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A “Trojan virus” is the common name for Trojan horse malware: software that pretends to be legitimate or useful so someone will install it. Unlike a virus or worm, a Trojan does not spread on its own. Once installed, it may steal information, spy on activity, install more malware, enable fraud, or let an attacker control the device—but no one behavior is guaranteed in every case.
What is a Trojan virus?
Technically, a Trojan is a type of malware, not a self-replicating virus. It relies on deception: it may be disguised as an ordinary app, file, or other useful software. Microsoft explains that Trojans “can’t spread on their own”; they must be downloaded manually or delivered and installed by another malware program. See Microsoft’s definition of Trojans.
A Trojan can carry or install other malware, including a virus, but that does not make self-replication a defining feature of the Trojan itself. Microsoft also describes Trojans as malware that attempts to appear harmless and may steal personal information, download other malware, or provide an attacker with access to a device: Microsoft’s Trojan overview.
What can a Trojan do after installation?
The effects depend on the specific malware. Possible outcomes include:
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- Stealing credentials or personal information: It may capture passwords, sign-in details, or other data and transmit them to an attacker.
- Monitoring activity: Some Trojans can record keystrokes or browsing activity.
- Installing more malware: A Trojan may download additional malicious software onto the device.
- Enabling fraud or unauthorized access: It may support fraudulent activity or give an attacker control of the device.
These are possible capabilities, not a checklist that applies to every Trojan. The disguise is the common thread: a person is persuaded to install software that does something other than, or in addition to, what it appears to do.
How do I know if I have a Trojan?
A slow computer, pop-ups, or another isolated problem does not establish that a Trojan is present. Such symptoms do not provide a reliable diagnosis by themselves. If you suspect malware, use an up-to-date security tool and follow its official instructions rather than trying to identify a Trojan from one symptom.
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For Windows, Microsoft points users to Defender Antivirus and Microsoft Safety Scanner as detection and removal resources. Start with the current guidance at Microsoft’s Defender Antivirus help page and Microsoft Safety Scanner. A scan is a useful step, but no single scan can be promised to find or remove every threat.
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How do I remove a Trojan virus?
- Use a trusted device for sensitive accounts. If malware may be present, avoid entering passwords on the suspected computer. From a different, trusted device, change passwords for accounts that could be exposed. The FTC gives this advice in its malware guidance.
- Run a current security scan. On Windows, use Defender Antivirus or Microsoft Safety Scanner and follow the tool’s current instructions. If a tool reports a threat, follow its recommended removal or quarantine steps.
- Check important accounts. From the trusted device, review account activity and use each service’s official account-security steps if you find changes you did not make.
- Use recovery guidance if scanning is not enough. If security software cannot remove malicious files, wiping or reformatting the device and reinstalling its operating system may be an option. This can erase files and installed programs. Consult current device-maker or IT guidance before taking that step.
How to reduce the risk of installing a Trojan
- Get apps from trusted sources. Prefer official app stores or the software maker’s own site, and be cautious about unexpected download links.
- Check what you are installing. Read the app name, publisher, and requested permissions before proceeding. Stop if the source or purpose does not make sense.
- Keep security protection updated. Microsoft recommends regularly updated antivirus protection and identifies Defender Antivirus as built into modern Windows versions. The FTC also recommends reputable security software configured to update automatically.
- Keep recoverable backups. Maintain backups that are separate from the device, so they remain useful if the computer is compromised. Scan files before restoring them, as recommended in the NIST computer security incident-handling guide.
What the often-cited 62% statistic means
Microsoft reported that, during the historical period from January 2005 through February 2006, a backdoor Trojan was present on 62% of the 5.7 million unique computers from which its Windows Malicious Software Removal Tool removed malware. That figure comes from a particular Windows tool’s selected population and an old time period; it is not a current estimate of Trojan prevalence among all computers or people. The report is Microsoft’s 2006 MSRT report.
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