Sigma
Table of Contents
Overview
Ransom:Win32/Sigma is a severe threat classified as Ransomware. It is designed to extort victims by strongly encrypting critical personal or corporate data, appending a specific extension (often `.sigma` or similar variations) to the files, and demanding payment in cryptocurrency for the decryption key.
Understanding Sigma
To an end-user, a Sigma infection represents an immediate crisis: documents and databases are suddenly inaccessible, and a ransom note (often a `.txt` or `.hta` file) is presented on the desktop. For an enterprise, it triggers a 'Code Red' incident response, requiring immediate network isolation to prevent lateral movement and the encryption of enterprise file shares and backups.
Execution and Extortion Mechanics
Sigma is frequently distributed via targeted phishing (`T1566.001`), exploit kits, or dropped by other initial access malware. Upon execution, it achieves persistence (`T1547.001`) and actively attempts to impair endpoint defenses (`T1562.001`). Crucially, before encryption begins, it executes commands like `vssadmin.exe delete shadows /all /quiet` to destroy Windows Volume Shadow Copies, preventing easy system restoration (`T1490`). It then rapidly iterates through the filesystem and mapped network drives, encrypting files matching high-value extensions (e.g., `.docx`, `.sql`, `.pdf`) using robust encryption algorithms (`T1486`).
Indicators of Compromise & Impact
The impact is catastrophic data loss. Host-based IoCs include massive spikes in disk I/O and CPU usage during the encryption phase, the sudden appearance of ransom note files in every directory, and EDR alerts for `vssadmin.exe` or `wmic.exe` attempting to delete shadow copies. Network IoCs may include anomalous outbound traffic if the ransomware incorporates a data exfiltration module.
MITRE ATT&CK Techniques
Observed techniques used by this family, mapped to our technical analysis hubs and the official MITRE ATT&CK framework:
| Technique | Name | Tactic | Official Spec |
|---|---|---|---|
T1486 | Data Encrypted for Impact | Impact | ATT&CK ↗ |
T1490 | Inhibit System Recovery (Deleting Shadow Copies) | Impact | ATT&CK ↗ |
T1562.001 | Impair Defenses: Disable or Modify Tools | Defense Evasion | ATT&CK ↗ |
T1566.001 | Phishing: Spearphishing Attachment | Initial Access | ATT&CK ↗ |
Tactical Mitigations
Based on the techniques used by this family, consider the following defensive strategies:
- 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):
| D3FEND ID | Countermeasure Name | Tactic | Countered ATT&CK | Defensive Guidance |
|---|---|---|---|---|
D3-FAED | File Alteration Early Detection | Detect | T1486 | Deploy canary files and monitor mass rename/encryption I/O spikes to terminate ransomware threads immediately. |
D3-VSS | Volume Shadow & Snapshot Hardening | Harden | T1490 | Block execution of vssadmin.exe and wmic shadowcopy delete; maintain air-gapped immutable backup repositories. |
D3-STA | Security Tool Anti-Tamper Enforcement | Harden | T1562.001 | Block unauthorized modification of antivirus registry flags and prevent stopping security agent services. |
D3-FS | File Sandboxing (Attachment Detonation) | Isolate | T1566.001 | Detonate macro-enabled documents, ISOs, and password-protected archives inside an isolated hypervisor. |
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:
Tier 1 Primitive Artifact Rule (Fast Triage)
Matches raw strings and file markers. Vulnerable to binary repacking and simple obfuscation.
rule MALWARE_WIN_SIGMA_T1_ARTIFACT {
meta:
description = "Tier 1 Primitive Artifact: Raw string matches for Sigma"
tier = "T1 - Primitive Artifact"
confidence = "Low-Medium (Fast Triage)"
author = "SystemHelpdesk Threat Research Desk"
date = "2026-10-14"
strings:
$s1 = "sigma" ascii wide nocase
condition:
uint16(0) == 0x5a4d and any of them
}Tier 2 Behavioral Context Rule (Process Chains & Sigma)
Tracks parent-child process anomalies and suspicious scripting telemetry. Resilient against file renames.
title: SystemHelpDesk Detection - Sigma (Ransomware) Execution & Triage
id: e8abcb2e-86e3-553c-8ef7-2a62de7aab1a
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
with the Sigma 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/sigma
- 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.t1562_001
- attack.t1566_001
- attack.t1490
- attack.t1486
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: criticalTier 3 Attribution Fingerprint Rule (High Fidelity)
Deep signature matching distinctive operational fingerprints and payload characteristics.
rule MALWARE_WIN_SIGMA_T3_ATTRIBUTION {
meta:
description = "Tier 3 Attribution Fingerprint: Heuristic detection for Sigma 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 = "sigma" 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.
- Immediately physically disconnect the infected endpoint from the network (pull the Ethernet cable) to stop the ransomware from encrypting network shares and spreading laterally.
- Do NOT reboot the infected machine; if possible, capture a live memory image (RAM dump) as the encryption keys might temporarily reside in memory.
- Restore the encrypted systems from secure, offline backups that were isolated from the network at the time of the attack.
- Verify the integrity of Active Directory; ransomware is often deployed network-wide by attackers who have already compromised Domain Admin credentials.
What to Avoid
Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.
- Do not pay the ransom immediately; paying funds criminal syndicates, and there is no guarantee the provided decryptor will actually work.
- Avoid reconnecting restored machines to the network until the initial entry vector has been completely secured.
References & External Analysis
- Search "sigma" on VirusTotal (External Analysis)
- Search "sigma" on Any.Run (Sandbox Sandbox)
- Search "sigma" on MalwareBazaar (Malware Samples)
Frequently Asked Questions
What is Sigma and what threat category does it belong to?
Sigma is classified as a Ransomware. Documented across security telemetry, it is engineered to execute unauthorized operations, establish persistent footholds on target endpoints, and facilitate malicious campaigns targeting enterprise and individual environments.
How does Sigma establish persistence and evade defensive controls?
Sigma typically establishes persistence through registry run keys, scheduled tasks, or service installations, while evading signature-based antivirus solutions through code obfuscation, memory injection, or anti-analysis sandbox checks.
What are the essential incident response steps for mitigating a Sigma compromise?
Security teams responding to Sigma should immediately isolate affected endpoints from the network, perform memory triage to capture volatile forensic artifacts, audit registry and startup persistence mechanisms, revoke exposed user credentials, and restore systems from verified, clean backups.
What cryptographic algorithms and file targeting techniques does Sigma employ during execution?
Sigma typically leverages hybrid cryptographic architectures combining symmetric stream or block ciphers (such as AES-256 or ChaCha20) for rapid file encryption with asymmetric algorithms (such as RSA-2048 or Curve25519) to securely protect the session keys. It aggressively targets sensitive file extensions including corporate documents, databases, virtual disks, and backups while systematically executing commands to delete Volume Shadow Copies (vssadmin delete shadows /all /quiet) and disable system recovery options.
Are decryptors available for Sigma, and what is the recommended recovery procedure?
Decryption availability for Sigma depends on whether security researchers or law enforcement (such as the No More Ransom initiative) have recovered the master private keys or discovered flaws in its key generation routines. Victims should never pay the extortion demand, as payment fuels criminal operations without guaranteeing file restoration. The recommended recovery procedure entails isolating infected hosts, conducting forensic disk imaging, identifying the root access vector, and restoring validated systems from immutable offline backups.
How do I remove the Sigma Ransomware from Windows?
Manual removal of Sigma is highly discouraged as it may leave persistence mechanisms intact. We recommend disconnecting the device from the internet and utilizing a professional incident response service or enterprise-grade EDR software to conduct a full forensic sweep.
Is Sigma a virus or a Ransomware?
Sigma is classified as a Ransomware. Unlike traditional viruses that infect files, modern malware like Sigma typically operates as a standalone payload designed to compromise systems, steal data, or deploy secondary stage implants.
What are the main symptoms of a Sigma infection?
Symptoms of Sigma can include unexpected system slowness, unauthorized outbound network traffic to unknown IP addresses, disabled security software, and suspicious background processes running from AppData or Temp directories.
Protect Your Network Against Ransomwares
Want to prevent Sigma and similar threats from compromising your organization? Read our comprehensive defensive guide: Ransomware Containment & Incident Response.
📣 Threat Intel Correction or Update?
Notice an outdated IOC, unlisted alias, or disputed attribution? Threat researchers are invited to propose an update on GitHub or contact the SystemHelpDesk research desk. All community revisions are credited in our public CHANGELOG.
Machine-readable
Get this profile as JSON: https://malware-families-catalog.systemhelpdesk.com/api/sigma.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.