Executive Summary

Black Shrantac is a modern, highly organized ransomware operation that appeared in the second half of 2025. It is not a “spray and pray” virus. It is a targeted, double-extortion ransomware, meaning attackers steal data first and then encrypt systems. Victims are pressured to pay not only to recover files but also to prevent sensitive data from being leaked or sold.

What makes Black Shrantac dangerous is not just the encryption itself, but how quietly it gets inside, how long it stays hidden, and how methodical the attackers are. In most cases, the ransomware is only the final step of a longer intrusion.

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How Black Shrantac Enters a Network

Black Shrantac rarely enters directly. Instead, it is delivered by a precursor malware, most commonly CastleRAT, which acts as a loader and backdoor.

Common Entry Methods

1. Malicious Emails (Malspam)
   • Fake invoices, shipping notices, legal documents
   • Attachments disguised as PDFs, ZIP files, or Office documents
   • Links leading to fake download pages
2. Cracked Software & Keygens
   • Pirated software installers
   • “Free” versions of paid tools
   • Fake activation tools that silently install CastleRAT
3. Fake Updates & Drive-By Downloads
   • Compromised websites prompting browser or software updates
   • Silent downloads triggered by malicious ads or scripts

Once the user executes the file, CastleRAT installs itself quietly, often without triggering antivirus alerts.

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What Happens After Initial Infection (Attack Chain)

Stage 1: Foothold & Persistence (CastleRAT)

After infection:

• CastleRAT establishes persistent access
• Creates scheduled tasks to survive reboots
• Collects system details (username, domain, IP address)
• Fetches the public IP using online services
• Begins monitoring:
  • Keystrokes
  • Clipboard data
  • Screenshots

At this stage, no files are encrypted yet, which is why early detection is critical.

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Stage 2: Privilege Escalation & Control

CastleRAT attempts to:

• Bypass User Account Control (UAC)
• Run with elevated privileges
• Inject itself into legitimate Windows processes
• Disable or evade security tools

This allows attackers to move freely inside the system and prepare the ransomware deployment.

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Stage 3: Data Exfiltration (Double Extortion)

Before encryption:

• Sensitive data is copied out of the network
• This may include:
  • HR data
  • Financial records
  • Client databases
  • Source code
  • Contracts and emails

The attackers later prove this theft by listing stolen files during ransom negotiations.

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Stage 4: Ransomware Deployment (Black Shrantac)

Only after data theft is complete does Black Shrantac activate.

Actions performed:

• Terminates running processes (databases, Office apps, services)
• Deletes Volume Shadow Copies to prevent rollback
• Encrypts files across local drives and mapped network shares
• Renames files completely, not just extensions

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Encryption Behavior (What Victims See)

File Renaming

Black Shrantac destroys the original filename.

Example:

invoice_2024.docx  →  0WeRZQJSTkOAnYP4.shrt

Why this matters:

• Victims cannot identify files by name
• Even partial recovery becomes extremely difficult

File Extension

• All encrypted files end with: .shrt

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Ransom Note

A ransom note named:

shrt.readme.txt

Contents include:

• Header: BLACK-SHRANTAC
• Statement that data was stolen and encrypted
• Instructions to contact attackers
• Threats of public data leaks
• Language framed as a “business transaction”

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Visual Impact

• Desktop wallpaper is changed to display ransom instructions
• Ensures the victim immediately notices the attack

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Communication & Extortion Method

Black Shrantac uses professional-style negotiation channels:

• Tor-based negotiation portals
• Tox encrypted chat

Characteristics:

• Payment demanded in Bitcoin only
• Attackers often:
  • Provide proof of stolen data
  • Offer to decrypt 2–3 small files for free
  • Apply pressure with leak deadlines

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Known Victims & Targeted Sectors

By late 2025, Black Shrantac had dozens of confirmed victims, showing rapid growth.

Frequently Targeted Industries

• Manufacturing & supply chain
• Construction
• Telecommunications
• Cybersecurity & IT services

These sectors are chosen because:

• Downtime is expensive
• Data sensitivity is high
• Business pressure increases ransom payment likelihood

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Indicators of Compromise (IoCs)

File-Based Indicators

• Files ending in .shrt
• Presence of shrt.readme.txt
• Sudden wallpaper change

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Network Indicators

• Tor network connections
• Tox peer-to-peer traffic
• Encrypted outbound traffic to unknown IPs
• Systems querying public IP lookup services unexpectedly

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System & Behavior Indicators

• New scheduled tasks with suspicious names
• Executables running from:
  • AppData
  • Temp directories
• rundll32.exe executing DLLs by ordinal
• ComputerDefaults.exe launched from cmd or PowerShell

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How to Detect Black Shrantac on a Windows Network

Early Detection (Before Encryption)

This is the most important window.

Watch for:

• New scheduled tasks (Event ID 4698)
• Tasks triggered on logon or startup
• Unusual UAC bypass behavior
• High-entropy encrypted outbound traffic
• Clipboard access by unknown processes

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Endpoint Clues

• rundll32.exe behaving abnormally
• Processes spawning from user-writable folders
• Sudden termination of security tools
• Unexpected system slowdowns before encryption

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Steps to Contain an Active Infection

Immediate Actions

1. Isolate the System
   • Disconnect network cable
   • Disable Wi-Fi
   • Prevent lateral movement
2. Do Not Reboot
   • Rebooting may trigger persistence tasks
   • Hibernate if possible to preserve memory
3. Disable Compromised Accounts
   • Temporarily lock affected user accounts
   • End active sessions

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Remove Persistence

• Inspect Task Scheduler for suspicious entries
• Check registry run keys
• Identify and stop CastleRAT processes
• Preserve disk image before cleanup

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Post-Incident Actions

• Reset credentials
• Review logs across the network
• Hunt for additional infected systems
• Restore from clean, offline backups

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Prevention Measures (What Actually Works)

Technical Controls

• Application allow-listing
• Disable macro execution by default
• Block cracked software and P2P downloads
• Monitor scheduled task creation
• Restrict PowerShell and script execution

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Network Controls

• Block Tor and Tox traffic
• Monitor outbound encrypted connections
• Segment critical systems

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Human Controls

• User awareness training
• Phishing simulations
• Clear policy against pirated software

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YARA Rules (Basic Detection)

These are generic detection rules intended for internal hunting and lab environments.

Ransom Note Detection

rule BlackShrantac_RansomNote
{
    meta:
        description = "Detects Black Shrantac ransom note"
    strings:
        $a = "BLACK-SHRANTAC"
        $b = "business transaction"
        $c = "shrt.readme.txt"
    condition:
        all of them
}

Encrypted File Pattern

rule BlackShrantac_Encrypted_File
{
    meta:
        description = "Detects Black Shrantac encrypted files"
    strings:
        $ext = ".shrt"
    condition:
        $ext
}

Behavioral Loader Detection (CastleRAT)

rule CastleRAT_Persistence
{
    meta:
        description = "Detects CastleRAT persistence behavior"
    strings:
        $schtask = "schtasks.exe"
        $onlogon = "/sc onlogon"
        $highest = "/rl highest"
    condition:
        all of them
}

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Final Assessment

Black Shrantac is not just ransomware, it is a full intrusion campaign.
If encryption has started, the attackers have likely been inside the network for days or weeks already.

There is no free decryptor, and paying the ransom does not guarantee safety or data deletion.

The only reliable defense is:

• Early detection
• Strong backups
• Tight endpoint and network monitoring
• Rapid containment when warning signs appear