Technical Advisory: Deconstructing the .hamster15 / .hamster20 Ransomware Series (MedusaLocker Lineage)

An aggressive enterprise file-locking campaign has been identified actively deploying payloads across global business sectors, leaving target infrastructures compromised by files appended with incrementing extensions such as .hamster15, .hamster20, .hamster30, .hamster40, and .hamster50. Extensive binary reverse-engineering and forensic attribution confirm that this distinct naming convention is directly tied to the evolutionary lineage of the MedusaLocker ransomware family. Rather than relying on static legacy extensions, the affiliates operating this specific campaign utilize systematically incremented numerical tags to track distinct attack waves, affiliate distributions, and target groupings.

MedusaLocker has maintained a persistent presence in the cyber threat landscape by operating under a structured Ransomware-as-a-Service (RaaS) model. Developers lease the core cryptographic engine to independent network intrusion affiliates who handle the initial breach, privilege escalation, and lateral movement phases. The recent surge in the “Hamster” extension series indicates a highly organized push by specialized affiliate subgroups targeting small-to-medium enterprises (SMEs), healthcare providers, and local government networks. Understanding the precise mechanical triggers, file modification behaviors, and encryption architecture of this variant is essential for incident response teams working to contain an active breach and evaluate viable data recovery pathways.

Table of Contents

• Threat Architecture & The “Hamster” Incrementing Pattern
• Technical Infection Mechanics & Service Termination
• The Hybrid Encryption Engine & Brute-Force Limitations
• Threat Indicators & Ransom Note Structure
• Primary Infiltration Vectors: RDP & Lateral SMB Traversal
• Immediate Enterprise Containment Mandates
• Forensic Data Restoration & Non-Extortion Pathways

Threat Architecture & The “Hamster” Incrementing Pattern

When the MedusaLocker engine executes on a host machine, it initiates a comprehensive filesystem discovery routine designed to identify all locally mounted physical disks, secondary logical partitions, and accessible network file shares. Unlike many modern ransomware families that randomize extensions per host or adopt a single corporate brand extension, this specific affiliate cluster utilizes a distinct, structured naming convention: a common root word (hamster) followed by double-digit increments (e.g., 15, 20, 30, 40, 50).

Forensic analysis across multiple intrusion cases reveals that these incrementing numerical suffixes serve an operational purpose for the threat actors. The numbers act as internal campaign trackers, allowing affiliates to segregate victim clusters, monitor the success rates of specific phishing or brute-force waves, and automate their internal ransom negotiation tables. Regardless of the specific numerical value attached, the core behavior of the executable remains identical across the series.

During the encryption phase, the malware modifies target files by appending the designated campaign string directly onto the existing file extension. The observed transformation protocol across enterprise environments follows this consistent structural pattern:

• Financial_Report_Q3.xlsx → Financial_Report_Q3.xlsx.hamster15
• Employee_Database.mdf → Employee_Database.mdf.hamster20
• Network_Architecture.vsdx → Network_Architecture.vsdx.hamster30
• System_Backup_Archive.zip → System_Backup_Archive.zip.hamster40

To ensure that the compromised operating system remains stable enough to boot, display the ransom note, and maintain network connectivity for negotiation communications, MedusaLocker enforces a hardcoded exclusion list. The encryption engine systematically bypasses core system directories, including \Windows\, \Program Files\, \Program Files (x86)\, \ProgramData\, and the user’s \AppData\ folder. Furthermore, it ignores critical executable and system file extensions such as .exe, .dll, .sys, .ini, .lnk, and .rdp. This selective processing guarantees that the machine is not rendered unbootable, forcing the victim to confront the ransom demand directly on their desktop.

Technical Infection Mechanics & Service Termination

The survival and efficiency of a MedusaLocker deployment depend heavily on its ability to disable host defenses and release open file locks before the encryption thread initializes. Upon gaining execution privileges, the binary executes an aggressive preparatory sequence utilizing built-in Windows administrative utilities and command-line interfaces.

First, the malware attempts to elevate its execution privileges and bypass User Account Control (UAC) enforcement. In many documented deployments of this lineage, the binary abuses the native Windows Connection Manager Profile Installer (cmstp.exe) or utilizes scripted Windows Management Instrumentation (WMI) routines to silently execute commands with elevated system rights without triggering user prompts. Once elevated, it executes a series of commands to terminate enterprise services that might block read/write access to high-value databases or back up data in real time:

By neutralizing database engines (like Microsoft SQL Server) and enterprise backup agents (such as Veeam and Acronis), the ransomware ensures that large `.mdf`, `.ldf`, and backup archive files are fully accessible for encryption. Simultaneously, it issues specific WMI commands to permanently destroy local operating system recovery points and Volume Shadow Copies:

Finally, to maintain persistence across system reboots and ensure that any newly attached network drives or external storage devices are subsequently encrypted, MedusaLocker modifies the Windows Registry. It creates a persistent run key—frequently utilizing the signature value name MDSLK or generating an obfuscated randomized string under HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run—pointing directly to the active malicious binary stored within user profile subdirectories.

The Hybrid Encryption Engine & Brute-Force Limitations

A central question for organizations hit by the `.hamster15` series is whether the encryption can be forcibly broken or reversed using computational mathematical brute-force methods. An analysis of the cryptographic implementation used by the MedusaLocker family confirms that the ransomware employs a highly secure **hybrid cryptography schema** combining symmetric and asymmetric algorithms.

When the encryption routine commences, the malware generates a unique, cryptographically secure AES-256 (Advanced Encryption Standard) symmetric key for each individual file or block of files. AES-256 is an industry-standard encryption cipher that operates at extraordinary speeds, allowing the ransomware to encrypt gigabytes of data in a matter of seconds. However, because symmetric encryption relies on the same key for both locking and unlocking, the threat actors must protect this AES key from being recovered by forensic analysts.

To secure the AES keys, MedusaLocker embeds an **RSA-2048 public key** directly within the payload’s compiled code. As each file is encrypted with its unique AES-256 key, that AES key is immediately encrypted using the attacker’s RSA-2048 public key. The encrypted version of the AES key is then appended directly to the header or footer of the locked file alongside a specialized identifier block. The plaintext AES key is then wiped from volatile system memory.

Because RSA is an asymmetric encryption system, a key encrypted with an RSA public key can only be decrypted using the corresponding **RSA private key**, which is generated and stored exclusively on offline command-and-control (C2) servers managed by the MedusaLocker developers. Factoring an RSA-2048 modulus to derive the private key without access to the attacker’s server would require immense computational resources over timeframes spanning decades, rendering mathematical brute-force attempts unfeasible with modern hardware. Therefore, legitimate recovery must rely on alternative forensic data extraction techniques or immutable backup infrastructures.

Threat Indicators & Ransom Note Structure

In every directory where files have been modified, the `.hamster15` variant deposits an ASCII text ransom note containing specific instructions for the victim. While legacy MedusaLocker builds frequently used note filenames such as !!!_READ_ME_MEDUSA_!!!.txt or HOW_TO_RECOVER_DATA.html, the “Hamster” affiliate series commonly deploys localized text notes titled How_to_back_files.txt, Recovery_Instructions.txt, or simply Readme.txt.

The structure of the note follows a standardized template designed to establish communication channels, display a unique hexadecimal infection ID, and threaten the victim with data publication (double-extortion) if compliance is not met within a strict 72-hour window. Below is an authentic structural representation of the text note associated with the `.hamster15` / `.hamster20` campaign:

Observed Ransom Note Text Reference

Threat Parameters Summary Table

Primary Infiltration Vectors: RDP & Lateral SMB Traversal

Understanding how the `.hamster15` MedusaLocker series breaches network perimeters is essential for preventing re-infection during the recovery phase. Threat intelligence monitoring confirms that affiliates deploying this specific variant rely overwhelmingly on two primary infiltration methodologies: **Remote Desktop Protocol (RDP) exploitation** and the acquisition of credentials from **Initial Access Brokers (IABs)**.

In a significant majority of investigated cases, initial access is achieved by scanning the internet for exposed RDP endpoints operating on default port 3389 or non-standard routed ports. Attackers utilize automated brute-force tools and credential-stuffing matrices to crack weak administrative passwords. Alternatively, affiliates purchase pre-compromised corporate RDP credentials, VPN session tokens, or web shell access directly from dark web marketplaces.

Once interactive desktop access is established on Patient Zero, the affiliate performs manual reconnaissance using native “Living-off-the-Land” (LotL) utilities. They run network scanners to map internal active subnets, enumerate Domain Controllers, and dump credentials from system memory using tools like Mimikatz or ProcDump. Upon securing elevated domain privileges, the actors deploy the MedusaLocker payload laterally across Server Message Block (SMB) network shares, Administrative Shares (C$, ADMIN$), and Remote Procedure Call (RPC) protocols, executing the encryption binary simultaneously across dozens of internal servers.

Immediate Enterprise Containment Mandates

If your cybersecurity monitoring systems or internal users report the sudden appearance of `.hamster15`, `.hamster20`, or related extension strings, incident response command must execute immediate containment protocols to arrest horizontal propagation across the enterprise mesh:

1. Sever Physical and Logical Network Interfaces: Immediately disconnect infected workstations, file servers, and virtual hosts from local switches, Wi-Fi access points, and routing trunks. Do not power the machines off immediately, as volatile RAM contains critical process memory and potential cryptographic key artifacts that may aid forensic investigation.
2. Isolate Network Storage Arrays and NAS Units: Temporarily disable SMB/NFS sharing services on Network-Attached Storage (NAS) devices, Storage Area Networks (SAN), and backup servers. Because MedusaLocker aggressively crawls network paths, isolating shared repositories prevents the automated encryption of shared corporate backups.
3. Audit perimeter Access Rules & Disable External RDP: Log into edge firewalls and gateway routers immediately to disable all external port forwarding rules mapping to internal RDP (port 3389) or SMB (port 445) services. Enforce multi-factor authentication (MFA) across all remote access VPN tunnels.
4. Terminate Malicious Persistence Keys: On isolated systems, inspect the Windows Registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run and HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run. Identify and remove unauthorized startup strings matching the MDSLK parameter or referencing unrecognize executables located within %AppData% or %Temp% folders.

Forensic Data Restoration & Non-Extortion Pathways

Because the AES-256 and RSA-2048 algorithmic implementation in stable MedusaLocker builds cannot be brute-forced, IT leadership must evaluate legitimate, non-extortion technical recovery workflows to restore operational continuity:

• Restoration from Offline Immutable Backups: The primary and most reliable recovery path is restoring systems from verified, cold-stored offline backups or immutable cloud snapshots that were physically isolated from the primary network domain during the intrusion window. Before restoring data, target servers must undergo complete disk sterilization and re-imaging from clean baseline OS media to ensure no persistent backdoor trojans remain.
• Volume Shadow Copy & Unallocated Cluster Recovery: While MedusaLocker issues WMI commands to delete Volume Shadow Copies, execution failures, permission conflicts, or third-party snapshot software (such as hardware-level VSS providers) may cause the deletion command to fail on specific database servers. Specialized forensic data carving tools should be deployed to analyze unallocated disk space and shadow storage sectors for uncorrupted system snapshots.
• Database Structure Carving and Repair: In environments running large relational databases (such as Microsoft SQL Server or Oracle), the encryption process may be interrupted by network timeouts or large file-size constraints. In specific scenarios, database recovery specialists can extract intact tabular data from unencrypted internal data blocks, repair torn page headers, and reconstruct production tables without requiring a decryption key.
• Verification of Public Decryption Repositories: Before making any operational decisions, incident responders should cross-reference the exact binary hash and extension variant with verified global ransomware repositories such as the No More Ransom Project or the Cybersecurity and Infrastructure Security Agency (CISA) advisories. While current `.hamster15` builds remain secure against public tools, law enforcement server seizures periodically yield master RSA private keys that enable the development of public decryption utilities.