How To Remove Zero Access Trojan Virus?

  Category:  TROJAN VIRUS | 25th February 2026 | Author:  CSI'S TEAM

In-Depth Overview

ZeroAccess, Also Known As Sirefef, Was One Of The Most Sophisticated Peer-to-peer Botnet Trojans Active Primarily Between 2010 And 2014. It Combined Rootkit Capabilities, Decentralized Command-and-control (C2), Click Fraud Monetization, And Later Cryptocurrency Mining Into A Resilient Cybercrime Platform. At Its Peak, It Infected Millions Of Computers Worldwide And Generated Substantial Illicit Revenue For Its Operators.

1. Background And Emergence

ZeroAccess First Appeared Around 2010 And Quickly Evolved Into A Large-scale Botnet Threat. Unlike Many Earlier Trojans That Relied On Centralized Servers For Command And Control, ZeroAccess Adopted A peer-to-peer (P2P) Architecture, Making It Significantly Harder For Law Enforcement Or Cybersecurity Companies To Dismantle.

The Malware Was Designed Primarily For Financial Gain. Its Operators Earned Money Through:

• Click Fraud Campaigns

• Bitcoin Mining (in Later Versions)

• Installing Additional Malware (pay-per-install Schemes)

• Fake Antivirus Distribution

ZeroAccess Became Notable Not Just For Scale, But For Technical Sophistication, Particularly In How It Embedded Itself Deep Within Infected Windows Systems.

Aliases: Trojan-Dropper.Win32.PMax.a (Kaspersky, Trojan.Horse (Symantec), TrojanDropper:Win32/Sirefef.A (Microsoft), Trojan.Win32.Agent.csaf [Kaspersky], Trojan.Cryptor.A [BitDefender]Trojan.Win32.Agent.csaf [F-Secure], Mal/Crot-A [Sophos], Trojan.Agent.csaf (CAT-QuickHeal), Crot.gen.b [McAfee] , TrojWare.Win32.Agent.csaf [Comodo]

Installation

The Trojan Has Been Distributed By Various Ways Such As Exploit Kits ( E.g. Blackhole Kit ), Malware/TrojanDownloaders ( Such As TrojanDownloader: Win32/Beebone.gen!A , TrojanDownloader:Win32/Karagany.I And Win32/Dofoil Family Variants ) , And Use Of Pirated Softwares Related To 'crack' And 'keygen'. Some Of The Softwares Are:

• "Download Nokia Dongle.exe"
• "Facebook Password Cracker.exe"
• "autocad_2007_full_crack.exe".

The Files Dropped By Sirefef Are As Follows:

Location: c:\recycler\ [Redacted]
Files Dropped:

• "@" - This File Contains Information That Sirefef Can Use To Find Other Infected Computers
• "n" - This File Contains The Malicious Code For Peer-to-peer (P2P) Communication

The Registry Changes Made By The Trojan Sirefef To Ensure Its Persistence Are As Follows:

In Subkey: HKLM\Software\Classes\clsid\{5839fca9-774d-42a1-acda-d6a79037f57f}\InprocServer32
Modifies Value: "(Default)"
From Data: " < System Folder > \wbem\wbemess.dll"
With Data: " " (For Example : "c:\recycler\ \n" )

To Intercept And Hijack Network Traffic, It Drops The Following Files:

• %windir%\assembly\GAC\desktop.ini Or
• %windir%\assembly\GAC_32\desktop.ini

It Stops And Attempts To Delete Windows Security Services Like Windows Defender (windefend), IP Helper (iphlpsvc),Windows Security Center(wscsvc), Windows Firewall (mpssvc), Base Filtering Engine (bfe)

2. Infection Vectors

ZeroAccess Spread Through Multiple Attack Methods:

Drive-by Downloads

Users Visiting Compromised Or Malicious Websites Would Unknowingly Trigger Downloads Via Exploit Kits Targeting Browser Vulnerabilities.

Malicious Email Attachments

Phishing Campaigns Delivered Infected Attachments Disguised As Invoices, Shipping Documents, Or Software Updates.

Pirated Software & Keygens

Cracked Software Distributed Through Torrent Sites Often Contained ZeroAccess Payloads.

Secondary Payload

Other Malware Families Frequently Installed ZeroAccess As A Secondary Infection.

3. Technical Architecture

Rootkit Capabilities

ZeroAccess Installed Itself As A Kernel-mode Rootkit. This Meant:

• It Modified System Drivers.

• It Injected Malicious Code Into Legitimate Windows Processes.

• It Hid Files, Registry Entries, And Network Connections.

• It Disabled Security Software.

Because It Operated At Such A Low System Level, Removal Was Extremely Difficult Without Specialized Tools Or A Full System Reinstall.

Peer-to-Peer Botnet Design

Traditional Botnets Rely On Central Command Servers. When Those Servers Are Seized, The Botnet Collapses.

ZeroAccess Avoided This Weakness By Using A Decentralized P2P Structure:

• Each Infected Machine Communicated With Other Infected Machines.

• Commands Were Distributed Across The Network.

• Updates Were Propagated Peer-to-peer.

• No Single Failure Point Existed.

This Resilience Made Takedown Efforts Complex And Prolonged.

4. Click Fraud Operations

One Of ZeroAccess’s Primary Monetization Strategies Was Click Fraud.

Here’s How It Worked:

1. The Malware Injected Code Into Browsers.

2. It Silently Loaded Advertisements In The Background.

3. It Simulated User Clicks On Pay-per-click Ads.

4. Advertisers Paid For Fake Traffic.

Victims Often Noticed:

• Slower Internet Performance

• High CPU Usage

• Increased Network Traffic

• Suspicious Outbound Connections

Click Fraud Generated Millions Of Dollars In Fraudulent Advertising Revenue.

5. Bitcoin Mining Variant

Later Versions Of ZeroAccess Included Cryptocurrency Mining Modules.

At The Time, Bitcoin Mining Was Still Feasible On Consumer Hardware. The Malware:

• Used Infected Computers’ CPU Or GPU Resources.

• Ran Mining Processes Silently In The Background.

• Generated Cryptocurrency For Attackers.

This Caused:

• Overheating

• Sluggish System Performance

• Increased Electricity Usage

• Hardware Strain

6. File System Manipulation

ZeroAccess Was Known For Modifying:

• NTFS Structures

• System Drivers

• Registry Keys

• Firewall Rules

It Sometimes Created Hidden File System Entries That Standard Antivirus Scanners Couldn’t See. In Certain Variants, It Deleted Legitimate Windows Components And Replaced Them With Malicious Versions.

This Deep System Tampering Often Required Advanced Forensic Tools For Complete Cleanup.

7. Security Evasion Techniques

ZeroAccess Used Several Stealth Techniques:

• Kernel-mode Rootkit Drivers

• Code Obfuscation

• Encrypted Network Communication

• Self-healing Mechanisms (re-infection If Partially Removed)

• Disabling Windows Defender And Security Updates

If An Antivirus Removed Part Of The Infection, Peer Nodes Could Re-download Missing Components.

8. Law Enforcement Disruption

In 2013, Coordinated Action Between Cybersecurity Firms And Law Enforcement Agencies Significantly Disrupted The ZeroAccess Botnet Infrastructure.

Although The Peer-to-peer Network Complicated Takedown Efforts, Sinkholing And Traffic Interception Weakened Its Operations. Over Time, Infections Declined As:

• Security Software Improved

• Patches Closed Exploit Vectors

• Public Awareness Increased

9. Differences From SEO Spam Malware

ZeroAccess Primarily Targeted Windows Operating Systems, Not Websites Directly.

If You Are Seeing:

• Thousands Of Spam Blog Posts

• SEO Keyword Injection

• Pharma Spam Pages

• Japanese Or Gambling Spam Content

That Is Usually Caused By:

• CMS Backdoors (e.g., Compromised WordPress Installations)

• Database Injection

• Stolen FTP Credentials

• Server-side PHP Malware

ZeroAccess Itself Does Not Typically Generate Blog Paragraphs On Websites. However, An Infected PC Could Leak Credentials Used To Compromise A Hosting Account.

10. Indicators Of Infection

Signs That A System May Have Been Infected With ZeroAccess Included:

• Disabled Antivirus Software

• Firewall Turned Off Unexpectedly

• Unexplained Outbound Network Traffic

• High CPU Usage When Idle

• Unknown Services Or Drivers

• DNS Manipulation

11. Removal Challenges

Due To Its Rootkit Nature, Standard Removal Often Failed.

Recommended Response Historically Included:

1. Disconnect From The Internet.

2. Use Offline Antivirus Rescue Media.

3. Scan From A Clean Boot Environment.

4. Back Up Essential Files.

5. Perform A Full OS Reinstall If Integrity Is Uncertain.

6. Change All Passwords After Cleanup.

In Many Cases, Security Professionals Recommended Complete Reinstallation Because Rootkits Compromise Trust In System Integrity.

12. Lessons Learned

ZeroAccess Demonstrated:

• The Effectiveness Of Decentralized Botnets.

• The Profitability Of Click Fraud.

• The Danger Of Kernel-level Persistence.

• The Need For Layered Security.

It Also Influenced Later Malware Families That Adopted Similar Peer-to-peer Resilience Techniques.

13. Modern Relevance

Although ZeroAccess Itself Is No Longer A Dominant Active Threat, Its Design Principles Live On In:

• Modern Botnets

• Cryptocurrency Mining Malware

• Modular Malware Frameworks

• Decentralized Command Infrastructures

The Cybersecurity Industry Continues To Evolve Detection Methods To Counter These Techniques.

14. Prevention Best Practices

To Prevent Similar Infections:

• Keep Operating Systems Updated.

• Use Reputable Security Software.

• Avoid Pirated Software.

• Enable Firewall Protection.

• Use Multi-factor Authentication.

• Monitor Unusual Outbound Network Traffic.

• Maintain Regular Backups.

Conclusion

ZeroAccess Was A Landmark Trojan In The Evolution Of Botnets. Its Peer-to-peer Command Architecture, Kernel-level Stealth, And Monetization Strategies Made It Both Technically Impressive And Financially Damaging. While Largely Neutralized Today, It Remains A Case Study In Resilient Malware Design And The Importance Of Proactive Cybersecurity Practices.

Malware Removal Guide For PC

Malware Removal Guide For Web Browsers

Prevent Future Malware

Summary - Malware Removal Guide

Guide For VPN Uses

Malware Removal Guide – PC And Web Browser

PART 1: Remove Malware From Your PC (Windows)

Step 1: Boot Into Safe Mode

• Restart Your PC And Press F8 (or Shift + F8 For Some Systems) Before Windows Loads.

• Choose Safe Mode With Networking.

Safe Mode Prevents Most Malware From Loading.

Step 2: Uninstall Suspicious Programs

1. Press Win + R, Type appwiz.cpl, And Press Enter.

2. Sort By Install Date And Uninstall Unknown Or Recently Added Programs.

Step 3: Run A Malware Scan

Use A Trusted Anti-malware Tool:

Malwarebytes – https://www.malwarebytes.com

Screenshot Of Malwarebytes - Visit Links

Microsoft Defender – Built Into Windows 10/11

HitmanPro, ESET Online Scanner, Or Kaspersky Virus Removal Tool

ZoneAlarm Pro Antivirus + Firewall NextGen

VIPRE Antivirus - US And Others Countries, | India

VIPRE Antivirus - Mac

F-Secure Total - Global

Run A Full Scan And Delete/quarantine Detected Threats.

Step 4: Delete Temporary Files

1. Press Win + R, Type temp → Delete All Files.

2. Press Win + R, Type %temp% → Delete All Files.

3. Use Disk Cleanup: cleanmgr In The Run Dialog.

Step 5: Reset Hosts File

1. Go To: C:\Windows\System32\drivers\etc

2. Open hosts File With Notepad.

3. Replace With Default Content:

Step 6: Check Startup Programs

1. Press Ctrl + Shift + Esc → Open Task Manager

2. Go To Startup Tab

3. Disable Any Suspicious Entries.

Step 7: Reset Network Settings

1. Open Command Prompt As Administrator.

2. Run These Commands:

netsh Winsock Reset

netsh Int Ip Reset

ipconfig /flushdns

PART 2: Remove Malware From Web Browsers

? Common Signs Of Malware In Browser:

• Unwanted Homepage Or Search Engine

• Pop-ups Or Redirects

• Unknown Extensions Installed

Step 1: Remove Suspicious Extensions

For Chrome:

• Go To: chrome://extensions/

• Remove Anything Unfamiliar

For Firefox:

• Go To: about:addons → Extensions

• Remove Suspicious Add-ons

For Edge:

• Go To: edge://extensions/

• Uninstall Unknown Add-ons

Step 2: Reset Browser Settings

Chrome:

• Go To chrome://settings/reset → "Restore Settings To Their Original Defaults"

Firefox:

• Go To about:support → "Refresh Firefox"

Edge:

• Go To edge://settings/resetProfileSettings → "Reset Settings"

Step 3: Clear Cache And Cookies

All Browsers:

• Use Ctrl + Shift + Del → Select All Time

• Clear Cookies, Cached Files, And Site Data

Step 4: Check Search Engine & Homepage Settings

Make Sure They Are Not Hijacked.

• Chrome: chrome://settings/search

• Firefox: about:preferences#search

• Edge: edge://settings/search

Step 5: Use Browser Cleanup Tools (Optional)

• Chrome: chrome://settings/cleanup

• Use Malwarebytes Browser Guard For Real-time Browser Protection.

FINAL TIPS: Prevent Future Malware

• Always Download Software From Trusted Sources.

• Keep Windows, Browsers, And Antivirus Updated.

• Avoid Clicking Suspicious Links Or Ads.

• Use ad Blockers And reputable Antivirus Software.

• Backup Your Files Regularly.

Short Summary: Malware Removal Guide (PC & Web Browser)

To Remove Malware From Your Windows PC, Start By Booting Into Safe Mode, Uninstalling Suspicious Programs, And Scanning With Trusted Anti-malware Tools Like Malwarebytes. Clear Temporary Files, Reset Your Network Settings, And Check Startup Apps For Anything Unusual.

For web Browsers, Remove Unwanted Extensions, Reset Browser Settings, Clear Cache And Cookies, And Ensure Your Homepage And Search Engine Haven’t Been Hijacked. Use Cleanup Tools Like Chrome Cleanup Or Browser Guard For Added Protection.

?? Prevention Tips: Keep Software Updated, Avoid Suspicious Downloads, And Use Antivirus Protection Plus Browser Ad Blockers. Regular Backups Are Essential.

VPN - How To Use IT

1. Choose A Trusted VPN Provider

• Why It Matters: Not All VPNs Offer Malware Protection.

• What To Look For: Providers With built-in Malware/ad/tracker Blockers (e.g., NordVPN’s Threat Protection, ProtonVPN’s NetShield).

• Nord VPN
• Hide.me VPN

2. Enable Kill Switch

• Purpose: Prevents Data Leaks If Your VPN Connection Drops.

• Benefit: Ensures Your Real IP And Browsing Activity Aren’t Exposed To Malware-distributing Websites.

3. Use VPN With DNS Leak Protection

• Why It Matters: DNS Leaks Can Expose Your Online Activity To Attackers.

• Solution: Enable DNS Leak Protection In Your VPN Settings Or Use A Secure DNS Like Cloudflare (1.1.1.1).

4. Avoid Free VPNs

• Risk: Free VPNs Often Contain Malware, Sell User Data, Or Lack Security Features.

• Better Option: Use Reputable Paid VPNs That Offer security Audits And Transparent Privacy Policies.

5. Use VPN With Anti-Phishing Tools

• Some VPNs Block Known Phishing And Malicious Sites.

• Example: Surfshark’s CleanWeb, CyberGhost’s Content Blocker.

6. Keep Your VPN App Updated

• Reason: Security Patches Fix Known Vulnerabilities.

• Tip: Enable Auto-updates Or Check For Updates Weekly.

. Use VPN On All Devices

• Scope: Malware Can Enter Through Phones, Tablets, Or IoT Devices.

• Solution: Install VPN Apps On Every Internet-connected Device.

8. Don’t Rely On VPN Alone

• Fact: VPNs Do Not Remove Or Detect Malware On Your System.

• Complement It With:

  • Antivirus Software

  • Firewall

  • Browser Extensions For Script Blocking

9. Avoid Clicking Unknown Links While VPN Is On

• VPN Encrypts Traffic But Can’t Stop Malware From Executing If You Download Infected Files.

10. Use VPN With Split Tunneling Cautiously

• Split Tunneling Allows Certain Apps/sites To Bypass VPN.

• Tip: Never Exclude Browsers, Email Clients, Or Download Managers From VPN Tunneling.

Short Note - VPN Uses

A VPN (Virtual Private Network) Enhances Your Online Privacy By Encrypting Your Internet Traffic And Masking Your IP Address. It Protects Your Data On Public Wi-Fi, Hides Browsing Activity From Hackers And ISPs, And Helps Bypass Geo-restrictions. VPNs Also Add A Layer Of Defense Against Malware By Blocking Malicious Websites And Trackers When Using Advanced Features. However, A VPN Does Not Remove Existing Malware Or Act As Antivirus Software. For Full Protection, Combine VPN Use With Antivirus Tools, Regular Software Updates, And Cautious Browsing Habits. Always Choose A Reputable VPN Provider With Strong Security And Privacy Policies.