Malicious Codes

In Brief

Malware or malicious code is software that infects a user’s machine without their knowledge. The impact of infection can be multiple, such as the loss of confidentiality, integrity or availability of an entity’s assets. A malicious code infection is not always noticeable, sometimes it is even very difficult to detect.

The infection can take place by inciting the victim by methods of social engineering for example, to open a file annexed to an electronic mail, to open a file being on a removable media, or simply to motivate the victim to visit a malicious web page.

Malicious codes can have different impacts. They present a considerable threat and can, depending on the type, either exploit human or technical vulnerabilities.


A virus is a software, or a part of software which, in order to be able to spread, attaches itself to any type of file or other software with the aim of infecting the machine concerned, as well as others, without the knowledge of users.

A virus is generally activated as soon as a person opens the file (attached to an e-mail or located on a removable medium) or runs the software hosting the virus. As a result, the virus creator will try to trick the potential victim into opening the infected file by exploiting human vulnerabilities.

As soon as a virus has been launched, it will try two things:

  1. launch its payload, that is to say, its malicious functionality;

  2. try to spread using distribution channels such as the corporate network, e-mail or removable media.

It is, therefore, strongly advised to ignore attached files from people who are unknown to us or content in emails or on computer media that cause our distrust.


Unlike the virus, a worm does not need human intervention to infect a machine. It has an ‘engine’ (automation) which initially allows it to automatically deliver and execute its malicious code.

The Morris Worm named after its creator (Robert Morris) was one of the first worms (1988) to spread quickly on the Internet and to infect a large number of machines.

Trojan Horse

The term ‘Trojan horse’ appears in Greek mythology, especially in Homer’s Iliad which tells the story of the Greeks who decided to invade the city of Troy. Using a ruse to assault this well-protected city, they built a large wooden horse and sent it to Troy as a gift and a sign of peace. The people of Troy appreciated this gesture and took the horse into the city. During the night some Greek soldiers, sheltered inside the wooden horse, emerged from their hiding place and opened the gates of the city to the rest of the army, which seized it.

By analogy, a Trojan horse is spyware installed on a machine to open a back door to the attacker. Unlike a virus or a worm, a Trojan horse is not intended to reproduce or spread. But like viruses, Trojans hide in harmless files and often exploit human vulnerabilities to get launched.

In general, the Trojan is used to create and maintain permanent unauthorised access to a machine, when the latter is connected to the Internet. The number of ‘trojans’ is as impressive as the variety of actions they allow attackers on target computers.

Some simply open access to machine files, others allow real interaction with the infected machine from the Internet or a local network.


Spyware is a program designed to collect personal data on users without authorisation, and sending them to its designer, or a third party, via the Internet or any other computer network. The primary objective of spyware is to spy on the behaviour of the Internet user and to transmit the information collected to the creators and software editors to feed a gigantic database.

Spyware is usually ‘caught’ by browsing the Internet, as well as by downloading software.

There are different types of spyware:


Commercial spyware collects data and interacts visibly with their users, by managing the display of targeted advertising banners, by triggering the appearance of pop-ups, or even by modifying the content of the websites visited to add commercial links, for example. These are the most common spyware. Their existence is generally mentioned in the licence to use the software concerned, but often in an ambiguous manner and/or in a foreign language, which means that the user is not clearly informed.


Cookies also collect data about their users but do so with the utmost discretion. The monitoring and possible reuse of the data collected are done without the knowledge of the users, generally, for the purpose of collecting statistics, marketing information, debugging or technical maintenance, or even cyber-surveillance. The existence of these cookies is deliberately hidden from users.


Integrated (or internal) spyware is an executable program included in the source code of the software with its own function, to give it the possibility of collecting and transmitting information over the Internet. This spyware can be downloaded separately or are offered for installation at the same time as other free programs, themselves usually spyware, thanks to agreements between software publishers.


Outsourced spyware is a standalone application that dialogues with the main software associated with it, and whose sole function is to take care of the ‘customer relationship’: collection and transmission of information, display of advertising banners, etc. This spyware is designed by advertising agencies or specialised companies.

How to Protect Yourself?


  • It is essential to follow behavioural advice related to e-mail, advice on handling removable media and those concerning malicious websites. Follow good practices concerning malware.


The organisation must draft and enforce several sectoral policies:


The use of antivirus can help against infection by malicious codes.

Network segmentation using a firewall can protect against infection by malicious code type ‘worms’ and against data extraction by Trojans.

Updating the patches of all applications, as well as the operating system, can help fight against infection by malicious ‘worm’ codes.