Diving into the Sun — SunCrypt: A new neighbour in the ransomware mafia

The first time I heard about SunCrypt I was just enjoying my time off preparing some stuff to get dinner ready. My colleagues sent me over some weird samples that were flagged as SunCrypt that were also beaconing to Maze C2 infrastructure, however they weren’t Maze but shared some similarities and that’s basically why they did it as they know my interest in ransomware and my personal fascination with Maze.

As I was checking a couple of hashes, I saw very interesting things that made me go hands-on to investigate further and I ended up spending a lot of time not only analysing but also hunting and creating some detections for this malware.

Initial Open Source Intelligence shows that SunCrypt made its first appearance around October 2019 and it has been active since then. The SunCrypt team is active on underground forums where they look for affiliates for their program just like the rest of ransomware operating under the RaaS model.

SunCrypt forum advertisement

In their advertisement in a popular forum, shared in Twitter by ShadowIntelligence, can be observed some of the features they offer for their product such as crypto independency from the Windows CryptoAPI, “bypass” of 70% of the AV engines, which is not a lot..but I guess at least they didn’t lie selling it as FUD as other threat actors, asynchronous search and encryption, speed etc.

Additionally to the lock service, the SunCrypt team provides DDoS if they consider it as part of the extortion to get the ransom paid.

SunCrypt shaming website
SunCrypt shaming site where they expose victims, status and data dumps

For some journalists and websites related to infosec, they are part of what they call “The Maze Cartel” and they also drew some shy lines associating this team with Maze.

My personal opinion: I don’t believe such thing as a Cartel exists but sounds fancy. This would imply a high degree of collaboration, potential relationships and involvement in the money-laundry circuit, extortion and affiliation methodology… this is very different from a good or neutral relationship with potential collaborations to share resources and infrastructure. I wanted to see with my own eyes the degree of relationship and sophistication of this ransomware family.

That said, to start the analysis I grabbed two samples that are available not only in VT but also in AnyRun and from there I started the analysis to figure out how it works and hunting more samples for code comparison.

This was fundamental to me to construct a solid opinion and conclusions that I may share publicly with the community.

SunCrypt PowerShell loader analysis

SunCrypt ransomware has been spotted in many cases using PowerShell loaders for delivery and deploy following the tendency marked by other groups offering ransomware service.

After the analysis of some of these loaders, I could observe that share some similarities or reminds to me the structure and functionality of other loaders such a Netwalker PowerShell Loader scripts. The SunCrypt loaders contains an embedded resource in plain text with two export functions that are called by the script after the compilation of the C# code, heavy obfuscation and a lot of junk code and useless data to harden the analysis and detection.

The obfuscation of the script includes arithmetical operations, encoding and string manipulation not only for anti-analysis but probably to avoid detection by segmenting the base64 strings.

SunCrypt ransomware powershell
PoweShell Loader script obfuscation

Jumping to the analysis, the sample md5 c171bcd34151cbcd48edbce13796e0ed is a PowerShell loader containing the heavy obfuscation mentioned above and contains an embedded small PE file md5 479712042d7ad6600cbe2d1e5bc2fa88 coded in C# that is compiled in runtime and dropped to disk.

This DLL is used to assist with Process Injection of the payload by adding a class with obfuscated names for the exported functions to call the functions VirtualAlloc and EnumDesktopW. These are used to allocate memory and enumerate desktops associated with the process.

The PowerShell script spawns an additional process instance of Powershell which is directly invoked with the following arguments after the deobfuscation on the fly of the parameters:

C:\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell.exe -ep bypass -file <filepath of the script>

The main structure of script contains two heavily obfuscated payloads base64 encoded in multiple strings that are subsequently deobfuscated by multiple operations and then decoded.

PowerShell uses the dropped .NET DLL to allocate memory space and copies the resultant buffers to the newly created PowerShell process. The injected bytes are consistent with shellcode and a PE32 file that are injected into PowerShell’s memory address space.

To analyze the payload the buffer containing the deobfuscated payload was dumped to disk to carve the bytes.

A basic triage of the shellcode showed that it contains functions related to process injection such as VirtualAlloc and VirtualProtect, likely indicating that the shellcode is assisting the PowerShell script to leverage the injection of the payload.

SunCrypt payload Analysis

Most analyzed samples share the same code structure and characteristics but one of them had a simpler structure that facilitated the analysis as the only major modification that most samples contain was the addition of more obfuscation layers.

In this case I will jump directly to the analysis of the code shared between multiple samples followed by a summarisation and conclusion.

From the PowerShell loader md5 c171bcd34151cbcd48edbce13796e0ed, the payload md5 0a0882b8da225406cc838991b5f67d11 was dumped and the bytes carved for analysis. This PE file is consistent with an executable file likely to be coded in C and contains capabilities to encrypt the filesystem, delete backups and to gather and exfiltrate user and host information.

One of the first noticeable difference of the sample md5 0a0882b8da225406cc838991b5f67d11 is that it does not contain commandline arguments unlike other analyzed samples like md5 3d756f9715a65def4a302f5008b03809 (payload carved from the PowerShell loader with MD5 d87fcd8d2bf450b0056a151e9a116f72) which contains multiple arguments .

Most samples contain these command line arguments just like the one I just mentioned above that are intended to modify the behaviour of the ransomware. One of the key characteristics of SunCrypt is that contain an embedded configuration that is likely to be inserted and then encoded by the builder.

These command line arguments seen for example in md5 0a0882b8da225406cc838991b5f67d11 that are expected share some similarities with the ones seen in other malware like Maze ransomware.

SunCrypt commandline arguments are:

  • -noshares
  • -noreport
  • -nomutex
  • -log
  • -path

The triaged samples that expect command line arguments contain obfuscated parameters and SunCrypt uses FNV hashing to obfuscate these parameters.

SunCrypt arguments flow
SunCrypt arguments flow
IDA flow with each hash value and parameter

FNV (Fowler–Noll–Vo) is a non-cryptographic hash function created by Glenn Fowler, Landon Curt Noll, and Kiem-Phong Vo. FNV functions contains two primary operations: XOR and multiplication. These can be spotted also by identifying the hash values for FNV Prime (0x01000193) and the FNV offset basis (0x811c9dc5).

In this case the identified version that SunCrypt is using is FNV-1a hashing.

SunCrypt arguments flow
Above the FNV hashing can be observed to obfuscate the commandline arguments, however the value for “-noreport” is not hashed.

The next step the ransomware takes is to decode the configuration.

This configuration added and encoded by the builder contains the following parameters:

  • Mutex/implant ID
  • C2 address/addresses
  • Ransom note
  • Identifier

The whole section containing the configuration is decoded in multiple steps to save some of its values for its use in different routines, however the whole configuration is decoded by multiple loops that decode the data with a single-byte xor key 0x11.

SunCrypt ransom note decoded
SunCrypt ransom note decoded
Decoded data containing the HTML code for the ransom note.

One of the most noticeable things on the ransom note is that is written in multiple languages including Spanish(Latin American Spanish), German, French and English. This note contains an hardcoded identifier in hex format that is intended to be used to start negotiating the payment of the ransom along with the TOR Hidden Service URL where SunCrypt hosts its webpage and Shaming list containing the victim names, proofs and data dumps.

SunCrypt ransom note
SunCrypt ransom note

After the initial analysis of the configuration its data showed that both the offset of the ransom note containing the individual key in hex format and other values that are used later are not generated on the fly indicating that they are likely to be added by the builder during the construction process.

One of the decoded details is a string in hex format that is re-encoded at a later stage and sent within the POST data that is likely to be an implant identifier for the attackers.

The fact that all the data is precomputed or hardcoded is not always typical in ransomware and popped up some questions in my head about the crypto used that I had to figure out later.

Once the C2 are decoded the malware looks for multiple addresses in order to save them for its use later or just continues if only one is used.

The image below shows that after the configuration decoding and extraction, the next step taken by SunCrypt is a quick system check to delete the Shadow Copy Volumes. Microsoft’s shadow copy technology allows Windows systems to create snapshots-backups of files and volumes. This step ensures no possible recovery.

SunCrypt ransomware configuration
After decoding the embedded configuration, it saves the C2 addresses for its use

In most samples all the data such a function names, DLL names and other data are dynamically loaded and obfuscated using xor, sub or add operations with random values likely to be inserted by the builder combined with the use of StackStrings for anti-analysis and evasion.

The function in charge of deleting the Shadow Copy Volumes works in the same way and all its steps are obfuscated. SunCrypt retrieves the system version to look for its architecture to handle this via WMI query executing the query “select * from Win32_ShadowCopy” to then delete all the available volumes.

SunCrypt shadowcopy deletion
SunCrypt shadowcopy deletion
obfuscated flow to decode strings used to retrieve system architecture and to handle the task via WMI

One of the most interesting things I noticed is the fact that so far most samples create a mutex but not all of them. In this case, the below screenshot of the sample 3d756f9715a65def4a302f5008b03809 creates it using this hex string that was mentioned above and that is decoded during the configuration extraction.

SunCrypt pulls system version and create mutex
SunCrypt pulls system version and create mutex
After the config decoding, gets the System Version and creates the mutex

Once SunCrypt deletes the Shadow Volumes, it scans for available drives following the keyboard scheme from Q: to M:. In the case of the sample md5 0a0882b8da225406cc838991b5f67d11 it directly contains the hardcoded volume letters instead of using other techniques to harden detection such a incrementing loops. However most samples like md5 3d756f9715a65def4a302f5008b03809 contains a different technique using FNV-hashing again to obfuscate this phase, indicating that the developer team modified this to make its detection more complicated.

SunCrypt drive scanning
SunCrypt drive scanning
In most samples, drive scanning phase is obfuscated using FNV hashing, funnily enough, 0a0882b8da225406cc838991b5f67d11 contains the strings.

File encryption and Crypto walkthrough

SunCrypt uses a particular way to encrypt the filesystem and this is somehow reflected in their advertisement where they specify that SunCrypt uses a completely independent cryptography from the system API. Well, this is true and stepping through the crypto made it complicated as there’s only a few visible calls and it does completely rely on itself instead of the Windows CryptoAPI to generate the keys because the implementation is almost completely manual.

Another interesting feature observed during the crypto walkthrough is that skips files that are empty or less the 512 bytes size to skip potential garbage files. The Crypto is basically identical in all the samples and it consists on a first loop to scan all the directories ensuring the drive C: is present and is encrypted. However SunCrypt scans all the available network drives such as Windows SMB File Shares in order to encrypted them if they are connected and available.

The malware in most cases contains two lists:

  • Whitelist of directories: SunCrypt contains a list of directories that skips from the encryption process to ensure its recovery. These directories are: “Windows”, “Program Files”, “Program Files (x86)”, “$Recycle.bin”, “System Volume Information”.
  • Blacklist of file extensions: A huge list of file extensions are used to ensure only vital files are maintained intact.
SunCrypt file extension check via FNV
SunCrypt file extension check via FNV
note again the use of FNV hashing and the FNV prime hash above. The number 13:19:00 corresponds to the end of the array containing the file extensions.

These two arrays of lists are used with the FNV hashing technique again to ensure obfuscation, however md5 0a0882b8da225406cc838991b5f67d11 again saves the day containing all this information in plain text. The directories and file extensions don’t change between samples and versions.

Another file extension that is skipped is if the filename contains “YOUR_FILES_ARE_ENCRYPTED.HTML” which is used for the ransom note to ensure its delivery to the victim, however there is a problem that is not resolved.

If the ransomware is executed twice, it would re-encrypt the filesystem as the file extension of the encrypted files is generated on the fly. This embarrassing scenario indicates that if a previous instance of SunCrypt encrypts the filesystem, another execution would re-encrypt all the encrypted files, making this very hard to solve.

Once the directory is picked for encryption SunCrypt decodes the DLL name “advapi32.dll” and a function to be loaded and used. The malware executes a GetProcAddress to use the function SystemFunction036(RtlGenRandom), this will be our RNG to generate 32 random bytes without using CryptGenRandom function call or an insecure generator.

SunCrypt RNG
After decoding the DLL and the function that will be called, SunCrypt uses SystemFunction036 to create 32 byte keys.

This 32-byte buffer is validated after it’s generation to create a secure private key for the Elliptic Curve algorithm Curve25519. Curve25519 is a Diffie-Hellman function suitable for a wide variety of applications and is used by many software applications.

Curve25519 works as follows according to the official documentation:given a user’s 32-byte secret key, Curve25519 computes the user’s 32-byte public key. Given the user’s 32-byte secret key and another user’s 32-byte public key, Curve25519 computes a 32-byte secret shared by the two users. This secret can then be used to authenticate and encrypt messages between the two users”.

After the generation of this private session key, SunCrypt jumps to the curve function to compute its session public key where this session private key is passed to the function along with a basepoint constant of 9 followed by all zeros. This session public key will be converted to hex format and used as file extension for the encrypted file to allow the recovery as each time a file is encrypted the session private key is destroyed. Next step of SunCrypt is to load its embedded Curve25519 public key to compute a shared secret.

The analyzed sample md5 0a0882b8da225406cc838991b5f67d11 contains the following public key in hex format:

c75d83161c3768477c859b15cfe3f6c7bf707976bfed511af7015d04f7066558

The other analyzed sample used in the blog to compare SunCrypt versions (MD5:3d756f9715a65def4a302f5008b03809) containing the improvement and obfuscation shared with most of the observed SunCrypt samples contains the public key: 695c567285a5b331dcf1d61bb291ce850e92c57111678fe79a2e5c2e399c9310

The implementation of Curve25519 looks manual and it’s likely to be using code from any of the multiple open source implementation of Curve25519,

The shared secret is computed by calling the curve function and passing the attacker’s public key, the generated private key and the basepoint as parameters. This shared secret is used by another algorithm to encrypt the file, allowing its recovery with the attacker’s Curve25519 private key hosted in the SunCrypt C2 servers and the session public key that is the file extension of each file.

Summarising, each time a file is selected for encryption after the aforementioned checks, SunCrypt creates Curve25519 session keys, stores each public key as file extension and destroys the session private key.

SunCrypt key generation
session public key is formatted and used as extension, then the shared key is computed with the session private key and the attacker’s public key

Once the session keys are created and the shared key is computed for a given file, the required data is sent via completion I/O port to the encryption thread which takes a pointer to the file and its new generated extension. To handle encryption threading, SunCrypt gets the number of processors and the maximum number of available threads that the operating system can allow to process I/O completion packets for I/O completion port.

This provide an efficient threading for multiple asynchronous I/O requests for encryption. Again, this was referenced in their forum ad.

SunCrypt asynchronous multithreading
CreateIOCompletionPort initialisation

For the file encryption SunCrypt uses a very fast and secure algorithm which in this case is ChaCha. The implementation of ChaCha20 stream cipher is also manual and makes sense its use, because it does not rely on statically linked libraries or the CryptoAPI avoiding suspicious calls and it’s very fast and secure. This cipher is used by other popular ransomware like Maze. Again another similarity.

The actual file encryption happens after the above steps. The malware calls ReadFile function to get the content of the file and pushes a pointer to the buffer with the file content, the file extension and the computed shared key for that file to ChaCha20 function that encrypts the file partially. This limitation is intended to speed up a bit more the encryption as is enough to make the file useless.

After reading the file, the flow redirects the data containing the computed shared key and the file to the ChaCha20 encryption routine

C2 communications

SunCrypt contains a network module unlike many other ransomware and in my opinion this is also influenced by Maze. The ransomware uses the IP addresses that are embedded in the configuration and attempts to push different information from the compromised host.

It’s worth noting that the observed IP addresses in SunCrypt are in many cases consistent with previous associated Maze infrastructure’s subnet, reinforcing the idea of some sort of collaboration between ransomware dev groups.

SunCrypt gathers information from the Windows system gathering the Minor Version, Major Version and Buildnumber of the compromised system. The system survey also consists of gathering username and hostname information by calling GetUserNameA and GetComputerA functions.

SunCrypt information gathering
The network module gathers the information and creates a buffer that is later encoded with a hardcoded key

In the above image can be observed the information mentioned plus the fact of the use of this offset containing the bytes used for the mutex or implant id.

One of the things that raised my attention is that this step is identical between versions, meaning that these bytes are always used and intended to be sent in the network traffic, however, they are also used as mutex if the sample generates one, which is not always the case. That’s why I’m naming it implant-id and mutex at the same time. Is some sort of identifier that they may find useful.

Additionally, SunCrypt adds to this information (Versions and implant/mutex) the victim’s information as mentioned above.

The network buffer contains the following example structure:

  • 4 bytes: 0x50120108
  • Implant ID/Mutex
  • MinorVersion
  • MajorVersion
  • BuildNumber
  • 3 bytes: 0x3a0130
  • Length of username
  • Username
  • 1 byte: 0x4a
  • Length of hostname
  • Hostname

The above mentioned bytes are likely to be related to a manually implemented library used by the network routine to create the network buffer, however the library was not identified. Below an example of the network buffer prior to it’s encoding using an hardcoded single-byte xor key 0x11.

Afterwards the C2 address that was stored during the configuration decoding is loaded and pushed into the stack to send all this information to the routine handling the HTTP request.

SunCrypt network buffer
SunCrypt network buffer
SunCrypt network buffer entering the encoding loop stage

Below an example of what SunCrypt HTTP traffic looks like in WireShark, containing the encoded data in the POST data.

SunCrypt network traffic
SunCrypt network traffic

Another interesting detail that was mentioned before was the use of C2 infrastructure associated to the Maze team, reinforcing the idea of collaboration between ransomware teams which may indicate not only an stable relationship between threat actors but also a potential cooperation to share infrastructure, resources and techniques.

Summary

After the analysis of the samples, can be appreciated some but not a lot of evolution and changes. So far only one sample was found using clear-text strings (md5:0a0882b8da225406cc838991b5f67d11) which contains vital strings such as the user-agent and the drive letter whereas the rest of the identified samples contains some of the following modifications:

  • Use of StackStrings and more obfuscation hiding all the relevant data
  • FNV hashing implementation
  • Mutex

From all the observed samples, most of them has been spotted using PowerShell as initial vector to obfuscate the payload and to load the binary into memory and “honours” all the functionalities that the developer team announce in their forum post like the non-use of the CryptoAPI for file encryption and thread features. By looking at the PowerShell loaders I personally found it similar to Netwalker PowerShell loaders, which makes me think that the SunCrypt team got some inspiration from them.

SunCrypt contains multiple manual implementations on the source code to avoid detections but despite the notable differences, it can be noticed that the team got a lot of inspiration from Maze. SunCrypt copied part of its cryptoscheme like the use of ChaCha20 stream-cipher and modified its session key implementation by replacing RSA for the Curve25519 algorithm probably for two main reasons:

  • Key generation is way faster and uses smaller keys than RSA, which allows a very fast key generation and operation.
  • Non dependant of the Windows CryptoAPI. This allows SunCrypt to hide function calls and hardens the analysis. RSA could be implemented manually, but I guess they considered that it wasn’t probably worth.

Additionally, to increase the encryption speed the SunCrypt team sends file data via completion I/O port to the encryption thread.

Overall I have to say SunCrypt is not a very sophisticated ransomware unlike some of its competitors like Maze, Egregor, Ragnar or Wastedlocker and the main motivation I have to say this, is due to the evolution, techniques, language(C over OOP like C++) and obfuscation techniques point to this direction but they got a lot of inspiration to make something similar to Maze. In fact when I saw SunCrypt for the first time I thought: “someone from the team just left and started its own project(?)” but after diving into it and checking more samples I could notice that the level of sophistication and experience, tooling and techniques used are not the same, but I feel confident saying that they were a source of inspiration for sure.

However, what has in common with Maze? It shares some of the command line arguments , the use of a network module for data exfiltration, algorithm for file encryption and other references, however the main differences with Maze are:

  • Different implementation of key generation, algorithms and session keys.
  • No storage of any temporary data. Maze drops a file in %PROGRAMDATA% to store session keys.
  • No anti-disassembly
  • C instead of C++
  • No Anti-Analysis and different evasion techniques
  • Maze contains a complex network module and URI scheme whereas SunCrypt’s is simple
  • Different Shadow Copy deletion technique
  • No CIS country protection check.

It will be interesting to keep an eye on this malware to see its evolution and modifications to survive as the RaaS model service is a fast paced ecosystem full of very capable competitors.

MITRE ATT&CK

  • T0127-Evasion: Obfuscated Files or Information
  • T1497-Evasion: Virtualization/Sandbox Evasion::System Checks
  • T1083-Discovery: File and Directory Discovery
  • T1082-Discovery: System Information Discovery
  • T1033-Discovery: System Owner/User Discovery
  • T1129-Execution: Shared Modules

Samples

C2

  • hxxp://91[.]218[.]114[.]31
  • hxxp://91[.]218[.]114[.]30

Kimchi and Ransomware. Incident Responder and sort of malware analyst in my free time. Personal blog, opinions are my own.

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