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Trojan horse

Trojan Horse: Can It Be Detected before It’s Too Late?

 By Charles Joseph | Cybersecurity Researcher
 Published on November 23rd, 2022
This post was updated on November 25th, 2023

We all know the story of the original Trojan horse: during the Trojan War, Odysseus and the Greeks hide their soldiers in a wooden horse that’s left outside the enemy city of Troy.

The Trojans, thinking the horse is a gift, bring it inside the city walls. Once in position, the soldiers emerge from the horse, defeat the Trojan army, destroy the city, and end the war.

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It’s an inspiring story if you’re on the side of the Greeks.

But the modern version of the story is reenacted every day on our computers with Trojan horse malware. This time, cybercriminals play the role of the Greeks — and their victims are ordinary, unsuspecting people.

What Is a Trojan Horse?

Trojan horses, often abbreviated to Trojans, are pieces of malware (malicious software) that, like the original wooden horse, disguise themselves as desirable or benign files or programs.

In actuality, it’s all misdirection: the Trojan’s true purpose is to wreak havoc on you and your device.

How Trojan Horses Spread


Unlike viruses or worms, Trojans can’t replicate themselves. They require their victims to download them, either voluntarily or unintentionally, before they can start working.

This necessitates a clever disguise — and often, a bit of social engineering.

Trojans may come bundled with cracked software, luring you in with the promise of a free version of an expensive program. Sometimes, they even include a working copy of that program, making it all the more easy to distract you from the malicious portion of the download.

But even non-pirated programs, like free games, browser extensions, or screensavers, can actually be Trojans.

Some especially insidious Trojans appear to be antivirus programs or important security patches, urging you to act immediately due to a serious vulnerability. The only real threat, though, is the Trojan itself preying on your sense of caution.

Other Trojans spread through emails and IMs, often in the guise of an important attachment like an unpaid invoice or a personal letter. These messages may appear to come from people you know, increasing your willingness to open them.

And the stealthiest Trojans lurk on sketchy websites, then sneak onto your computer via a drive-by download — a download that initiates itself invisibly and automatically, without you needing to click on anything.

Types of Trojan Horses and What They Do

Once they enter your computer, Trojans are free to carry out their real attacks.

These attacks take on many forms depending on the exact variety of Trojans. Here are a few of the most common types.

Downloader Trojans

Some Trojans save the true carnage for later. Instead, they act as entry points for other malware, downloading infected programs that you would never have downloaded on your own.

These other malicious programs could be anything from more Trojans to adware to keyloggers to ransomware — the possibilities are limitless.

Other types of Trojans may also include a downloader component, causing their own damage while also paving the way for more malware to join in.

Backdoor Trojans

Backdoor Trojans create secret access points for attackers to take remote control of your computer. They often do this by taking advantage of security vulnerabilities in your OS or installed programs.

A backdoor lets the attacker see and modify your files, conduct malicious uploads or downloads, steal your personal data, and corrupt or erase your hard drive. Some attackers even use backdoors to conduct criminal activity using other people’s computers, setting the victim up as the “fall guy.”

DDoS Trojans

DDoS attacks

Many hackers conduct distributed denial-of-service (DDoS) attacks using computers that have been compromised by Trojans.

The Trojan turns the computer into a zombie whose only purpose is to bombard a target network or web server with traffic. As more computers become infected with the Trojan, they get sucked into this “botnet,” further increasing the traffic and eventually causing the target to error out and go offline.

This strategy allows the attacker to create a virtual army with which to carry out attacks against personal, political, or economic enemies.

Banker Trojans

Banker Trojans are designed to seek out financial information like bank statements, account numbers, credit card info, and electronic payment platform credentials.

They may scan your files for valuable data or spy on your browser activity to extract banking login info or credit card numbers.

The attacker can then use this stolen data to break into your accounts, deplete them of funds, and disappear without a trace.

Ransomware Trojans

Ransomware Trojans are among the most unsettling Trojans out there, and they’re becoming more and more common with the rise of cryptocurrency.

A ransomware Trojan encrypts some or all of your files and then demands that you pay a ransom to unlock them, often within a short timeframe of just a few days. It warns that if you don’t pay up, the encryption key will be deleted, and your files will be permanently unrecoverable.

The urgent nature of the demand makes many victims panic and pay up, sometimes giving the attacker thousands of dollars to get their files back.

Email or Mailfinder Trojans

Mailfinder Trojans collect email addresses from your address book and email programs.

In some cases, they sign all of these emails up for spam mailing lists or use them to create fake accounts on various websites.

But some email Trojans hijack your email app and assume your identity, then send all of your contacts malicious attachments or harmful messages. You become a puppet in the attacker’s game — and a vector through which they can find new victims.

3 Trojan Horse Examples

1. Fake Software

One common way that cybercriminals distribute Trojans is by disguising them as legitimate, popular software downloads. This could be something like a free word processor, a movie player, or even a security update. The user, hoping to acquire useful software or update their current one, inadvertently downloads the Trojan onto their computer.

The dangerous aspect of this situation is that the Trojan seems harmless. Unbeknownst to the user, they’ve actually downloaded harmful software. Once installed, the Trojan can operate in the background undetected, carrying out its malicious intent. The actions range from stealing sensitive data to allowing other harmful software to infiltrate the system.

Because of their deceptive nature, these disguised Trojans often slip past cybersecurity measures. It’s essential to be cautious while downloading software. Always prefer trusting and verified sources to prevent falling victim to such disguises.

2. Email Attachments

Email attachments are another common delivery method for Trojans. In this scenario, a hacker might pretend to be a trusted institution, such as your bank or a popular online store. They send an email with an attachment, often masquerading as a receipt, a statement, a coupon, or an important document.

When you download and open this seemingly harmless attachment, the Trojan is quietly installed on your computer. From there, it could start monitoring your activities, stealing your sensitive information, or even taking over your entire system without your knowledge.

It is critical to be skeptical of unexpected emails, especially those with attachments. Even if an email seems to come from an authentic source, double-check the sender’s email, look for spelling errors, or reach out to the organization directly if you have doubts. By maintaining this vigilance, you can avoid unintentionally inviting a Trojan into your system.

3. Links in Messages

Another sneaky way Trojans can infiltrate your computer is through links sent via instant messages or social networks. Here’s how it might work: You get a message from a friend who suggests you check out a fun game or watch a funny video through a link they’ve shared.

Trusting your friend, you click on the link. However, instead of taking you to the expected funny video or engaging game, the link triggers a Trojan download. Without even knowing it, you’ve now been given access to harmful software that runs in the background of your system, completing its harmful activities.

This urgent need for cautious clicking extends to social networks, too. Even if a friend has sent the link, they might not be aware it’s a trap. If a link looks suspicious or seems out of character for the sender, it’s better to ask them directly about it before clicking. In this digital age, vigilance can go a long way in maintaining your online security.

Key Trojan Horse Takeaways

A trojan horse
  • Trojan horses are a type of malware that disguise themselves as non-malicious downloads.
  • Many Trojans mask themselves as free pirated software, though they may also appear as legitimate apps or email attachments from trusted contacts.
  • Trojans can act as middlemen between your computer and other, more directly harmful malware.
  • Other Trojans conduct their own malicious activity, including stealing financial info, ransoming your files, turning your computer into a “zombie” in a botnet, or giving an attacker direct control over your computer.

History of Trojan Horses

The original Trojan Horse attack was conducted over 3,000 years ago. Back then, nobody could have predicted the new form this strategy would take in 1974 when the first computer Trojan made its debut.

The ANIMAL program, created by John Walker, was a 20-questions style game that guessed your favorite animal. And while it performed its stated function, it also secretly spread itself to other file directories, combining a Trojan-style package with the self-replicating mechanism of a virus.

But this relatively harmless nature wouldn’t last long. In the late ’80s and early ’90s, as the internet became more accessible, criminals began using Trojans to conduct truly malicious activities.

The 1989 AIDS Trojan arrived on 20,000 floppy disks, handed out at the World Health Organization’s AIDS conference by creator Joseph Popp. It claimed to contain information about AIDS but was actually designed to encrypt the user’s files and demand a payment of $189 to decrypt them.

In the 2000s, with personal computers and email accounts now commonplace, Trojans were hiding everywhere. The ILOVEYOU Trojan disguised itself as a love letter, while the Zlob Trojan purported to be a Microsoft ActiveX video codec.

Today, Trojans continue to insinuate themselves into virtually every new computing development, from filesharing to video streaming to cryptocurrency.

Trojan Horses by the Numbers

Famous Trojan Horse Attacks

The Emotet Trojan

Emotet first appeared in 2014, disguised as an email containing an invoice or payment confirmation. In reality, the attachment was a banking Trojan that used email-finding techniques to reach more victims.

Sporting a money transfer module and the ability to download other banking Trojans, Emotet quickly racked up millions of dollars in damages. It struck indiscriminately, hitting hospitals, city governments, schools, and ordinary citizens alike.

By the time the Trojan was finally subdued in 2021, it had infected over 1.5 million computers and cost its victims over $2.5 billion.

The Petya and NotPetya Trojans

First seen in 2016, the Petya Trojan encrypted the victim’s boot record, rendering the computer unusable and the files inaccessible. It then demanded a Bitcoin ransom to restore access to the machine.

But its successor, NotPetya, took the destruction to the next level in 2017. It specifically targeted agencies and companies in Ukraine, including banks, energy companies, airports, gas stations, transportation networks, and even the Chornobyl nuclear plant.

NotPetya demanded a relatively small ransom of $300 USD, but upon payment, many victims found that their systems were not actually restored. It’s thus believed that the Russian government was behind the attack, aiming not to make money but to damage Ukraine’s infrastructure.

All told Petya and NotPetya caused over $10 billion in damages worldwide, making it possibly the most destructive cyberattack in history.

Related Questions

1. How can I protect myself from Trojans?

The best way to protect from Trojans is to use updated antivirus software, avoid clicking on suspicious links or email attachments, and only download software from verified and trusted sources.

2. Can Trojans affect mobile phones too?

Yes, Trojans can also affect mobile phones. Just like on computers, they hide within legitimate-looking apps or come bundled with other software and can cause similar harm.

3. Can a Trojan virus be removed?

Yes, a Trojan virus can be removed using robust antivirus software. It’s always best to maintain your antivirus software with the latest updates to effectively detect and eliminate Trojans.

4. What harm does a Trojan virus cause?

A Trojan virus can cause various types of damage, including stealing personal information such as bank details or passwords, damaging the computer’s system software, or granting remote control access to the attacker.

5. Can a Trojan spread on its own like other viruses?

No, unlike other viruses, Trojans cannot replicate or spread on their own. They rely on the unsuspecting user to install them, usually by disguising themselves as something attractive or useful.

What Is a Trojan Horse and How Does It Work? (Video)

"Amateurs hack systems, professionals hack people."
-- Bruce Schneier, a renown computer security professional
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