Cutwail

Overview

Trojan:Win32/Cutwail (often associated with the Pushdo botnet) is a historically massive, highly resilient spam botnet and trojan downloader responsible for sending billions of spam emails daily and aggressively distributing severe secondary malware payloads.

Understanding Cutwail
To the end-user, a Cutwail infection is largely invisible, though the system may experience severe network lag. For incident responders, Cutwail represents a massive infrastructure threat. Operating at its peak, the Pushdo/Cutwail botnet was responsible for a significant percentage of all global spam. It is highly modular; the 'Pushdo' component handles initial infection and payload delivery, while the 'Cutwail' component is the specialized spam-sending engine.

Execution and Evasion Strategies
Cutwail is primarily distributed via drive-by downloads from exploit kits or dropped by other malware families. Upon execution, it employs a rootkit (often leveraging a malicious driver, like `TDSS`) to hide its files, registry keys, and network connections from the operating system and legacy antivirus. It establishes a connection to a decentralized C2 infrastructure. The Cutwail engine then utilizes an integrated, highly optimized SMTP engine to blast out spam campaigns (phishing, pump-and-dump stock scams, or malspam) directly from the infected host, spoofing headers to evade basic spam filters.

Indicators of Compromise & Impact
The impact is massive bandwidth consumption, participation in global cybercrime, and a high likelihood of the corporate IP being blacklisted. Incident responders will observe a massive, continuous spike in outbound traffic on TCP port 25, 465, or 587 originating from rootkit-hidden processes. EDR platforms frequently flag Cutwail based on 'Suspicious Rootkit Activity' or 'Anomalous SMTP Spikes'. Memory forensics is often required to detect the hidden Cutwail modules.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the 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.

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

Sigma Rule

title: Suspicious Cutwail Activity
id: 577322c875519fb19ad924a3287c6299
status: experimental
description: Detects generic indicators of the cutwail malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*cutwail*"
    condition: selection
level: medium

Containment & Response Steps

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

1. Quarantine the endpoint immediately to halt the outbound mass-mailing campaign and prevent the corporate IP from being blacklisted.
2. Enforce strict egress firewall rules blocking outbound SMTP traffic (TCP 25) from all endpoints, forcing traffic through authorized mail relays.
3. Utilize specialized anti-rootkit scanners or live memory forensics to identify and remove the hidden Cutwail drivers and injected processes.
4. Assume total endpoint compromise; perform a clean OS rebuild, as Cutwail's rootkit capabilities make manual remediation extremely unreliable.

What to Avoid

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

1. Do not ignore the infection as a 'low severity' spam issue; being blacklisted will cause all legitimate corporate emails to be blocked globally.
2. Avoid relying on standard, user-mode antivirus scans; the rootkit component actively hides the malware from the operating system.

References & External Analysis

• Search "cutwail" on VirusTotal (External Analysis)

Frequently Asked Questions

Related Families (Category: trojan)

Explore other malware families in the same category:

• Monitoringtool
• Vindor
• Faketool
• Wrar
• Zepfod

Machine-readable

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