Incomputing, atrojan horse (or simplytrojan;[1] often capitalized,[2] but see below) is a kind ofmalware that misleads users as to its true intent by disguising itself as a normal program.
Trojans are generally spread by some form ofsocial engineering. For example, a user may be duped into executing anemail attachment disguised to appear innocuous (e.g., a routine form to be filled in), or into clicking on a fake advertisement on theInternet. Although their payload can be anything, many modern forms act as abackdoor, contacting a controller who can then have unauthorized access to the affected device.[3]Ransomware attacks are often carried out using a trojan.
Unlikecomputer viruses andworms, trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.[4]
It is unclear where and when the computing concept, and this term for it, originated; but by 1971 the firstUnix manual assumed its readers knew both.[5]
Another early reference is in a US Air Force report in 1974 on the analysis of vulnerability in theMultics computer systems.[6]
The term "Trojan horse" was popularized byKen Thompson in his 1983Turing Award acceptance lecture "Reflections on Trusting Trust",[7] subtitled: "To what extent should one trust a statement that a program is free of Trojan horses? Perhaps it is more important to trust the people who wrote the software." He mentioned that he knew about the possible existence of trojans from a report on the security of Multics.[8][9]
The computer term "Trojan horse" is derived from the legendaryTrojan Horse of the ancient city ofTroy. For this reason "Trojan" is often capitalized, especially in older sources. However, many modernstyle guides[10] and dictionaries[1] suggest a lower-case "trojan" for this technical use.
Once installed, trojans may perform a range of malicious actions. Many tend to contact one or moreCommand and Control (C2) servers across the Internet and await instruction. Since individual trojans typically use a specific set of ports for this communication, it can be relatively simple to detect them. Moreover, other malware could potentially "take over" the trojan, using it as a proxy for malicious action.[11]
In German-speaking countries,spyware used or made by the government is sometimes calledgovware. Govware is typically used to intercept communications from the target device. Some countries like Switzerland and Germany have a legal framework governing the use of such software.[12][13] Examples of govware trojans include the SwissMiniPanzer and MegaPanzer[14] and theGerman "state trojan" nicknamed R2D2.[12] German govware works by exploiting security gaps unknown to the general public and accessing smartphone data before it becomes encrypted via other applications.[15]
Due to the popularity ofbotnets 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 byBitDefender from January to June 2009, "Trojan-type malware is on the rise, accounting for 83% of the global malware detected in the world."[16] BitDefender has stated that approximately 15% of computers are members of a botnet, usually recruited by a trojan infection.[17]
Recent investigations have revealed that the trojan-horse method has been used as an attack oncloud computing systems. A trojan attack on cloud systems tries to insert an application or service into the system that can impact the cloud services by changing or stopping the functionalities. When the cloud system identifies the attacks as legitimate, the service or application is performed which can damage and infect the cloud system.[18]
Trojans are usually classified based on their primary function or intended impact. One major category is thebanking trojan, which is designed to steal financial information such as online banking credentials, credit card numbers, or cryptocurrency wallet keys.[19]Zeus andTrickBot are among the most studied examples of this type. Another category is theremote 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.[20]
Some trojans are primarilydownloaders, meaning they serve as an initial infection stage by installing other forms of malware once inside a system. These often pave the way forransomware,spyware, orbotnet recruitment.[21] 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.[22]
Because trojans can be adapted for multiple purposes, many modern types have overlapping features, mixing elements ofbackdoors, spyware, and downloaders.[23] This adaptability has been a major factor in their prevalence as one of the most common forms of malware.
A trojan horse is aprogram that purports to perform some legitimate function, yet upon execution it compromises the user's security.[24] One simple example[25] is the following malicious version of the Linuxls command. An attacker would place this executable script in a publicly writable and "high-traffic" location (e.g.,/tmp/ls). Then, any victim who tried to runls from that directory —if and only if the victim's executable searchPATH unwisely[25] included the current directory. — would execute/tmp/ls instead of/usr/bin/ls, and have their home directory deleted.
#!/usr/bin/env bashrm-rf~2>/dev/null# Remove the user's home directory, then remove self.rm$0
Similar scripts could hijack other common commands; for example, a script purporting to be thesudo command (which prompts for the user's password) could instead mail that password to the attacker.[24]
In these examples, the malicious program imitates the name of a well-known useful program, rather than pretending to be a novel and unfamiliar (but harmless) program. As such, these examples also resembletyposquatting andsupply chain attacks.
Preventing trojan infections requires a mix of user awareness, technical safeguards, and proactive security practices. Educating users about the dangers of downloading unverified files or executing unknown programs remains an effective way to prevent attacks.[26] Security software such as antivirus and anti-malware programs can help detect, quarantine, and remove trojans when kept up to date.[27]
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.[28] Additionally, maintaining regular software and operating system updates ensures that known vulnerabilities are patched, making it harder for trojans to exploit weaknesses.[29]
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.[30]
There have been many well-known trojans that have played an important role in the history of cybersecurity. An early example is theAIDS 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.[31] Another famous example is theZeus trojan, first identified in 2007. Zeus mainly targeted Microsoft Windows systems and was designed to steal banking credentials throughman-in-the-browser attacks, which infected a user's browser to intercept and manipulate data, leading to widespread financial losses and data breaches.[32]
In 2016, theMEMZ 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.
Privacy-invasive software – Malware that collects and transmits user information without their knowledgePages displaying short descriptions of redirect targets
^Thompson, Ken; Ritchie, Dennis M."Unix Programmer's Manual, November 3, 1971"(PDF). p. 5. RetrievedMarch 28, 2020.Also, one may not change the owner of a file with the set—user—ID bit on, otherwise one could create Trojan Horses able to misuse other's files.
^Paul A. Karger; Roger R. Schell (June 1974)."Multics Security Evaluation: Vulnerability Analysis, ESD-TR-74-193"(PDF).HQ Electronic Systems Division: Hanscom AFB, MA.2. 4.2.1. Archived fromthe original(PDF) on July 9, 2011. RetrievedDecember 24, 2017....some sort of protection from user written applications programs that may contain "Trojan Horses" [sic]
^Karger and Schell wrote that Thompson added this reference in a later version of his Turing conference speech:Ken Thompson (November 1989), "On Trusting Trust.",Unix Review,7 (11):70–74
^Tim Grance; Karen Kent (November 2008)."Guide to Malware Incident Prevention and Handling"(PDF). NIST Special Publication 800-83 Revision 1. National Institute of Standards and Technology. RetrievedOctober 5, 2025.
