Grenam

Overview

Worm:Win32/Grenam (also known as a generic network worm or **File Infector**) is a self-replicating malware family that aggressively spreads across network shares and removable media. While older, its mechanics are highly destructive. Beyond propagation, Grenam acts as a polymorphic File Infector (`.exe` and `.dll` files), physically altering legitimate binaries to embed its malicious code, ensuring maximum persistence and complicating remediation.

Understanding the Grenam Worm
To an infected user, the computer might become highly unstable, and antivirus engines will suddenly start quarantining hundreds of previously safe programs. For security analysts, Grenam represents a severe incident. Because it alters legitimate system files, manually deleting the 'infected' files will destroy the operating system or the user's installed applications. The network worm aspect means the infection will rapidly spread to any unprotected node on the LAN.

Propagation and File Infection Mechanics
The infection cycle often begins via an infected USB drive (`T1091`) or an open network share (`T1080`). Upon execution (`T1204.002`), Grenam establishes persistence via multiple Registry Run keys. It begins scanning the local network for vulnerable SMB shares to copy itself into. Concurrently, it acts as a File Infector (`T1055.001`), searching the hard drive for executable files (`.exe`, `.scr`). It opens the target file, modifies the PE header, and appends its malicious, often polymorphically packed code. It then alters the entry point of the executable to point to the viral code. When the user later runs the 'infected' legitimate application, the worm code executes first, infects more files, and then passes control back to the legitimate application.

Indicators of Compromise & Impact
The primary impact is the widespread corruption of executable files, severe system instability, and network congestion. The most glaring IoC is an avalanche of EDR alerts indicating that hundreds of distinct, legitimate executables across the `C:\` drive have changed their file hashes and sizes. Network logs will show a massive spike in SMB (Port 445) traffic as the worm attempts lateral movement.

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_GRENAM {
    meta:
        description = "Detects Grenam (advanced_threat)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "grenam" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Grenam Activity
id: a42a70d7629d1e8803dbeb4692cb004e
status: experimental
description: Detects generic indicators of the grenam malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*grenam*"
    condition: selection
level: medium

Containment & Response Steps

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

1. Isolate the endpoint from the network instantly to halt the SMB propagation engine and stop it from infecting other workstations.
2. Disable AutoRun and AutoPlay enterprise-wide via Group Policy for all removable media.
3. Because Grenam is a file infector, you must use an enterprise antivirus solution capable of 'cleaning' (disinfecting) the appended malicious code from legitimate `.exe` files.
4. If disinfection fails or the system is heavily corrupted, a complete OS rebuild from secure backups is required.

What to Avoid

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

1. Do not blindly delete all files flagged by the AV; if they are infected system files, deleting them will render Windows unbootable.
2. Avoid plugging a clean USB drive into the infected machine to transfer data, as the worm will immediately infect the drive.

References & External Analysis

• Search "grenam" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: advanced_threat)

Explore other malware families in the same category:

• Iwin
• Navipromo
• Fakerean
• Silentall
• Nspm

Machine-readable

Get this profile as JSON: https://jordanricky1604-ship-it.github.io/malware-families-catalog/api/grenam.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.