h–p://apk.hiapk.com is one of many un-official Android Marketplace from China.

While all these can be considered “fairly” normal behavior for an “Google PlayStore”, there are also other suspicious activities. Like at certain intervals (even when the app is not in use) encrypted data are being posted to urls like h–p://log6.devopenserv.com

uuplay-burpsuite-encrypteddata

The URL to which the data is posted is not hard-coded.  Meaning, sometimes the data are being posted to other URLs. But the key point is data (judging from the long list of permissions, it could be anything the app has access to) is being encrypted and were being ex-filtrated from the device.

[ Conclusion ]

In conclusion, uuplay is an application that masquerades as the official Google PlayStore application. It does similar activities like installing applications fetched mainly from the following urls:

– h–p://apk.hiapk.com

-h–p://agoldcomm.plat96.com

At the same time, it also posts out personal information masked as log data to the following urls:

-h–p://cloud6.devopenserv.com

-h–p://pus7.devopenserv.com

-h–p://log6.devopenserv.com

In short, install at your own risk!!

David Billa (@billa316)

[ Technical Tear Down: SIMPLELOCKER, ANDROID Ransomware ]

[ Sample used in the analysis ]
MD5: FD694CF5CA1DD4967AD6E8C67241114C
SHA1: 808DF267F38E095492EBD8AEB4B56671061B2F72

This is one of the latest Android ransomware that we got. It can be found in Mila’s website and you can take a look at the VirusTotal analysishere. The ransomware is TOR enabled and goes about encrypting files in the Android system and subsequently requesting for ransom before the key will be given to decrypt the files. Now let’s look at the technical details of the ransomware.
I’ve attached the link to the file here for anyone interested to try analysing it themselves.
The password to the attachment is “infected29A

[ Tools Used ]
Cebero Profiler is used to disassemble the apk file to analyse the smali code.
Dex2Jar and Java Decompiler are used to decompile the apk file to a jar file and subsequently to get the java code for analysis.
Android Emulator is used for the dynamic analysis portion.

[ Files ]
Below is the list of files after using Cebero Profiler to disassemble the apk file.

simplelocker-filelist

[ Manifest File ]

Below is the full manifest file extracted from the .apk file:

simplelocker-manifest

[ Permissions ]
Now let’s take a look at the AndroidManifest.xml file. When installing the application, the user is prompted with a list of permissions the application requires.

simplelocker-manifest-permissions

The permissions are more or less harmless in nature and doesn’t throw a redflag as in the case with other malwares. The permissions that could raise any kind of suspicion are android.permission.RECEIVED_BOOT_COMPLETED and android.permission.WAKE_LOCK. RECEIVED_BOOT_COMPLETED permission is required to allow an application to receive a broadcast that the system has finished booting up. WAKE_LOCK permission is required to keep the processor from sleeping or screen from dimming.
Both these permissions look harmless at first look but could be used to run an application upon bootup and keep it running without being pushed to suspended state by the Android operating system.

[ Source Code Analysis ]
From the manifest file, Main.class file is first launched upon starting of the app. The Main.class checks whether the MainService.class is running and if not calls it. MainService.class is linked to 5 other classes. MainService1.class, MainService2.class, MainService3.class, MainService4.class and MainService5.class. These classes are all interlinked.  What it does:

• Checks whether the TOR connection is established.
• If not try to establish a TOR connection.
• Next it gets a list of predetermined file extensions..
• Gets the filename of all files in the external storage that has any of the file extension.
• Does an AES encryption of the files.
• Displays a message presuming asking for an ransom.

Now let’s look at 2 class files that is of special interest. As it reveals more information about how the ransomware works. First is the Constants.class file. Look at the screenshot below.

simplelocker-constants

The first constant ADMIN_URL which point to http://xeyocsu7fu2vjhxs.onion/ is the TOR site the ransomware connects to. This explains why it checks and tries to establish TOR connection upon starting. 2nd piece of information we can find from the “Constants” class is that it contains a variable EXTENSIONS_TO_ENCRYPT which contains the following file extensions: “jpeg”, “jpg”, “png”, “bmp”, “gif”, “pdf”, “doc”, “docx”, “txt”, “avi”, “mkv”, “3gp”, “mp4”. As the name of the variable suggest, this is the file extensions which the Ransomware would encrypt.

Another key piece on information is the CIPHER_PASSWORD, “jndlasf074hr“. Is this the key used to encrypt the files? If yes, could it also decrypts the files? For this, we take a look at the FileEncryptor.class. Look at the screenshot below of two of the functions:

simplelocker-fileencryptor

As you can see from the functions, the encrypt and decrypt routine is fairly straightforward, AES using “jndlasf074hr“. Now let’s take a look at the AesCrypt class.

simplelocker-aescrypt

AesCrypt has both the encrypt and decrypt functions. Thus for example if you have installed the app by mistake and your files are now encrypted, you can re-use the functions to decrypt the files without the need to pay the ransom!!!

2 other interesting classes are “HttpSender” & “HttpSender$1”, as it send data & fetches JSON data from “http://xeyocsu7fu2vjhxs.onion/“.

[ Dynamic Analysis ]

I installed the app onto an emulator to test how it works. As it was executed on the emulator it could not perform all the ransomware “features”. But from the errors thrown, you can confirm the flow of the features. And also another reason is so that I need not turn one phone into a brick just in case it does some irreversible damage to the device.

Upon installing, it is displayed as a porn app as shown from the name of the icon.

simplelocker-icon

When the app is run, it shows a message in Russian and this screen just keeps on popping up consistently even when the app is closed.

simplelocker-firstscreen

So what does actually it do in the background. Let’s check the logcat.

simplelocker-torconnection

Obviously it is trying to establish a TOR connection which it can’t and this is repeated continuously.

simplelocker-netwkstat2

There are also a lot of error messages thrown by the external libraries (which are actually *.so files needed for TOR connection) and also error that the app could not excess the External Storage which it needs to access in order to encrypt the files.

These confirms the flow of the ransomware features/actions as discussed earlier in the static analysis portion.

[ Conclusion ]

In conclusion, the ransomware is most probably marketed as an porn app and upon installing will establishes a TOR connection to the C&C and  encrypts the victim’s files and will subsequently asks for an ransom in order to decrypt back the files. However on closer observation, a technically savvy person could decrypt the files without paying the ransom as the decryption key is hardcoded within the app itself. But to be absolutely safe, don’t install porn apps 🙂

David Billa (@billa316)

 

 

 

[Technical Tear Down] Fiesta Exploit Kit – Silverlight Exploit (CVE-2013-3896 & CVE-2013-0074)

While analysing the Fiesta Exploit Kit, a number of “java applets” and a Silverlight application was downloaded by the exploit kit. This post will describe the Silverlight exploit. The purpose of this post is to give a better technical understanding of how exploits makes use of vulnerabilities and play with memory to acheive shellcode execution.

[ Sample used in the analysis ]
MD5: a920d4ead958746e4886859fc28dbcdb
SHA1: 12a3f08144d3d9f317a62cfefb4c54fe823e9724

[ Part 1: Getting Started ]

For those who want to follow along, this is a link to the Fiesta-EK-silverlight-exploit. Do note, this is a MALICIOUS file, so please run it in a “safe” environment. The password to the attachment is “infected29A”

Silverlight applications come in a .xap file. As .XAP file is a .zip compressed file that contains the client DLL for the Silverlight application as extra DLLs of assemblies that the Silverlight application could be using. This can be the author’s own assemblies of user controls or assemblies that are not part of the Silverlight runtime by default that gets installed on the client.

External resources like images or other types of files can also be added to the .XAP file if you do embed them into your client DLL. By renaming the .xap file to .zip you can view its contents. Inside the .xap file and we will see a manifest file and a DLL file:

The manifest file describes the entry point of the application so let’s take a look at that first.

In this case, the manifest file is quite straightforward. The “EntryPointAssembly” tells us that the application starts in the tuyngled30.dll and the “EntryPointType” tells us that it starts in the App class of that package.

The next step would be to decompile the dll file. For this purpose, I used .Net Reflector. Throw the DLL into it and you will get back the C# source of the internal classes. Let’s take a look at the entry point, the App class.

02_appclasscode

The constructor calls the InitializeComponent which actually loads in components based on a xaml file. Normally, you would want to look at the xaml file to see if it loads in any additional suspicious components but for this case, the xaml file does not load in anything extra. The Application_Startup method is also called when the Silverlight application is started. In this case, it creates a new “jake” object and passes it the InitParams (We will get back to these params later)

[ Part 2: Overview ]

This exploit uses 2 vulnerabilities to achieve code execution.

CVE-2013-3896 (Memory Disclosure vulnerability) bypasses ASLR by disclosing adjacent memory such that ROP gadgets can be reliably located within mscorlib.ni.dll.

After which, CVE-2013-0074 (Double-Dereference vulnerability) achieves code execution. DEP is also bypassed by overwriting the shellcode into legitimate executable memory addresses.

Also, this exploit seems to target only x86 (32-bit) architecture.

[ Part 3: Bypassing ASLR with Memory Disclosure CVE-2013-3896 ]

Let’s look at the “jake” class.

04_jakecons

The constructor of the “jake” class assigns the init parameter of “dies” into the byte array named “curd” and then calls the “wake” method (we will get back to this “dies” param later).

05_jakewake

The first interesting point of the wake method is on line 103. The “brab” method basically exploits the memory disclosure vulnerability and returns the address of the ROP gadget. For this part, we will examine the “brab” method. Part 4 will describe the rest of the code.

06_jakebrab

First, take a look at line 23 and 24. A “polk” object is created. Reading the source of the “polk” class will reveal that it is a subclass of the MemoryStream object. It will be used together with the BitmapImage and WriteableBitmap class to trigger this vulnerability.

The vulnerability lies in the BitmapImage.SetSource(MemoryStream ms) method. This method creates a BitmapImage based on a memory stream. The amount of memory that is read in is based on the “length” attribute within the stream. This value is controlled by user and can be manipulated to a larger value to read in adjacent memory.

In this case, the memory stream is manipulated to look like a PNG file with a declared PLTE chunk of 0x300 bytes. Those unfamiliar with the PNG format can read about it at the following links. These 2 chapters show how a PNG is represented and the idea of chunks within a PNG file. (http://www.w3.org/TR/PNG/#5DataRep and http://www.w3.org/TR/PNG/#11Chunks)

c01_pngheader

The constructor first writes 0x29 bytes into the memory stream. These 0x29 bytes is the header of a PNG file up till the PLTE chunk header.

07_polkread

The “racy.tunc” method then calls the BitmapImage.SetSource method which will call polk.Read method which has been overridden such that it will declare additional arrays in a specific order (specifically the scum and raps array). When these arrays are declared, they are stored right after the already written PNG header.

c02_pngfooter

The last few array elements will store the PNG Footer.

When viewed as a contiguous memory chunk, it looks like a legit PNG file. The resultant memory layout of the “polk” object is shown below.

c03_polk

The main objective of this arrangement is to have the additional arrays be stored in the PLTE chunk of the PNG. If we look at the PNG specifications at http://www.w3.org/TR/PNG/#11Chunks under the PLTE Palette section, it says that

“The first entry in PLTE is referenced by pixel value 0, the second by pixel value 1, etc.”

On line 29 of the “brab” method, “racy.papa” method creates a WriteableBitmap object with the affected BitmapImage object. The disclosed memory in the PLTE Chunk can then be accessed with the WriteableBitmap.Pixels array. The method transforms this pixels array (which stores 4 bytes/pixel in a ARGB format) into a contiguous byte array and returns it. So why is the disclosed memory important? Let’s take a look at what is disclosed.

c04_add

If you are not familiar with how C# stores objects/primitives in memory or about Runtime Type Information (RTTI), read this first ( http://www.abhisheksur.com/2011/09/internals-of-net-objects-and-use-of-sos.html) . Don’t worry if your RTTI addresses are different from mine, this is because these addresses are ASLR-ed. Basically, all objects in C# has this RTTI pointer. The RTTI stores type information that can be used for dynamic type-casting.

The objective here is to get the RTTI address. The RTTI address points to somewhere in the mscorlib.ni.dll which is ASLR enabled. However, this RTTI is always loaded at the same offset from the base address of the mscorlib.ni.dll. Therefore, with the RTTI address, the base address of the DLL can be calculated. With that, the exact location of the ROP gadget can be reliably determined thus bypassing ASLR.

[ Part 4: Achieving code execution with CVE-2013-0074 (Double De-reference vulnerability) ]

06_jakebrab

Looking back at the “brab” method again, these are what the other functions do.

  • vamp(p1, p2) – Reads 4 bytes from p1 array starting from offset p2 (Read Pointer)
  • oils(p1,p2,p3) – Writes 4 bytes (p3) into p1 array starting from offset p2 (Write Pointer)
  • racy.lisp() – Returns the offset to find the ROP Gadget on different Silverlight versions.
    • Based on this method, it seems to target the following Silverlight versions

08_racylisp

The rest of the “brab” method does the following

  • Save the RTTI pointer (num)
  • Save the raps[] pointer (num2)
  • Save the scum[] pointer (mews)
  • Save the offset to ROP gadget (num3)
  • Calculate and save the memory location of ROP gadget (num6)
  • Get the memory address of 41 bytes before the end of the PNG (c) (Take note of this!)
  • Writes a total of 8 bytes starting from memory address c in the following format
  • c05_8bytes
  • Returns the value of c “pointer to itself” (Take note of this!)

Let’s return back to the “wake” method.

05_jakewake

The next important step is on line 109. The “diam” class exploits the CVE-2013-0074 vulnerability. The vulnerability lies in the ScriptObject.Initialize method. Fortunately, this is a protected method, meaning only child classes can call it. Unfortunately, ScriptObject has a public child class called HTMLObject. By sub-classing the HTMLObject class, it is now possible to call the protected Initialize method.

09_diam

The “diam” class inherits from HTMLObject and wraps the Initialize method in the “gain” method. Calling the “gain” method with “(IntPtr) a” will lead to a “call [a+4]” instruction, meaning the next pointer after “a” will be called.

c05_8bytes

Recall that in the “brab” method, the above 8 bytes was written in the IDAT chunk, 41 bytes before the end of the PNG? Line 109 calls the “gain” method with “pointer to itself” as the parameter. This would lead to a “call [ptr to ROP gadget]”, thus executing the ROP gadget.

So, what ROP gadgets are being used? Interestingly, this ROP chain only consists of 1 gadget. The gadget consists of the following byte sequence “83-49-34-40-B8-01-00-00-00-C2-04-00”. Passing this sequence into Capstone yields the following result.

c06_rop

The gadget simply performs an OR operation with the value in [ecx+0x34] with 0x40.

Analysing the memory with WinDBG reveals that ecx at this moment contains the value of the “pointer to itself”. Therefore, [ecx+0x34] actually points to somewhere after the PNG “file”. In this case, it points to the most significant byte of the length of the next adjacent array, jake.chap[].

By performing this OR operation, it changes the length of the jake.chap[] array from 0x00000003 to 0x40000003, effectively allowing jake.chap[] array to access the entire 32-bit process memory space.

[ Part 5: Executing the Shellcode ]

05_jakewake

With access to the entire memory space, the next step would be to get the exploit to run the shellcode payload. From line 114 of the “wake” method, a “jake.loci” object is created. This object is responsible for putting the shellcode into memory and executing it.

10_loci

Take note of the “bust” method. It is the first method declared after the constructor and it contains a lot of code that does nothing. It performs a whole lot of mathematical operations and the result is neither returned nor used.

11_locipore

On line 115, the “loci.pore” method is called next with the parameter “curd”. The “curd” here refers to “jake.curd”. Recall the initParams that was passed into this Silverlight application in the beginning? “jake.curd” contains the value passed in through the “dies” parameter. The “loci.pore” method basically transfers “jake.curd” into “loci.curd” variable.

This is actually the shellcode itself, it is Base64 encoded and passed in through the initParams of the Silverlight Application. In this way, the shellcode could be changed or modified without needing to recompile the whole Silverlight application.

12_locignaw

On line 116, the “gnaw” method is called. The comments basically explain what this method is trying to do. Here is a summary. (Note: [“pointer”] means dereference the pointer)

  • Create “bust” method to make sure a vTable entry is created.
  • Create a new array “scum” which contains a marker value and the pointer to this “loci” object
  • Iterate through the jake.chap[] (with 0x40000003 length) to find the marker value
  • “loci” pointer = next pointer from the marker value
  • vTable pointer = [“loci pointer”] + 0x2c
  • “bust” function pointer = [“vTable pointer”] + 0x8 (I believe the value 0x8 is used because “bust” is the second function in the “loci” class)
  • Exchange the bytes from “loci.curd” (Shellcode) and the bytes at [“bust pointer”]
  • Call the “bust” method (Shellcode Execution Here!!!)
  • Exchange the bytes back from “loci.curd” and the bytes at [“bust pointer”]
  • Restore the length of jake.chap[] to its original value (Element 0x3fffffff causes an integer overflow, essentially becoming jake.chap[-1])

Shellcode execution is achieved by making use of the jake.chap[] array to override the “bust” function instruction bytes with the shellcode bytes and then calling the “bust” function. In this way, there is no need to bypass DEP as the shellcode is residing in a legitimate executable memory.

Although I am unable to confirm this, I believe that the length of the initial “bust” function instruction bytes must be the same as the length of the shellcode that will replace them. This would explain why there are a whole lot of useless instructions initially declared in the “bust” function.

[ Conclusion ]

I have discussed how this exploit made use of 2 vulnerabilities to achieve shellcode execution. I hoped that you gained a better understanding of how the exploit was able to work. Also, an additional point to note is that the exploit code is similar to the POC code initially released by Packet Storm for these 2 vulnerabilities.

This is also my first time writing such a technical article and there are some parts of the article that I found it difficult to explain the technical details in words. If you have any advice/suggestions on how to improve this, please let me know through the comments.

If not, I hoped you enjoyed the article!

Glenn (@_graypanda)