What is a Trojan (Trojan horse malware)?
Definition
A Trojan horse is a program that looks useful but has a hidden, malicious function, so it does harm once you install and run it.
- Category
- Attacks
- Also called
- trojan, trojan virus, malware trojan
- Last updated
NIST defines a Trojan horse as a computer 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 authorizations of whoever runs it. A second NIST definition puts it as a useful or seemingly useful program that contains hidden malicious code, which executes when the program is invoked.
The name comes from the Greek story: the gift looks harmless, and the danger is inside.
What that looks like in practice
You download a free PDF converter, a game crack or a "video player update" from a site that isn't the official source. It does what it promises, or seems to. Behind the scenes, the hidden part runs with your permissions. Depending on the code, that could mean copying files, watching what you type or giving someone else access to your device.
It works because you hand it the access yourself. You run the program, so it runs as you.
Trojan vs. virus vs. worm
- A Trojan needs you to run it, because it pretends to be something else.
- A virus or worm is malware built to copy or spread itself. A Trojan on its own doesn't have to do that.
Malware is the umbrella term for all of them.
Why it matters
Once a Trojan runs on your phone or computer, anything on it can be at risk: saved passwords, messages, documents, photos. That data can be used for account takeover or identity theft, and it often ends up in the hands of people who sell or share it.
How to protect yourself
- 1
Install software only from official sources
Use the developer's own site or your device's official app store, and skip cracked software and "required update" pop-ups.
- 2
Think before you open attachments
Unexpected invoices, shipping notices and "voice message" files are a common way to get you to run a program. Check with the sender through another channel.
- 3
Keep your system and apps updated
Turn on automatic updates for your operating system, browser and security software.
- 4
Use a normal account for daily work
A program that runs as you has your permissions. If your everyday account isn't an administrator, a Trojan can do less.
- 5
Change passwords from a clean device if you were hit
If you ran something suspicious, remove it, then change your important passwords from a device you trust and turn on two-step verification. Serus monitors breach databases and the dark web and alerts you if your email or passwords show up.
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 a virus?
Not exactly. A virus is malware that copies itself into other files. A Trojan is defined by disguise: it looks like a useful program but hides a malicious function. People often use the words loosely.
How do Trojans get onto a device?
You run them, believing they're something else. That might be a download from an unofficial site or a file attached to a message. The program itself is what's hidden.
Can my phone get a Trojan?
Any device that runs programs can. Stick to your device's official app store and keep it updated.
Sources
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