Krserv

Overview

KRServ (classified across antimalware research as Ransom:Win32/KRServ, Trojan:Win32/KRServ, and Win32/KRServ) is a specialized ransomware and trojan service family observed targeting corporate servers and enterprise infrastructure. Distributed via compromised remote access credentials, unpatched server vulnerabilities, or lateral movement tools, KRServ installs itself directly as a background Windows Service (`krserv.exe`) configured with Automatic startup under `HKLM\System\CurrentControlSet\Services\KRServ`. Executing under high-privilege `NT AUTHORITY\SYSTEM` context, KRServ deploys asynchronous worker threads to traverse all local fixed disks, mounted network volumes, and removable media. It encrypts non-system files using robust AES-256 encryption, appending custom extensions and dropping ransom notes across traversed directories. Because it operates as an autonomous background service, KRServ persists across user logoffs and reboots, continuously monitoring the filesystem to encrypt newly created or modified files on the fly.

Known aliases

Threat reports may refer to this family under multiple names:

• Ransom:Win32/KRServ
• Trojan:Win32/KRServ
• Win32/KRServ
• KRServ Ransomware
• KRServ Service Trojan

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to our technical analysis hubs and the official MITRE ATT&CK framework:

Tactical Mitigations

Based on the techniques used by this family, consider the following defensive strategies:

• T1105: Implement network intrusion detection systems (NIDS) and host-based firewalls to block unauthorized inbound or outbound file transfers.
• T1566.001: Scan email attachments for malicious macros, scripts, or suspicious archive files.

MITRE D3FEND™ Defensive Countermeasures

Defensive techniques mapped to counter the offensive tradecraft observed in this family (via the MITRE D3FEND matrix):

Tiered Detection Rules (Cisco CAIRN Model)

Modern threat detection requires multi-layered rule engineering. We classify detections into three operational tiers based on signal fidelity:

rule MALWARE_WIN_KRSERV_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Krserv"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "krserv" ascii wide nocase
        $s2 = "ransom:win32/krserv" ascii wide nocase
        $s3 = "trojan:win32/krserv" ascii wide nocase
        $s4 = "win32/krserv" ascii wide nocase
        $s5 = "krserv ransomware" ascii wide nocase
        $s6 = "krserv service trojan" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Krserv (Ransomware) Execution & Triage
id: 29738802-d35c-563b-92f0-44d44d95f79b
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Krserv malware family. Detects volume shadow copy deletion, boot status
  tampering, and recovery inhibition characteristic of modern ransomware operations.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/krserv
- https://malware-families-catalog.systemhelpdesk.com/defensive/ransomware-protection
- https://malware-families-catalog.systemhelpdesk.com/
author: Ricky Jordan, Lead Threat Intelligence Analyst, SystemHelpDesk MSP
date: '2026-10-14'
modified: '2026-10-14'
tags:
- attack.ransomware
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1486
- attack.t1543_003
- attack.t1490
- attack.t1083
- attack.t1135
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \vssadmin.exe
    - \wmic.exe
    - \wbadmin.exe
    - \bcdedit.exe
    CommandLine|contains:
    - delete shadows
    - shadowcopy delete
    - resize shadowstorage
    - recoveryenabled no
    - ignoreallfailures
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: critical

rule MALWARE_WIN_KRSERV_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Krserv tradecraft"
        tier = "T3 - Attribution Fingerprint"
        confidence = "High (Confirmed Family Signature)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        // Core family identifier & heuristic payload signatures
        $attrib_str = "krserv" ascii wide nocase
        $pe_magic = "MZ" ascii
        $net_marker = "http" ascii nocase
    condition:
        uint16(0) == 0x5a4d and ($attrib_str and $net_marker) and filesize < 15MB
}

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

1. Isolate the endpoint immediately to prevent Krserv from downloading and executing its secondary payloads (e.g., ransomware).
2. Audit the Windows Task Scheduler and Registry Run keys to identify and remove the Krserv persistence mechanisms.
3. Review firewall and proxy logs to identify the C2 domains Krserv attempted to contact, and block them enterprise-wide.
4. Capture a live memory image (RAM dump) to extract the injected Krserv modules and identify the secondary payloads.

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

1. Do not close an incident simply because the initial Krserv dropper was quarantined; always verify if secondary payloads were downloaded.
2. Avoid relying solely on manual file deletion, as the injected processes will likely just recreate the dropped binaries.

References & External Analysis

• Microsoft Defender Threat Encyclopedia - Ransom:Win32/KRServ
• Kaspersky Threat Intelligence - Trojan-Ransom Overview
• MITRE ATT&CK - Create or Modify System Process: Windows Service (T1543.003)
• Search "krserv" on VirusTotal (External Analysis)
• Search "krserv" on Any.Run (Sandbox Sandbox)
• Search "krserv" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Ransomware)

Explore other documented threats in the same operational category:

• Emathi
• Pstinb
• Bero
• Ksdler
• Jaiks
• Regub
• Yobdam
• Fraudkrypt

Machine-readable

Get this profile as JSON: https://malware-families-catalog.systemhelpdesk.com/api/krserv.json

Ecosystem & Interactive Environments

This profile is part of the Malware Families Catalog, a public dataset of 2,899 malware families. The catalog is also published across our ecosystem: Hugging Face, Kaggle, Zenodo, Replit, StackBlitz, CodeSandbox, and CodePen.