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What is a Trojan Virus and How can You Defend Against One?

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A few generations back, you may have heard the story of the wooden horse used to infiltrate Troy. In cybersecurity, a Trojan (or Trojan horse) is the modern version of that deception: software that looks legitimate, but hides a malicious purpose.

Trojans remain one of the most common ways attackers gain an initial foothold in a device or business network—often as the “first step” that later enables credential theft, data exfiltration, ransomware, and long-term remote access.

This guide breaks down what a Trojan is, how it works in 2026’s threat landscape, the most common Trojan types, warning signs, and—most importantly—how to defend your home or business against Trojan-based attacks using modern best practices and the ACSC Essential Eight (highly relevant for Australian organisations)

What Is a Trojan Horse?

A Trojan horse is a program that appears useful or harmless, while secretly performing malicious actions—often by taking advantage of the permissions you (or your device) grant it when you run it.

Is a Trojan a “virus”?

People often say “Trojan virus,” but it’s not technically a virus. A virus typically replicates by attaching itself to other files. A Trojan usually relies on deception and execution (you run it, open it, or approve it) rather than self-replication.

Why attackers love Trojans

A Trojan is often a delivery vehicle:

It can install additional malware (like ransomware or spyware)
It can create a backdoor for future access
It can steal credentials and session cookies
It can enroll devices into botnets
It can quietly surveil users and systems over time

How Trojans Work: The Modern Infection Chain

While the “look legitimate → run it → get compromised” idea still holds, modern Trojans typically follow a well-worn chain of events:

1) Delivery: how it reaches you

Common delivery paths include:

Phishing emails (attachments, invoices, shared documents)
Malicious links via SMS (“smishing”), social media, Teams/Slack-style messages
Fake software updates or fake security alerts
Cracked software / keygens and shady download sites
Malvertising (malicious ads) and drive-by downloads
Compromised third-party software (supply chain risk)

Email and social engineering remain major initial access vectors for ransomware and data theft campaigns—many of which begin with a Trojan-style payload.

2) Execution: the user (or system) runs it

This can look like:

Microsoft specifically notes that VBA macros have been widely abused to gain initial access for malware and ransomware—hence changes to block macros in files from the internet by default.

3) Persistence: it makes itself hard to remove

Once installed, a Trojan may:

Modern RATs (Remote Access Trojans) often prioritise stealth and persistence so they can remain in an environment longer and extract more value.

4) Command-and-control (C2): it “phones home”

Many Trojans connect to attacker-controlled infrastructure to:

5) Actions on objectives: the real damage

Depending on the attacker’s goal, that may include:

Common Types of Trojans (and What They Do)

Below are Trojan categories you’ll still see referenced—plus the modern equivalents your security team is likely tracking.

Backdoor Trojans

Designed to give attackers remote access to the system. Backdoors are often used to upload/download files, execute commands, and maintain long-term access.

Remote Access Trojans (RATs)

A subset of backdoor Trojans designed for full remote control—including surveillance features (screen capture, keylogging, clipboard capture) and data theft. Microsoft incident response has documented modern RATs that focus on stealth, persistence, reconnaissance, and sensitive data theft (including crypto-related data).

Banker Trojans

Built to steal financial credentials (online banking, payment apps, card data). Many “banking trojans” have evolved into broader loaders and botnets used to deliver other malware—QakBot is a widely cited example of this evolution.

Downloader / Loader Trojans

Their job is simple: get in, then fetch more malware. Many ransomware incidents begin with loader activity that establishes access and delivers follow-on payloads.

Rootkit Trojans

Rootkits are meant to hide malware activity and evade detection. They can interfere with security tools and conceal malicious processes.

DDoS Trojans

Used to join infected machines into coordinated attacks against targets (websites/services), flooding them with traffic.

Infostealers (modern “credential theft” Trojans)

These focus on:

Email / Contact Harvesters (e.g., “mailfinder” behaviour)

Designed to exfiltrate address books and email contacts to fuel spam, phishing, and business email compromise attempts.

Mobile Trojans (including SMS Trojans)

On mobile devices, Trojans may:

Signs Your Device or Network Might Be Infected

Signs that your device may be affected by a Trojan-Computing Australia Group

Trojans don’t always announce themselves. Many aim to be quiet. Still, common red flags include:

On a personal device

Noticeable slowdowns, overheating, frequent crashes
New programs you don’t remember installing
Browser redirects, strange extensions, or changed homepage/search engine
Repeated login prompts, locked accounts, suspicious password reset emails
Antivirus disabled “mysteriously” (a big warning sign)

In a business environment

Unusual outbound network traffic (especially to rare domains/IPs)
Multiple failed logins, new admin accounts, or unexpected privilege changes
Security logs showing PowerShell misuse or suspicious scheduled tasks
Devices communicating with external hosts at odd hours
Unapproved remote tools installed

If you suspect a Trojan, don’t rely on “Google the weird process name” alone. Attackers often mimic legitimate names. Treat symptoms as a cue to investigate properly.

For individuals

1. Disconnect from the internet(Wi-Fi off / unplug Ethernet).

2.Don’t log in to banking or email from the suspected device.

3. Run a full scan with reputable security software.

4. Update your OS and apps from official sources only.

5. From a clean device, change passwords for:

For businesses

1. Isolate the endpoint(EDR network isolation if available).

2. Preserve evidence (don’t wipe immediately if you need forensics).

3. Check for:

4. Reset credentials that may be compromised (especially privileged accounts).

5. Review backups and recovery readiness.

6. If ransomware is suspected, align response to established guidance (e.g., incident checklists and response playbooks).

How to Protect Yourself From Trojans: Modern Defence That Actually Works

“Install antivirus” is not enough anymore. Strong defence is layered: people + process + technology.

1) Patch operating systems and applications (fast)

Attackers routinely exploit known vulnerabilities—especially where patching is inconsistent. The ACSC Essential Eight places strong emphasis on patching applications and operating systems as core mitigations.

Practical tips:

2) Use multi-factor authentication (MFA) everywhere possible

MFA reduces the damage of credential theft—one of the most common Trojan outcomes. ACSC includes MFA in the Essential Eight for a reason.

Even better:

3) Block risky macro behaviour and harden user apps

Office macros have historically been a common Trojan delivery mechanism. Microsoft has moved to block macros from the internet by default in Office to reduce this risk.

For businesses:

This maps closely to Essential Eight controls like restrict Microsoft Office macros and user application hardening.

4) Apply least privilege and restrict admin rights

Many Trojans do more damage when they get admin privileges. Reducing admin access limits:

ACSC explicitly includes restrict administrative privileges as an Essential Eight strategy.

5) Use application control (stop unknown executables)

Application control can prevent many Trojan payloads from ever running. It’s a cornerstone mitigation in the Essential Eight.

If you’re a Microsoft environment, this often means implementing tools/policies that restrict execution to approved applications (paired with strong endpoint protection).

6) Maintain secure, testable backups

Trojans often pave the way for ransomware. Backups aren’t just “nice to have”—they’re critical resilience.

Best practices:

Essential Eight includes regular backups as a key mitigation.

7) Strengthen email and web filtering

Since many Trojans arrive via phishing, invest in:

The #StopRansomware guidance groups prevention recommendations around common initial access vectors like phishing and exposed services—reducing initial access reduces Trojan success rates.

8) Use endpoint detection and response (EDR) + monitoring

Modern Trojans can be modular and stealthy. Logging and detection matter:

9) Train staff for realistic threat behaviour

Because Trojans often rely on social engineering, invest in:

Trojan Defence Checklist (Quick Win Version)

If you want an actionable list you can implement quickly:

Patch OS and third-party apps (automate where possible)
Enforce MFA (especially for email + admin accounts)
Block or tightly control Office macros
Remove local admin rights from standard users
Implement application control/allowlisting
Deploy EDR and centralise logs for visibility
Filter email + web traffic; block risky attachments by policy
Back up data securely and test restores
Maintain an incident response plan aligned to recognised guidance

Staying vigilant, being aware of the latest threats, and a sound security system can help protect yourself from Trojans. Speak to our cybersecurity specialists today and let us help your business stay safe and secure. Contact us or email us at cybersecurity@computingaustralia.group.

Jargon Buster

Distributed Denial of Service attack – DDoS – Hackers crash a website or computer by overwhelming the website or server with too many requests or traffic.

Firewalls – is a network security system that observes and manages incoming and outgoing network traffic based on a predetermined set of rules.

Malware – is any malicious software intentionally designed to cause damage to a computer, server or network—Eg. Ransomware, Spyware etc.

Run as a standard user, not admin, for daily use.