What is a Trojan horse malware?
Definition
A Trojan horse is malicious software disguised as something useful. NIST defines it as a program that appears to have a useful function but also has a hidden, potentially malicious function that evades security mechanisms.
- Category
- Attacks
- Also called
- Trojan, Trojan malware, trojan virus
- Last updated
The name comes from the Greek story of the wooden horse: it looks like a gift and hides something harmful inside. A Trojan horse works the same way. NIST describes it as a program that appears to have a useful function, but also has a hidden and potentially malicious function that evades security mechanisms, sometimes by exploiting the legitimate permissions of the person who runs it.
How it works
You have to run it yourself, which is why the disguise matters. A Trojan might look like a free game, a cracked version of paid software, a PDF invoice, a browser extension or a "video player update". When you open it, the useful part may even work while the hidden part runs in the background.
What the hidden part does varies. It can log what you type, steal saved passwords, open a back door so someone else can control the device, or download more malware.
A typical case: you search for a free copy of a paid photo editor and run the installer from a random site. The editor opens, but a second program has also been installed that captures your browser logins.
Trojan vs. virus vs. worm
- A virus attaches itself to other files and spreads when those files are shared.
- A worm copies itself across networks without anyone running it.
- A Trojan doesn't spread by itself. It relies on you installing it because you trust what it looks like.
Why it matters
Trojans tend to go after the logins that open everything else, like your email. What they take gets used for fraud or identity theft, or sold on.
How to protect yourself
- 1
Install software only from official sources
Use your device's official app store or the developer's own site. Cracked software and "free" copies of paid tools are a common way to deliver Trojans.
- 2
Don't open unexpected attachments
If an invoice or document arrives that you weren't expecting, check with the sender through another channel before opening it.
- 3
Keep your system and security software updated
Updates fix the flaws malware leans on. Turn on automatic updates and keep your built-in or installed antivirus switched on.
- 4
Change your passwords from a clean device if you think you ran one
Disconnect the affected device, run a full scan, then change the passwords for your email and bank accounts from a different, trusted device and turn on two-factor authentication.
See where your personal data is exposed
Serus finds it on data brokers and people-search sites and removes it.
Frequently asked
Is a Trojan horse a virus?
Not exactly. A virus spreads by attaching itself to files. A Trojan doesn't spread by itself, it tricks you into installing it. Both are kinds of malware.
How do I know if I have a Trojan?
Warning signs include a device that runs unusually slowly, programs you didn't install, pop-ups, or accounts showing logins you don't recognize. A full scan with a reputable antivirus tool is the next step.
Can phones get Trojans?
Yes. Apps installed from outside the official stores are a common route, and malicious apps sometimes get through official stores too. Check an app's reviews and permissions before installing.
Sources
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