Mustang Panda’s Hodur: Previous methods, new Korplug variant

ESET researchers found a still-ongoing marketing campaign utilizing a beforehand undocumented Korplug variant, which they named Hodur because of its resemblance to the THOR variant beforehand documented by Unit 42 in 2020. In Norse mythology, Hodur is Thor’s blind half-brother, who’s tricked by Loki into killing their half-brother Baldr.

Key findings on this blogpost:

• As of March 2022, this marketing campaign continues to be ongoing and goes again to no less than August 2021.
• Recognized victims embrace analysis entities, web service suppliers, and European diplomatic missions.
• The compromise chain contains decoy paperwork which might be regularly up to date and relate to occasions in Europe.
• The marketing campaign makes use of a customized loader to execute a brand new Korplug variant.
• Each stage of the deployment course of makes use of anti-analysis methods and control-flow obfuscation, which units it aside from different campaigns.
• ESET researchers present an in-depth evaluation of the capabilities and instructions of this new variant.

Victims of this marketing campaign are probably lured with phishing paperwork abusing the newest occasions in Europe reminiscent of Russia’s invasion of Ukraine. This resulted in additional than three million residents fleeing the battle to neighboring nations, resulting in an unprecedented disaster on Ukraine’s borders. One of many filenames associated to this marketing campaign is State of affairs on the EU borders with Ukraine.exe.

Different phishing lures point out up to date COVID-19 journey restrictions, an authorized regional assist map for Greece, and a Regulation of the European Parliament and of the Council. The final one is an actual doc out there on the European Council’s web site. This reveals that the APT group behind this marketing campaign is following present affairs and is ready to efficiently and swiftly react to them.

Affected nations:

• Mongolia
• Vietnam
• Myanmar
• Greece
• Russia
• Cyprus
• South Sudan
• South Africa

Affected verticals:

• Diplomatic missions
• Analysis entities
• Web service suppliers (ISP)

Evaluation

Based mostly on code similarities and the numerous commonalities in Techniques, Strategies, and Procedures (TTPs), ESET researchers attribute this marketing campaign with excessive confidence to Mustang Panda (often known as TA416, RedDelta, or PKPLUG). It’s a cyberespionage group primarily focusing on governmental entities and NGOs. Its victims are largely, however not completely, positioned in East and Southeast Asia with a concentrate on Mongolia. The group can be identified for its campaign targeting the Vatican in 2020.

Whereas we haven’t been capable of establish the verticals of all victims, this marketing campaign appears to have the identical focusing on goals as different Mustang Panda campaigns. Following the APT’s typical victimology, most victims are positioned in East and Southeast Asia, together with some in European and African nations. Based on ESET telemetry, the overwhelming majority of targets are positioned in Mongolia and Vietnam, adopted by Myanmar, with only some within the different affected nations.

Mustang Panda’s campaigns regularly use customized loaders for shared malware together with Cobalt Strike, Poison Ivy, and Korplug (often known as PlugX). The group has additionally been identified to create its personal Korplug variants. In comparison with different campaigns utilizing Korplug, each stage of the deployment course of makes use of anti-analysis methods and control-flow obfuscation.

This blogpost accommodates an in depth evaluation of this beforehand unseen Korplug variant used on this marketing campaign. This exercise is a part of the identical marketing campaign recently covered by Proofpoint, however we offer extra historic and focusing on info.

Toolset

Mustang Panda is thought for its elaborate customized loaders and Korplug variants, and the samples used on this marketing campaign showcase this completely.

Compromise chains seen on this marketing campaign observe the standard Korplug sample: a respectable, validly signed, executable susceptible to DLL search-order hijacking, a malicious DLL, and an encrypted Korplug file are deployed on the goal machine. The executable is abused to load the module, which then decrypts and executes the Korplug RAT. In some circumstances, a downloader is used first to deploy these recordsdata together with a decoy doc. This course of is illustrated in Determine 2.

Determine 2. Overview of the deployment course of for the Hodur Korplug variant.

What units this marketing campaign aside is the heavy use of control-flow obfuscation and anti-analysis methods at each stage of the deployment course of. The next sections describe the conduct of every stage and take a deeper have a look at the protection evasion methods utilized in every of them.

Preliminary entry

We haven’t been capable of observe the preliminary deployment vector, however our evaluation factors to phishing and watering gap assaults as probably vectors. In cases the place we noticed a downloader, the filenames used counsel a doc with an fascinating topic for the goal. Such examples embrace:

• COVID-19 journey restrictions EU opinions record of third nations.exe
• State_aid__Commission_approves_2022-2027_regional_aid_map_for_Greece.exe
• REGULATION OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL.exe
• State of affairs on the EU borders with Ukraine.exe

To additional the phantasm, these binaries obtain and open a doc that has the identical title however with a .doc or .pdf extension. The contents of those decoys precisely mirror the filename. As proven in Determine 3, no less than considered one of them is a publicly accessible respectable doc from the European Parliament.

Determine 3. First web page of the decoy doc for the REGULATION OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL.exe downloader. It’s an actual doc out there on the European Council’s web site.

Downloader

Though its complexity has elevated over the course of the marketing campaign, the downloader is pretty simple. This enhance in complexity comes from extra anti-analysis methods, which we cowl later on this part.

It first downloads 4 recordsdata over HTTPS: a decoy doc, a respectable executable, a malicious module and an encrypted Korplug file. The mixture of these final three parts to execute a payload by way of DLL side-loading is typically known as a trident and is a way generally utilized by Mustang Panda, and with Korplug loaders generally. Each the server addresses and file paths are hardcoded within the downloader executable. As soon as every little thing is downloaded, and the decoy doc opened to distract the sufferer, the downloader makes use of the next command line to launch the respectable executable:

cmd /c ping 8.8.8.8 -n 70&&”%temp%<respectable executable>”

This ping command each checks web connectivity and introduces a delay (by way of the -n 70 possibility) earlier than executing the downloaded, respectable executable.

The downloader makes use of a number of anti-analysis methods, a lot of that are additionally used within the loader and last payload. Further obfuscation has been added to new variations over the course of the marketing campaign with out in any other case altering their objective.

In early variations of the downloader, junk code and opaque predicates have been used to hinder evaluation, as proven in Determine 4, however the server and filenames are plainly seen in cleartext.

Determine 4. Management circulate obfuscation in early variations of the downloader

In later variations, the recordsdata on the server are RC4 encrypted, utilizing the bottom 10 string illustration of the file measurement as the important thing, after which hex-encoded. This course of is illustrated within the Python snippet beneath. The other operations are carried out client-side by the downloader to get well the plaintext recordsdata. That is probably completed to bypass network-level protections.

from Crypto.cipher import ARC4
key = “%d” % len(plaintext)
rc4 = ARC4.new(key)
cipher_content = rc4.encrypt(plaintext).hex().higher()

These variations change the usage of cleartext strings with encrypted stack strings. They’re nonetheless hardcoded within the file, however the obfuscation surrounding them, and the usage of totally different keys, makes it onerous to decrypt them statically in an automatic method. This identical method is used closely within the subsequent levels. Encrypted stack strings are additionally used to obfuscate calls to Home windows API features.

First, the title of the goal perform is decrypted and handed to a perform. This perform obtains a pointer to the InMemoryOrderModuleList subject of the PEB (Process Environment Block). It then iterates over the loaded modules, passing every deal with to GetProcAddress together with the perform title till the goal perform is efficiently resolved. A part of this course of will be seen in Determine 5.

Determine 5. Obfuscation of Home windows API calls within the downloader. The screenshot reveals a name to WriteFile, however the identical sample is used for all API features.

Loader

As is widespread with Korplug, the loader is a DLL that exploits a side-loading vulnerability in a respectable, signed executable. We now have noticed many various purposes being abused on this marketing campaign, for example a susceptible SmadAV executable beforehand seen by Qurium in a campaign attributed to Mustang Panda that focused Myanmar.

The loader exports a number of features. The precise record varies relying on the abused utility, however in all circumstances, solely considered one of them does something of consequence. In all the loaders we noticed, that is the exported perform with the very best load handle. All the opposite exports, and the library’s entry level, both return instantly or execute some do-nothing junk code. Many of those exports have names that include random lowercase letters and level to the identical handle as proven in Desk 1.

Desk 1. Features exported by a Hodur loader. The createSystemFontsUsingEDL export is the one which masses the ultimate malware stage on this model.

The loader perform obtains the listing from which the DLL is operating utilizing GetModuleFileNameA and tries to open the encrypted Korplug file it accommodates. That filename is hardcoded within the loader. It reads the file’s contents right into a domestically allotted buffer and decrypts it. The loader makes this buffer executable utilizing VirtualProtect earlier than calling into it at offset 0x00.

Home windows API perform calls are obfuscated with a distinct method than that used within the downloader. Not like the loader, which accommodates the names of its features (as proven in Desk 1 above), solely the 64-bit hashes of the Home windows API perform calls are current within the binary. To resolve these features, the loader traverses the export lists of all loaded libraries by way of the InMemoryOrderModuleList of the PEB. Every export’s title is hashed, then in comparison with the anticipated worth. The FNV-1a hash algorithm, just lately introduced again into the mainstream by the Sunburst backdoor, has beforehand been utilized by Mustang Panda, in Korplug loaders documented by XORHEX, to resolve GetProcAddress and LoadLibraryA, though it was not recognized by title in that evaluation. On this model, nevertheless, it’s used for all API features.

Korplug backdoor

Korplug (often known as PlugX) is a RAT utilized by a number of APT teams. Despite it being so broadly used, or maybe due to it, few experiences extensively describe its instructions and the information it exfiltrates. Its performance is just not fixed between variants, however there does appear to exist a major overlap within the record of instructions between the model we analyzed and different sources such because the Avira report from January 2020 and the plugxdecoder undertaking on GitHub.

As beforehand talked about, the variant used on this marketing campaign bears many similarities to the THOR variant, which is why we now have named it Hodur. The similarities embrace the usage of the SoftwareCLASSESms-pu registry key, the identical format for C&C servers within the configuration, and use of the Static window class.

As anticipated for Korplug payloads, this stage is simply ever decrypted in reminiscence by the loader. Solely the encrypted model is written to disk in a file with a .dat extension.

Except said in any other case, all hardcoded strings mentioned on this part are saved as encrypted stack strings.

On this module, Home windows API features are obfuscated by way of a mix of the strategies utilized in earlier levels. LoadLibraryA and GetProcAddress are resolved by way of the FNV-1a hashing method and stack strings are decrypted and handed to them to acquire the goal perform.

Loading

As soon as decrypted, the payload is a sound DLL that exports a single perform. In nearly all noticed samples from this marketing campaign, this perform is called StartProtect. Nevertheless, launching it instantly by way of this export or its entry level won’t execute the primary payload and the loading course of is kind of intricate.

As defined within the earlier part, the file is decrypted in reminiscence as a steady blob by the loader and the execution begins at offset 0x00. The PE header accommodates shellcode, proven in Determine 6, that calls a selected offset that corresponds to the module’s single export.

Determine 6. Shellcode within the PE header that calls the exported perform

This perform parses the PE blob in reminiscence and manually maps it as a library right into a newly allotted buffer. This contains mapping the assorted sections, resolving imports and, lastly, utilizing DLL_PROCESS_ATTACH to name the DLL entry level. As soon as once more, opaque predicates and junk code are used to obfuscate the aim of this perform.

The entry level of the correctly loaded library is then known as with the non-standard worth of 0x04 for the fdwReason parameter (solely values from 0x00 to 0x03 are currently defined). This particular worth is required to get it to execute its primary payload. This straightforward examine prevents the RAT from being trivially executed instantly with a generic device like rundll32.exe.

The backdoor first decrypts its configuration utilizing the string 123456789 as a repeating XOR key. As soon as decrypted, the configuration block begins with ########. The format of the configuration varies barely between samples, however all of them comprise no less than the next fields:

• Set up listing title. Additionally used because the title of the registry key created for persistence. This worth roughly corresponds to the title of the abused utility with three random letters appended (e.g., FontEDLZeP or AdobePhotosGQp)
• Mutex title
• A worth that’s both a model or ID string
• Listing of C&C servers. Every entry contains IP handle, port quantity, and a quantity indicating the protocol to make use of with that C&C

The backdoor then checks the trail from which it’s operating utilizing GetModuleFileNameW. If this matches %userprofile%<set up listing> or %allusersprofile%<set up listing>, the RAT performance shall be executed. In any other case, it can undergo the set up course of.

Set up

To put in itself, the malware creates the aforementioned listing underneath %allusersprofile%. Utilizing SetFileAttributesW, it’s then marked as hidden and system. The susceptible executable, loader module, and encrypted Korplug recordsdata are copied to the brand new listing.

Subsequent, persistence is established. Earlier samples achieved this by making a scheduled process to be run at boot by way of schtasks.exe. Newer samples add a registry entry to SoftwareMicrosoftWindowsCurrentVersionRun, attempting the HKLM hive first, then HKCU. This entry has the identical title because the set up listing with its worth set to the newly copied executable’s path.

As soon as persistence has been arrange, the malware launches the executable from its new location and exits.

RAT

The RAT performance of the Hodur variant used on this marketing campaign largely strains up with different Korplug variants, with some extra instructions and traits. As we now have beforehand said, although, detailed analyses of Korplug instructions are few and much between, so we purpose to supply such an evaluation within the hopes of aiding future analysts.

When on this mode, the backdoor iterates by way of the record of C&C servers in its configuration till it reaches the tip or receives an Uninstall command. For every of these servers, it processes instructions till it receives a Cease command or encounters an error.

Hodur’s preliminary handshake will be completed over HTTPS or TCP. That is decided by a worth within the configuration for that individual C&C server. Subsequent communication is at all times completed over TCP utilizing a customized protocol that we describe on this part, together with the instructions that may be issued. Hodur makes use of sockets from the Home windows Sockets API (Winsock) that assist overlapped I/O.

Following the preliminary handshake, Hodur’s communications contain TCP messages that include a header, with the construction described in Desk 2, adopted by a message physique that’s often compressed utilizing LZNT1 and at all times encrypted with RC4. Messages whose Command quantity header subject have the 0x10000000 bit set (people who comprise file contents for the ReadFile and WriteFile instructions, described in Desk 3) have encrypted however not compressed message our bodies. All encrypted message our bodies use the hardcoded key sV!e@T#L$PH% with a four-byte random nonce (the worth at offset 0x00 within the header) appended to it.

Desk 2. Header format used for communication between the C&C and the backdoor

Hodur’s C&C message headers are transmitted within the clear, adopted by variably sized (the worth at offset 0x08 of the header) message our bodies. The format of the message physique varies per command, however as soon as decrypted and decompressed, values of variable size (like strings) are at all times at a message physique’s finish and their offset within the physique is saved as an integer within the corresponding message subject.

Just like the model described by Avira, Hodur has two teams of instructions – 0x1001 and 0x1002 – every with its personal handler. The C&C server can set which group to hear for by sending the corresponding ID because the command quantity when a consumer is just not already in one of many two modes. It should proceed to hear for a similar group till it receives the Cease command, or an error happens (together with receiving a message with an invalid Command quantity in its header).

The primary group, 0x1001, accommodates instructions for managing the execution of the backdoor and doing preliminary reconnaissance on a newly compromised host. As these instructions take no arguments, messages despatched by the C&C server consist solely of the headers. Desk 3 accommodates a listing of those instructions. The GetSystemInfo command is described in additional element beneath. Notice that no command names are current within the RAT; they have been both taken from earlier analyses or supplied by us.

Desk 3. Instructions in group 0x1001

The GetSystemInfo command collects in depth details about the system, as detailed in Desk 4. If it doesn’t exist already, the SoftwareCLASSESms-puCLSID registry secret’s set to the present timestamp, attempting HKLM first then HKCU. The worth of this secret’s then despatched within the response.

Desk 4. Response physique format for the GetSystemInfo response

The 0x1002 group accommodates instructions that present RAT performance, as detailed in Desk 5. A few of these take parameters supplied within the command’s message physique. The FindFiles command is described in additional element beneath. Once more, be aware that no command names are current within the RAT; they have been both taken from earlier analyses or supplied by us.

Desk 5. Instructions in group 0x1002

FindFiles command

Ranging from the supplied listing, this command searches for recordsdata whose names match the given sample. This sample helps the identical wildcard characters because the Home windows FindFirstFile API. For every matching file, the consumer sends a response message with its physique within the format described in Desk 6.

Desk 6. Format of the response physique for the FindFiles command

One response message with an empty physique is shipped as soon as the search is accomplished.

Conclusion

The decoys used on this marketing campaign present as soon as extra how rapidly Mustang Panda is ready to react to world occasions. For instance, an EU regulation on COVID-19 was used as a decoy solely two weeks after it got here out, and paperwork in regards to the battle in Ukraine began getting used within the days following the start of the launch of the invasion. This group additionally demonstrates a capability to iteratively enhance its instruments, together with its signature use of trident downloaders to deploy Korplug.

IoCs

Community

MITRE ATT&CK methods

This desk was constructed utilizing version 10 of the MITRE ATT&CK framework.