Refined new Android malware marks the most recent evolution of cellular ransomware

Screenshot of mobile ransom note in Russian language

Attackers are persistent and motivated to constantly evolve – and no platform is immune. That’s the reason Microsoft has been working to increase its industry-leading endpoint safety capabilities past Home windows. The addition of cellular risk protection into these capabilities implies that Microsoft Defender for Endpoint (beforehand Microsoft Defender Superior Menace Safety) now delivers safety on all main platforms.

Microsoft’s cellular risk protection capabilities additional enrich the visibility that organizations have on threats of their networks, in addition to present extra instruments to detect and reply to threats throughout domains and throughout platforms. Like all of Microsoft’s safety options, these new capabilities are likewise backed by a world community of risk researchers and safety specialists whose deep understanding of the risk panorama information the continual innovation of safety features and be sure that clients are shielded from ever-evolving threats.

For instance, we discovered a bit of a very subtle Android ransomware with novel strategies and conduct, exemplifying the fast evolution of cellular threats that we’ve additionally noticed on different platforms. The cellular ransomware, detected by Microsoft Defender for Endpoint as AndroidOS/MalLocker.B, is the most recent variant of a ransomware household that’s been within the wild for some time however has been evolving continuous. This ransomware household is thought for being hosted on arbitrary web sites and circulated on on-line boards utilizing numerous social engineering lures, together with masquerading as fashionable apps, cracked video games, or video gamers. The brand new variant caught our consideration as a result of it’s a sophisticated malware with unmistakable malicious attribute and conduct and but manages to evade many out there protections, registering a low detection fee towards safety options.

As with most Android ransomware, this new risk doesn’t truly block entry to recordsdata by encrypting them. As a substitute, it blocks entry to units by displaying a display that seems over each different window, such that the consumer can’t do anything. The stated display is the ransom be aware, which comprises threats and directions to pay the ransom.

Screenshot of mobile ransom note in Russian language

Determine 1. Pattern ransom be aware utilized by older ransomware variants

What’s progressive about this ransomware is the way it shows its ransom be aware. On this weblog, we’ll element the progressive methods during which this ransomware surfaces its ransom be aware utilizing Android options we haven’t seen leveraged by malware earlier than, in addition to incorporating an open-source machine studying module designed for context-aware cropping of its ransom be aware.

New scheme, similar purpose

Previously, Android ransomware used a particular permission referred to as “SYSTEM_ALERT_WINDOW” to show their ransom be aware. Apps which have this permission can draw a window that belongs to the system group and might’t be dismissed. It doesn’t matter what button is pressed, the window stays on prime of all different home windows. The notification was meant for use for system alerts or errors, however Android threats misused it to pressure the attacker-controlled UI to totally occupy the display, blocking entry to the gadget. Attackers create this state of affairs to influence customers to pay the ransom to allow them to achieve again entry to the gadget.

To catch these threats, safety options used heuristics that centered on detecting this conduct. Google later applied platform-level modifications that virtually eradicated this assault floor. These modifications embody:

  1. Eradicating the SYSTEM_ALERT_WINDOW error and alert window varieties, and introducing just a few different varieties as substitute
  2. Elevating the permission standing of SYSTEM_ALERT_WINDOW to particular permission by placing it into the “above harmful” class, which implies that customers should undergo many screens to approve apps that ask for permission, as a substitute of only one click on
  3. Introducing an overlay kill change on Android and later that customers can activate anytime to deactivate a system alert window

To adapt, Android malware advanced to misusing different options, however these aren’t as efficient. For instance, some strains of ransomware abuse accessibility options, a way that might simply alarm customers as a result of accessibility is a particular permission that requires customers to undergo a number of screens and settle for a warning that the app will be capable to monitor exercise through accessibility companies. Different ransomware households use infinite loops of drawing non-system home windows, however in between drawing and redrawing, it’s doable for customers to go to settings and uninstall the offending app.

The brand new Android ransomware variant overcomes these boundaries by evolving additional than any Android malware we’ve seen earlier than. To floor its ransom be aware, it makes use of a collection of strategies that make the most of the next elements on Android:

  1. The “name” notification, amongst a number of classes of notifications that Android helps, which requires fast consumer consideration.
  2. The “onUserLeaveHint()” callback technique of the Android Exercise (i.e., the standard GUI display the consumer sees) known as as a part of the exercise lifecycle when the exercise is about to enter the background because of consumer alternative, for instance, when the consumer presses the Residence key.

The malware connects the dots and makes use of these two elements to create a particular kind of notification that triggers the ransom display through the callback.

Screenshot of malware code

Determine 2. The notification with full intent and set as “name’ class

Because the code snippet exhibits, the malware creates a notification builder after which does the next:

  1. setCategory(“name”) – Which means that the notification is constructed as a vital notification that wants particular privilege.
  2. setFullScreenIntent() – This API wires the notification to a GUI in order that it pops up when the consumer faucets on it. At this stage, half the job is completed for the malware. Nonetheless, the malware wouldn’t need to rely on consumer interplay to set off the ransomware display, so, it provides one other performance of Android callback:

Determine 3. The malware overriding onUserLeaveHint

Because the code snippet exhibits, the malware overrides the onUserLeaveHint() callback perform of Exercise class. The perform onUserLeaveHint() known as at any time when the malware display is pushed to background, inflicting the in-call Exercise to be robotically delivered to the foreground. Recall that the malware hooked the RansomActivity intent with the notification that was created as a “name” kind notification. This creates a sequence of occasions that triggers the automated pop-up of the ransomware display with out doing infinite redraw or posing as system window.

Machine studying module signifies steady evolution

As talked about, this ransomware is the most recent variant of a malware household that has undergone a number of phases of evolution. The data graph beneath exhibits the varied strategies this ransomware household has been seen utilizing, together with abusing the system alert window, abusing accessibility options, and, extra not too long ago, abusing notification companies.

Knowledge graph showing techniques used by the Android rasomware family

Determine 4. Data graph of strategies utilized by ransomware household

This ransomware household’s lengthy historical past tells us that its evolution is way from over. We count on it to churn out new variants with much more subtle strategies. In reality, latest variants comprise code forked from an open-source machine studying module utilized by builders to robotically resize and crop photographs based mostly on display measurement, a precious perform given the number of Android units.

The frozen TinyML mannequin is beneficial for ensuring photographs match the display with out distortion. Within the case of this ransomware, utilizing the mannequin would be sure that its ransom be aware—usually faux police discover or express photographs supposedly discovered on the gadget—would seem much less contrived and extra plausible, growing the probabilities of the consumer paying for the ransom.

The library that makes use of tinyML shouldn’t be but wired to the malware’s functionalities, however its presence within the malware code signifies the intention to take action in future variants. We are going to proceed to observe this ransomware household to make sure clients are protected and to share our findings and insights to the group for broad safety towards these evolving cellular threats.

Defending organizations from threats throughout domains and platforms

Cellular threats proceed to quickly evolve, with attackers constantly trying to sidestep technological boundaries and creatively discover methods to perform their purpose, whether or not monetary achieve or discovering an entry level to broader community compromise.

This new cellular ransomware variant is a crucial discovery as a result of the malware displays behaviors that haven’t been seen earlier than and will open doorways for different malware to observe. It reinforces the necessity for complete protection powered by broad visibility into assault surfaces in addition to area specialists who observe the risk panorama and uncover notable threats that may be hiding amidst large risk information and alerts.

Microsoft Defender for Endpoint on Android, now usually out there, extends Microsoft’s industry-leading endpoint safety to Android. It detects this ransomware (AndroidOS/MalLocker.B), in addition to different malicious apps and recordsdata utilizing cloud-based safety powered by deep studying and heuristics, along with content-based detection. It additionally protects customers and organizations from different cellular threats, similar to cellular phishing, unsafe community connections, and unauthorized entry to delicate information. Study extra about our cellular risk protection capabilities in Microsoft Defender for Endpoint on Android.

Malware, phishing, and different threats detected by Microsoft Defender for Endpoint are reported to the Microsoft Defender Safety Middle, permitting SecOps to research cellular threats together with endpoint alerts from Home windows and different platforms utilizing Microsoft Defender for Endpoint’s wealthy set of instruments for detection, investigation, and response.

Menace information from endpoints are mixed with alerts from electronic mail and information, identities, and apps in Microsoft 365 Defender (beforehand Microsoft Menace Safety), which orchestrates detection, prevention, investigation, and response throughout domains, offering coordinated protection. Microsoft Defender for Endpoint on Android additional enriches organizations’ visibility into malicious exercise, empowering them to comprehensively forestall, detect, and reply to towards assault sprawl and cross-domain incidents.

Technical evaluation


On prime of recreating ransomware conduct in methods we haven’t seen earlier than, the Android malware variant makes use of a brand new obfuscation method distinctive to the Android platform. One of many tell-tale indicators of an obfuscated malware is the absence of code that defines the courses declared within the manifest file.

Malware code showing manifest file

Determine 5. Manifest file

The courses.dex has implementation for under two courses:

  1. The principle utility class gCHotRrgEruDv, which is concerned when the appliance opens
  2. A helper class that has definition for customized encryption and decryption

Which means that there’s no code akin to the companies declared within the manifest file: Principal Exercise, Broadcast Receivers, and Background. How does the malware work with out code for these key elements? As is attribute for obfuscated threats, the malware has encrypted binary code saved within the Property folder:

Screenshot of Assets folder with encrypted executable code

Determine 6. Encrypted executable code in Property folder

When the malware runs for the primary time, the static block of the principle class is run. The code is closely obfuscated and made unreadable by title mangling and use of meaningless variable names:

Determine 7. Static block

Decryption with a twist

The malware makes use of an attention-grabbing decryption routine: the string values handed to the decryption perform don’t correspond to the decrypted worth, they correspond to junk code to easily hinder evaluation.

On Android, an Intent is a software program mechanism that enables customers to coordinate the capabilities of various Actions to attain a activity. It’s a messaging object that can be utilized to request an motion from one other app element.

The Intent object carries a string worth as “motion” parameter. The malware creates an Intent contained in the decryption perform utilizing the string worth handed because the title for the Intent. It then decrypts a hardcoded encrypted worth and units the “motion” parameter of the Intent utilizing the setAction API. As soon as this Intent object is generated with the motion worth pointing to the decrypted content material, the decryption perform returns the Intent object to the callee. The callee then invokes the getAction technique to get the decrypted content material.

Determine 8. Decryption perform utilizing the Intent object to go the decrypted worth

Payload deployment

As soon as the static block execution is full, the Android Lifecycle callback transfers the management to the OnCreate technique of the principle class.

Malware code showing onCreate method

Determine 9. onCreate technique of the principle class decrypting the payload

Subsequent, the malware-defined perform decryptAssetToDex (a significant title we assigned throughout evaluation) receives the string “CuffGmrQRT” as the primary argument, which is the title of the encrypted file saved within the Property folder.

Malware code showing decryption of assets

Determine 10. Decrypting the property

After being decrypted, the asset turns into the .dex file. This can be a notable conduct that’s attribute of this ransomware household.

Comparison of code of Asset file before and after decryption

Determine 11. Asset file earlier than and after decryption

As soon as the encrypted executable is decrypted and dropped within the storage, the malware has the definitions for all of the elements it declared within the manifest file. It then begins the ultimate detonator perform to load the dropped .dex file into reminiscence and triggers the principle payload.

Malware code showing loading of decrypted dex file

Determine 12. Loading the decrypted .dex file into reminiscence and triggering the principle payload

Principal payload

When the principle payload is loaded into reminiscence, the preliminary detonator palms over the management to the principle payload by invoking the tactic XoqF (which we renamed to triggerInfection throughout evaluation) from the gvmthHtyN class (renamed to PayloadEntry).

Malware code showing handover from initial module to main payload

Determine 13. Handover from preliminary module to the principle payload

As talked about, the preliminary handover element referred to as triggerInfection with an occasion of appObj and a way that returns the worth for the variable config.

Malware code showing definition of populateConfigMap

Determine 14. Definition of populateConfigMap, which masses the map with values

Correlating the final two steps, one can observe that the malware payload receives the configuration for the next properties:

  1. quantity – The default quantity to be ship to the server (in case the quantity shouldn’t be out there from the gadget)
  2. api – The API key
  3. url – The URL for use in WebView to show on the ransom be aware

The malware saves this configuration to the shared preferences of the app information after which it units up all of the Broadcast Receivers. This motion registers code elements to get notified when sure system occasions occur. That is executed within the perform initComponents.

Malware code showing initializing broadcast receiver

Determine 15. Initializing the BroadcastReceiver towards system occasions

From this level on, the malware execution is pushed by callback capabilities which can be triggered on system occasions like connectivity change, unlocking the telephone, elapsed time interval, and others.


Dinesh Venkatesan

Microsoft Defender Analysis


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