Trojan
A Trojan, also known as a Trojan horse, is a type of malware that masquerades as legitimate programs or files to infect a computer or mobile device. Unlike viruses, Trojans do not replicate themselves but can open doors to other malicious activities such as data theft, additional malware installation, or remote device control.
What is a Trojan?
Trojan Definition
Trojan, also known as a Trojan horse, is a type of malware that masquerades as legitimate programs or files to infect a computer or mobile device. Unlike viruses, Trojans do not replicate themselves but can open doors to other malicious activities such as data theft, additional malware installation, or remote device control.
How Does a Trojan Work?
A Trojan works by hiding its malicious code in legitimate-looking applications or files. After installation on a device, a Trojan can perform various malicious actions, such as:
- Data Theft: Capturing login credentials, credit card numbers, and other confidential information.
- Remote Control: Allowing attackers remote access and control of the infected device.
- Additional Malware Installation: Downloading and installing other malicious programs.
- Spying: Monitoring user activity, including keystrokes and network traffic.
- Resource Exploitation: Using the infected device to conduct DDoS attacks or mine cryptocurrency.
Types of Trojans
- Trojan-Downloader: Downloads and installs other malware.
- Trojan-Dropper: Hides malware in legitimate files and installs it on the device.
- Trojan-Banker: Steals user banking data.
- Trojan-Spy: Monitors user activity and transmits collected information to the attacker.
- Trojan-Ransom: Encrypts user data and demands ransom for decryption.
- Trojan-Backdoor: Creates a backdoor enabling remote access to the infected device.
Trojan Infection Methods
- Downloading from unknown sources: Installing software from unknown or untrusted websites.
- Email attachments: Opening attachments in emails from unknown senders.
- Fake updates: Installing fake software updates.
- Advertisements and pop-ups: Clicking on suspicious advertisements and pop-up windows.
- Social media: Clicking on malicious links shared on social platforms.
Threats Associated with Trojans
- Identity Theft: Gaining access to personal and financial data.
- Data Loss: Encryption or deletion of user files.
- Remote Control: Attacker takeover of the device.
- Espionage: Monitoring user activity without their knowledge.
- Resource Exploitation: Using system resources to conduct attacks or mine cryptocurrency.
How to Detect a Trojan?
- System Slowdown: Unusual slowdown of computer or mobile device operation.
- Unknown Processes: Presence of suspicious processes in task manager.
- Pop-ups: Appearance of unexpected advertisements and pop-ups.
- Settings Changes: Unintended changes in system or browser settings.
- Unknown Programs: Appearance of unknown applications or files.
Trojan Removal Methods
- Antivirus Software: Using up-to-date antivirus software to scan and remove the Trojan.
- Safe Mode: Starting the computer in safe mode and performing a scan.
- Manual Removal: Identification and removal of infected files and registry entries (recommended for advanced users).
- System Restore: Restoring the system to a state before infection.
- System Reinstallation: In extreme cases, reinstalling the operating system.
Protection Against Trojans
- Software Updates: Regularly updating the operating system and applications.
- Antivirus Software: Installing and regularly updating antivirus programs.
- Safe Browsing: Avoiding suspicious websites and links.
- Download Caution: Downloading software only from trusted sources.
- Attachment Scanning: Scanning email attachments before opening.
Trojans on Mobile Devices
- Fake Applications: Installing malicious applications from unknown sources.
- In-app Advertisements: Clicking on malicious advertisements in free applications.
- SMS and MMS: Opening links in suspicious SMS and MMS messages.
- Rooting and Jailbreaking: Increased infection risk through rooting or jailbreaking the device.
Differences Between Trojans and Other Types of Malware
- Trojan vs. Virus: Viruses replicate themselves, while Trojans don’t have this capability.
- Trojan vs. Worm: Worms spread independently through networks, while Trojans require user interaction for infection.
- Trojan vs. Ransomware: Ransomware encrypts data and demands ransom, while Trojans can have various goals including data theft and remote control.
Related Terms
- Malware - broader category of malicious software
- Ransomware - Trojan-Ransom type trojans
- Phishing - main trojan distribution vector
- EDR - detecting and responding to trojans
Explore Our Services
Want to protect your organization against trojans? Check out:
- SOC 24/7 - detecting and blocking trojans
- Incident Response - infection removal
- Security Awareness Training - threat recognition
Trojans are a serious threat to computer and mobile device security. It’s important to be aware of how Trojans work and apply appropriate protection measures to prevent infections and minimize risk.
Frequently asked questions
+ What is a trojan in simple terms?
A trojan (or trojan horse) is malware that disguises itself as legitimate software to trick users into installing or running it. Unlike viruses, trojans don't self-replicate — they rely on social engineering: phishing emails with malicious attachments, fake software downloads, infected installers, malicious browser extensions, drive-by downloads from compromised websites. Once executed, a trojan can do almost anything the user can: steal credentials, install additional malware (including ransomware), open backdoors for remote control, log keystrokes, exfiltrate data, hijack the device for botnet activity. The name comes from the wooden horse the Greeks used to enter Troy — looked harmless, hid attackers.
+ What are the main types of trojans?
Eight common categories: (1) **Backdoor trojan** — opens hidden access for the attacker, (2) **RAT (Remote Access Trojan)** — full remote control of the device (DarkComet, NjRAT, AsyncRAT), (3) **Banking trojan** — steals online banking credentials and intercepts transactions (Emotet, TrickBot, Qakbot, IcedID, Zeus historically), (4) **Downloader / dropper** — small initial trojan that downloads and installs further malware, (5) **Infostealer** — exfiltrates passwords, browser data, crypto wallets (RedLine, Lumma, Vidar — exploding in 2024-2026), (6) **Keylogger trojan** — records keystrokes, (7) **DDoS trojan** — turns the device into a botnet client, (8) **Ransomware loader** — drops ransomware payload (often Emotet → TrickBot → Conti chains).
+ What are famous trojan examples?
Five high-impact: (1) **Emotet (2014-2025)** — banking trojan turned into a malware-as-a-service distribution platform; multiple law enforcement disruptions (Jan 2021) and resurgences, (2) **TrickBot (2016-2023)** — banking trojan, became ransomware loader for Conti; sanctioned and partially disrupted in 2022-2023, (3) **Qakbot (2007-2023)** — multi-decade banking trojan, dismantled by FBI Operation Duck Hunt in August 2023, (4) **IcedID** — emerged as Emotet replacement, used for ransomware delivery, (5) **RedLine Stealer (2020-)** — infostealer sold as Malware-as-a-Service, responsible for massive credential leaks; primary ammunition for stuffer attacks in 2024-2026. Modern trend: trojans are no longer about visible damage — they steal credentials silently, then sell access on dark markets to ransomware operators.
+ How do trojans infect computers?
Six common vectors: (1) **Phishing emails** — malicious attachments (Office macros, ISO/LNK files, ZIP, PDF), still the leading vector in 2026, (2) **Drive-by downloads** — compromised or malicious websites exploiting browser vulnerabilities, (3) **Malicious advertising (malvertising)** — ads on legitimate sites redirecting to exploit kits, (4) **Cracked / pirated software** — game cracks, key generators, pirated commercial software bundle trojans, (5) **Software supply chain** — Trojanised installers, poisoned updates (3CX 2023, MOVEit 2023), (6) **Malicious browser extensions** — fake productivity tools or privacy extensions exfiltrating data. AI-generated phishing has substantially raised infection rates in 2024-2026 by eliminating language flaws.
+ How to detect trojans on a computer?
Symptoms of trojan infection: (1) Slow performance, frequent crashes, unexpected reboots, (2) Antivirus disabled or unable to update, (3) Unfamiliar processes in Task Manager (especially with random names), (4) Unexpected network traffic to unknown servers, (5) Browser homepage / search engine changed without consent, (6) Unfamiliar programs in startup, (7) Mouse cursor moves on its own (RAT in use), (8) Banking transactions you didn't authorise. Detection methods: (1) **Modern EDR/XDR** — behavioural analysis catches what signature AV misses, (2) **Sandbox analysis** — submit suspicious files to VirusTotal, Hybrid Analysis, Joe Sandbox, (3) **Memory analysis** — Volatility, Rekall for fileless trojans, (4) **Network monitoring** — Zeek/Suricata flag C2 communication. For enterprise: 24/7 SOC + EDR is the gold standard.
+ How to remove a trojan?
Stepwise approach: (1) **Disconnect from network** to prevent further data exfiltration or lateral movement, (2) **Reboot in Safe Mode** to limit running processes, (3) **Run a full EDR or AV scan** with up-to-date definitions, (4) **Check startup items** (msconfig, Task Scheduler, Services) and remove unfamiliar entries, (5) **Reset browsers** — extensions, homepage, search engine, (6) **Change passwords** for every account accessed from the device — from a clean device, (7) **Monitor financial accounts** for fraudulent activity. **For confirmed serious infection**: complete OS reinstall is the safest option — modern advanced trojans (rootkits, UEFI implants) survive even disk reformat. Enterprise: invoke incident response, image the device for forensics, then re-image clean.
+ How to prevent trojan infections?
Eight controls: (1) **Modern endpoint protection** (NGAV + EDR) on every device — CrowdStrike, Microsoft Defender, SentinelOne, Sophos, (2) **Aggressive patching** — most trojans exploit known CVEs, (3) **Email security gateway** with sandbox detonation and impersonation detection, (4) **Restrict Office macros** — block from internet by default (Microsoft default since 2022), (5) **Application allow-listing** — Windows Defender Application Control, AppLocker, (6) **MFA on every account** — limits damage from stolen credentials, (7) **User awareness training** with phishing simulations and quarterly phishing tests, (8) **Browser hardening** — modern browsers (Edge, Chrome, Firefox) with Site Isolation, Enhanced Safe Browsing, ad blockers (uBlock Origin) reduce drive-by risk. Network-level: DNS filtering (Cloudflare 1.1.1.1 for Families, Quad9, NextDNS, Cisco Umbrella) blocks known C2 domains.