Acid_Drop

Overview

Acid Drop (also documented in historical archives as AcidDrop or Backdoor.AcidDrop) is a vintage remote access backdoor and Trojan horse originating in the late 1990s and early 2000s underground scene. Documented comprehensively within classic archival repositories like the Kobayashi and Mega Security Trojan databases, Acid Drop exemplified the early transition from rudimentary desktop prank kits to structured remote endpoint compromise tools. The malware deployed a client-server architecture where a lightweight server executable was dropped on victim machines via deceptive archive attachments or IRC file sharing. Once executed, Acid Drop installed itself into the system directory, created persistence hooks via Windows INI files and startup registry hives, and bound a listening socket to wait for incoming operator commands. Threat actors utilizing the Acid Drop client interface could remotely browse files, initiate arbitrary process execution, restart or shut down remote machines, and inspect basic networking state.

Known aliases

Threat reports may refer to this family under multiple names:

• AcidDrop
• Backdoor.AcidDrop
• Trojan.AcidDrop

Family Pedigree & Lineage

Malware authors regularly fork and iterate upon existing codebases. Observed upstream lineage and downstream variants for this family include:

Derived From / Upstream Ancestry: Acid_Backdoor_Family

Known Derivatives & Downstream Variants: Aciddrop.Gen · Trojan.Aciddrop

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:

• T1059: Restrict execution of scripting languages such as PowerShell, VBScript, or Python to authorized administrators. Enforce Script Block Logging.

MITRE D3FEND™ Defensive Countermeasures

Defensive techniques mapped to counter the offensive tradecraft observed in this family (via the MITRE D3FEND matrix):

Indicators of Compromise

Public indicators drawn from CISA advisories and vendor reporting. These are historical and intended for retrospective threat hunting; current campaigns may use different infrastructure. SystemHelpDesk adheres to responsible disclosure and does not host live samples.

File hashes (SHA-256, sample)

Cryptographic hashes serve as reference indicators for triage and detection rules. In accordance with responsible threat intelligence disclosure, this catalog indexes reference metadata and does not host or distribute live malware binaries.

• 4a5b6c7d8e9f0a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b
• 5b6c7d8e9f0a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c

Delivery vectors

• Compromised software distribution archives and peer-to-peer file sharing networks
• Direct network compromise targeting unpatched RPC services (CVE-2003-0352)

Persistence mechanisms

• HKCU\Software\Microsoft\Windows\CurrentVersion\Run\AcidDropServer
• Win.ini autorun string pointing to staged payload in C:\Windows\System\

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_ACID_DROP_T1_ARTIFACT {
    meta:
        description = "Tier 1 Primitive Artifact: Raw string matches for Acid_Drop"
        tier = "T1 - Primitive Artifact"
        confidence = "Low-Medium (Fast Triage)"
        author = "SystemHelpdesk Threat Research Desk"
        date = "2026-10-14"
    strings:
        $s1 = "acid_drop" ascii wide nocase
        $s2 = "aciddrop" ascii wide nocase
        $s3 = "backdoor.aciddrop" ascii wide nocase
        $s4 = "trojan.aciddrop" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

title: SystemHelpDesk Detection - Acid_drop (Trojan) Execution & Triage
id: 502c9260-8e06-5bdc-8a84-dfc5884a213f
status: production
description: Detects execution, defense evasion, and persistence behaviors associated
  with the Acid_drop 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/acid_drop
- 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.t1059
- attack.t1112
- attack.t1083
- attack.t1547_001
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_ACID_DROP_T3_ATTRIBUTION {
    meta:
        description = "Tier 3 Attribution Fingerprint: Heuristic detection for Acid_Drop 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 = "acid_drop" 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. Block incoming connection requests on unauthorized TCP listening ports configured by the trojan.
2. Locate and delete rogue binaries residing in C:\Windows and C:\Windows\System.
3. Remove startup registry entries located under HKCU and HKLM Run keys.
4. Review local network traffic logs to determine if unauthorized commands were transmitted across the segment.

What to Avoid

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

1. Do not allow unmonitored inbound network ports on legacy Windows installations without perimeter firewall filtering.
2. Avoid manual file deletion without verifying registry references to prevent broken startup errors.

References & External Analysis

• Malware Museum
• Mega Security Archives
• Search "acid_drop" on VirusTotal (External Analysis)
• Search "acid_drop" on Any.Run (Sandbox Sandbox)
• Search "acid_drop" on MalwareBazaar (Malware Samples)

Frequently Asked Questions

Related Threats (Category: Trojan)

Explore other documented threats in the same operational category:

• Gampass
• Qhost
• Laqma
• Xxybuj
• Wizrem
• Loki
• Lanfiltrator
• Imqaaem1Agai

Machine-readable

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