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In computing, a trojan horse or trojan is a kind of malware that misleads users as to its true intent by disguising itself as a normal program. Trojans are generally spread by some form of social engineering. Although their payload can be anything, many modern forms act as a backdoor, contacting a controller who can then have unauthorized access to the affected device. Trojan horses are named after the Trojan Horse. Another early reference is in a US Air Force report in 1974 on the analysis of vulnerability in the Multics computer systems.

Capitalization

The computer term "Trojan horse" is derived from the legendary Trojan Horse of the ancient city of Troy. For this reason, "Trojan" is often capitalized, especially in older sources; however, many modern style guides and dictionaries suggest a lower-case "trojan" for this technical use. Examples of govware trojans include the Swiss MiniPanzer and MegaPanzer and the German "state trojan" nicknamed R2D2.

Due to the popularity of botnets among hackers and the availability of advertising services that permit authors to violate their users' privacy, trojans are becoming more common. According to a survey conducted by BitDefender from January to June 2009, "Trojan-type malware is on the rise, accounting for 83% of the global malware detected in the world."

Classifications of Trojan horses

Trojans are usually classified based on their primary function or intended impact. One major category is the banking trojan, which is designed to steal financial information such as online banking credentials, credit card numbers, or cryptocurrency wallet keys. Zeus and TrickBot are among the most studied examples of this type. Another category is the remote access trojan, (RAT), which allows attackers to gain full control over an infected system, enabling them to install additional software, access files, or monitor user activity.

Some trojans are primarily downloaders, meaning they serve as an initial infection stage by installing other forms of malware once inside a system. These often pave the way for ransomware, spyware, or botnet recruitment. Information-stealing trojans are also common, collecting sensitive data such as browser cookies, stored credentials, or documents without the user’s knowledge. Furthermore, fake antivirus trojans imitate legitimate security software, tricking users into paying for unnecessary or harmful services. Because trojans can be adapted for multiple purposes, many modern types have overlapping features, mixing elements of backdoors, spyware, and downloaders. This adaptability has been a major factor in their prevalence as one of the most common forms of malware.

Linux ls example

A trojan horse is a program that purports to perform some legitimate function, yet upon execution it compromises the user's security. Security software such as antivirus and anti-malware programs can help detect, quarantine, and remove trojans when kept up to date. Safe browsing practices—such as avoiding suspicious links, refraining from downloading software from untrusted sources, and exercising caution with email attachments—are also key to reducing risk. Additionally, maintaining regular software and operating system updates ensures that known vulnerabilities are patched, making it harder for trojans to exploit weaknesses. In professional environments, network firewalls, intrusion detection systems (IDS), and email filtering tools are commonly used to block suspicious traffic and prevent trojans from communicating with external servers. Regular security checks, employee training programs, and routine data backups further strengthen defenses and help reduce potential damage in the event of an infection.

Modern developments and detection techniques

Modern trojan horses have changed beyond traditional file-based malware, progressively using techniques designed to avoid signature-based security tools. An example of this approach is fileless malware, in which trojans operate mainly in system memory and rely on legitimate administrative tools like PowerShell or Windows Management Instrumentation (WMI). By lowering or removing malicious files on disk, fileless trojans are more difficult for conventional antivirus software to detect.

In recent years, trojans have expanded to mobile and embedded platforms. Android trojans are often distributed through unofficial application stores or phishing campaigns and may request more permissions that allow access to messages, stored credentials, or financial information. Trojans targeting Internet of Things (IoT) devices often take advantage of weak authentication practices or unpatched firmware, which in turn allows compromised devices to be incorporated into large botnets used for distributed denial-of-service (DDoS) attacks.

To counter these developments, modern security systems increasingly rely on behavior-based detection techniques. Rather than depending only on known malware signatures, behavioral analysis monitors system activity for signs such as unauthorized privilege escalation or suspicious network communication patterns. Alongside behavior-based detection, machine learning–based methods have been developed to classify and detect trojans by identifying irregularities within large-scale system and network telemetry data. While these approaches can help detection rates, they also present challenges related to false positives and the clarity of automated decision-making systems

Notable examples

There have been many well-known trojans that have played an important role in the history of cybersecurity. An early example is the AIDS trojan, developed in 1989. It is considered one of the first forms of ransomware, as it encrypted filenames on infected computers and demanded payment to restore them.

thumb|Full animation of the "You Are An Idiot" Trojan horse

A famous example is the You Are An Idiot trojan, developed in 2002. It originated from a comedy sketch by American radio personality Rick Dees on his 1984 album Put It Where the Moon Don’t Shine. Specifically, the vocal jingle was featured during a prank call segment titled "Candid Phone: Dog Funeral." The audio later gained notoriety in the early 2000s when it was sampled for the YouAreAnIdiot.org website, which functioned as a browser-based trojan. This site overwhelmed users by displaying flashing black-and-white smiley faces while continuously looping the vocal track. It became a legendary "pop-up bomb" because it would spawn six new windows every time a user attempted to close one and disabled standard hotkeys like Alt+F4. While it did not cause permanent damage to files, it frequently exhausted system resources and caused computers to freeze. Safe versions of the website were later created without the spawn feature.

Another famous example is the Zeus trojan, first identified in 2007. Zeus mainly targeted Microsoft Windows systems and was designed to steal banking credentials through man-in-the-browser attacks, which infected a user's browser to intercept and manipulate data, leading to widespread financial losses and data breaches. In 2015, in Ukraine, an attacker group, by means of an authenticated computer that was controlled remotely using a remote access trojan, gained access to a controller server, that resulted in an electricity outage for 80.000 people. In 2016, the MEMZ trojan, a Windows-based program, became widely recognized for its complex and destructive payloads. MEMZ gained notoriety for displaying unusual visual effects on infected machines and ultimately rendering the systems unusable.thumb|Map of countries touched by ANOM

Private and governmental

  • ANOM – FBI
  • 0zapftis / r2d2 StaatsTrojaner – DigiTask
  • FinFisher – Lench IT solutions / Gamma International
  • DaVinci / Galileo RCS – HackingTeam
  • Magic Lantern – FBI
  • SUNBURST – SVR/Cozy Bear (suspected)
  • TAO QUANTUM/FOXACID – NSA
  • WARRIOR PRIDE – GCHQ

Publicly available

  • EGABTR – late 1980s
  • Netbus – 1998 (published)

See also

References

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Notes