Obfusransom

Overview

Ransomware:Win32/Obfusransom is a highly critical, heuristic detection string utilized by antivirus engines (specifically Microsoft Defender) to flag a file that is actively exhibiting ransomware behaviors (`T1486`), but whose static signature has been heavily obfuscated, packed, or encrypted (`T1027`) to bypass traditional detection. This detection indicates that a ransomware payload is likely executing *right now* on the endpoint.

Understanding Obfusransom (Heuristic Ransomware Detection)
To an end-user, this detection often happens too late—as files are actively being encrypted, or right before the ransom note is dropped. For an incident response team, an 'Obfusransom' alert is a 'Break Glass' emergency. It means a threat actor has already bypassed perimeter defenses, delivered a payload, and initiated the final, destructive phase of the attack.

Execution and Obfuscation Mechanics
The file triggering the 'Obfusransom' alert is typically a custom-packed variant of a known ransomware family (like LockBit, Conti, or BlackCat). The attacker uses commercial or custom packers (`T1027.002`) so the file hash doesn't match any known threats. However, when the file executes, the AV engine detects its *behavior*: the rapid opening, encrypting, and renaming of hundreds of user files in quick succession (`T1486`), the deletion of Volume Shadow Copies via `vssadmin.exe` (`T1490`), or the dropping of `.txt` or `.html` ransom notes.

Indicators of Compromise & Impact
The impact is immediate, catastrophic data loss and extortion. Host-based IoCs include EDR alerts for mass file modification, the execution of `vssadmin.exe delete shadows /all /quiet` or `wbadmin delete catalog -quiet`, and the presence of encrypted files with anomalous extensions (e.g., `.locked`, `.crypted`). Network IoCs may include large data exfiltration (double extortion) immediately preceding the encryption event.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the MITRE ATT&CK framework:

Generated Detections (Boilerplate)

These YARA and Sigma rules are auto-generated based on the family name and aliases. They must be heavily tuned before deployment in a production environment.

YARA Rule

rule MALWARE_WIN_OBFUSRANSOM {
    meta:
        description = "Detects Obfusransom (ransomware)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "obfusransom" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Obfusransom Activity
id: d3d898549680ee90169a599e15669015
status: experimental
description: Detects generic indicators of the obfusransom malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*obfusransom*"
    condition: selection
level: medium

Containment & Response Steps

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

1. IMMEDIATELY isolate the affected endpoint from the network by physically unplugging the Ethernet cable or disabling the NIC; do not gracefully shut it down.
2. Do NOT reboot the machine, as this will destroy the encryption keys that might currently reside in volatile memory (RAM).
3. Capture a full memory dump (RAM) immediately; researchers may be able to extract the symmetric encryption keys from memory to decrypt files.
4. Initiate a full, enterprise-wide incident response protocol to identify lateral movement and determine if data was exfiltrated.

What to Avoid

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

1. Do NOT reboot the affected machine or turn it off normally; pull the plug or isolate the NIC.
2. Do NOT pay the ransom without consulting professional incident response and law enforcement agencies.

References & External Analysis

• Search "obfusransom" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: ransomware)

Explore other malware families in the same category:

• Agiala
• Spnr
• Blakamba
• Hoax
• Bredo

Machine-readable

Get this profile as JSON: https://jordan123234-malware-families-explorer.static.hf.space/api/obfusransom.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.