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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →A Trojan, or Trojan horse, is malware disguised as a harmless or useful file or app. It relies on someone being tricked into running or installing it; once active, it may steal information, download other malware, or give an attacker access to the device. Unlike a virus or worm, a Trojan does not spread by itself.
What makes malware a Trojan?
The defining feature is deception: the program presents itself as something legitimate while concealing a malicious purpose. The NICCS Glossary defines a Trojan as “a type of malware that conceals its true content to fool a user into thinking it’s a harmless file.” Its related Trojan horse entry describes a program that appears useful but has a hidden, potentially malicious function.
A Trojan might be packaged as an ordinary-looking download or imitate a real application, even using the same file name. The name refers to the disguise, not to one specific kind of damage.
What can a Trojan do?
There is no single Trojan payload. Depending on the program, it may download additional malware, facilitate fraud, record keystrokes or websites visited, send passwords or browsing history to an attacker, or let an attacker control the device. These are possible behaviors, not a list of actions every Trojan performs.
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A Trojan is therefore a category defined by how malware is presented and executed, rather than a synonym for every type of malicious software. It does not necessarily open a backdoor or carry out every behavior listed above.
How is a Trojan different from a virus or worm?
A Trojan depends on a user being deceived into downloading or running it. Microsoft distinguishes it from viruses and worms by noting that a Trojan cannot spread on its own; it must be downloaded or delivered by another malware program. Viruses and worms are commonly distinguished by their ability to replicate or spread.
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How can you avoid installing one?
- Be cautious with unexpected downloads, attachments, and software offered by unfamiliar or untrusted sites.
- Keep your operating system, browser, and security tools current.
- Do not trust an unusual message claiming that a browser needs an update. Microsoft recommends opening the browser’s own settings and checking its Help or About page for updates instead.
- Use security software from a trusted source. An unsolicited warning that urges you to download a cleanup utility is not proof that the utility is safe.
What should you do if you suspect a Trojan?
If the affected device belongs to an employer, school, or other organization, contact its IT support and follow its instructions. For a personal Windows computer, Microsoft names Defender Antivirus and Microsoft Safety Scanner as tools for detecting and removing malware. Update Defender’s security intelligence and run a full scan; Microsoft Safety Scanner is another option. A scan cannot guarantee that every infection has been removed.
CISA’s general recovery guidance recommends disconnecting a suspected infected computer from the internet and using a trusted environment for scanning. It also warns that backed-up files may still be infected. If you need to restore files, scan them using trusted, up-to-date security software before opening them. Follow current support instructions from your organization or device platform when available, and obtain any security software directly from a trusted vendor rather than from an alert or unfamiliar download site.
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Sources
- NICCS Glossary, “Trojan” and “Trojan horse” entries.
- Microsoft Learn, “Trojan malware – Microsoft Defender for Endpoint” (updated October 29, 2024).
- Microsoft Support, “Protect your PC from unwanted software.”
- CISA, “Recovering from a Trojan Horse or Virus.”
- Microsoft Learn, “How Microsoft identifies malware and potentially unwanted applications.”
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