Mailcheck

Overview

Executive Summary

Infection Vector and Technical Capabilities

• Proxy Integration: To avoid rate-limiting, CAPTCHAs, and IP bans from the targeted email providers, MailCheck tools heavily integrate with massive lists of rotating SOCKS/HTTP proxies. This distributes the login attempts across thousands of IP addresses.
• Automated Authentication (IMAP/POP3/SMTP/Web): The tool ingests the combo list and rapidly attempts to authenticate against the targeted email provider's servers. It may use standard mail protocols (IMAP/POP3) or attempt to interact directly with the provider's web login API.
• Sorting and Output: The tool categorizes the results, generating text files separating the "Valid" (successful logins) accounts from the "Invalid" or "Locked" accounts. The attacker then uses the valid list for spamming, identity theft, or reselling on illicit markets.

Threat Assessment

Incident Response and Remediation

• Immediate Legal and HR Escalation: Isolate the endpoint immediately, but do not power it off to preserve volatile memory. Because the tool is used for explicit fraud/hacking, this incident must be immediately escalated to HR and Legal counsel.
• Forensic Chain of Custody: Treat the endpoint as a crime scene. A full forensic image of the hard drive must be captured to determine the origin of the combo lists and identify which specific email providers the attacker was targeting.
• Network Auditing: Review the corporate firewall logs for massive spikes in outbound traffic to IMAP/POP3 ports or anomalous DNS requests to proxy services, correlating this activity with the infected endpoint.

Known aliases

Threat reports may refer to this family under multiple names:

• HackTool.MailChecker
• Riskware.AccountChecker
• Tool.ComboChecker

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.

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_MAILCHECK_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Mailcheck"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "mailcheck" ascii wide nocase
        $s2 = "hacktool.mailchecker" ascii wide nocase
        $s3 = "riskware.accountchecker" ascii wide nocase
        $s4 = "tool.combochecker" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Mailcheck (Trojan) Execution & Triage
id: 6749e6f2-2546-5eb9-82cb-4057ccc68535
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Mailcheck malware family. Detects suspicious execution from temporary user
  directories with hidden window flags and background transfer utilities.
references:
- https://malware-families-catalog.systemhelpdesk.com/families/mailcheck
- https://malware-families-catalog.systemhelpdesk.com/defensive/downloader-dropper-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.trojan
- author.ricky-jordan
- source.systemhelpdesk-mfc
- attack.t1110_004
- attack.t1090
- attack.t1055
- attack.t1547_001
- attack.t1105
logsource:
  category: process_creation
  product: windows
detection:
  selection_process:
    Image|endswith:
    - \cmd.exe
    - \powershell.exe
    - \cscript.exe
    - \wscript.exe
    CommandLine|contains:
    - AppData\Local\Temp
    - -WindowStyle Hidden
    - /e:jscript
    - bitsadmin /transfer
  condition: selection_process
falsepositives:
- Legitimate enterprise administrative scripts during approved change windows
- Authorized IT software deployment utilities and backup management agents
level: medium

rule MALWARE_WIN_MAILCHECK_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Mailcheck 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 = "mailcheck" 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. Sever network connectivity for the flagged endpoint to terminate persistent command-and-control (C2) communication channels and stop Mailcheck from retrieving secondary stage payloads.
2. Capture process memory triage to detect and terminate malicious threads injected by Mailcheck into legitimate system processes such as explorer.exe or svchost.exe.
3. Audit Windows Registry Run keys (HKCU\Software\Microsoft\Windows\CurrentVersion\Run) and Task Scheduler entries to locate and purge Mailcheck persistence hooks.
4. Deploy updated enterprise EDR detection signatures and conduct an out-of-band endpoint scan to isolate dropped binaries and staged payloads.
5. Rotate all credentials, domain user passwords, and session cookies active on the system during the Mailcheck execution timeframe.

What to Avoid

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

1. Do not assume the incident is resolved simply because an antivirus tool removed the initial Mailcheck dropper; always verify that secondary payloads and persistence hooks have been eradicated.
2. Avoid logging in with Domain Admin or privileged credentials on a machine suspected of running Mailcheck until memory triage is complete.
3. Do not delay host isolation; automated lateral movement and credential dumping can occur within minutes of initial execution.

References & External Analysis

• Search "mailcheck" on VirusTotal (External Analysis)
• Search "mailcheck" on Any.Run (Sandbox Sandbox)
• Search "mailcheck" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Optix
• Seznam
• Anyprotect
• Bhoint
• Strongpity
• Njlogger
• Alyak
• Hvnc

Machine-readable

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