Malicious software: best practice

In brief

Malicious software represents one of the greatest threats to the whole IT system, regardless of its size. All types of operating systems are at risk, and all administrations, municipalities and companies can succumb to it.

Since they first came into existence, cybercriminals have become significantly more sophisticated in the way they work and target their victims; malicious software or malware, one of their main tools, has naturally benefited from this. Nowadays, you’ll no longer find malicious software written for purely entertainment purposes. Currently, the most widely used malicious codes on the market are variations on the Trojan horse, which grant access to devices so they can be used either for illicit purposes or to steal confidential information.

Today, these malicious codes are primarily used to:

They present a large and omnipresent threat. Without preventative measures, protective measures and curative measures, an entity risks being considerably impacted.

Infection vectors

There are a number of ways to infect a computer. The most commonly used include:

  • infecting genuine websites (preferably those with high visitor counts). These suffer from opportunistic or targeted attacks. The attacker will take advantage of the website’s technical vulnerabilities, often the result of bad management, by trying to access the website management system either by exploiting default passwords or as a result of weak security. They then install tools so they can exploit technical vulnerabilities in the visitors’ browsers, or incite users to install the malicious codes themselves (e.g. fake antiviruses or fake video players).
  • malicious websites specifically designed to exploit the technical vulnerabilities of their visitors’ web browsers. As the visitors aren’t familiar with these websites, the attacker has to draw them in. To do so, he can:
    • lure victims by sending them Spam containing links;
    • lure victims through phishing-type attacks;
    • lure victims through adverts posted on frequently visited websites.
  • emails with infected attachments. This technique is used for both targeted and opportunistic attacks. Social engineering methods are used to manipulate the victims into opening the infected files.
  • removable devices containing infected files. Malicious codes exploit the technical vulnerabilities of the targeted computers. This method is primarily used for targeted attacks, using social engineering techniques.


As there are malicious codes all over the Internet, the chances of encountering one are extremely high (EBIOS threats, remote listening, software trap).

The impacts caused are usually:

  • Financial
    • Access to online banking may result in a loss of money. Pay particular attention to multi-line applications, as transactions are frequently made from company accounts and procedures often aren’t noticed until days, or even weeks, afterwards.
    • Loss of confidential information (intellectual property theft, theft of trade secrets, theft of strategic company data, theft of customer information) often results in a loss of money.
  • Legal
    • If the infected computers are used for illicit purposes, the victim may face complaints (denial of service attacks, infection of visitor devices, hosting of illegal content, etc.).
    • The loss of confidential information, notably including data of a personal nature, may result in complaints received from the victims or from the CNPD.
  • Impact on reputation
    • The use of an organisation’s website to infect visitors’ devices may result in serious problems for its brand image. Just like any type of trade, e-commence thrives on its consumers’ trust. A security incident may negatively impact this trust.
  • Impact on “knowledge”
    • The theft or destruction of data relating to customer data or to trade secrets may result in a lack of knowledge.
  • Impact on “time”
    • Analysing the compromised system, as well as getting the infected machines up and running again, may result in a significant loss of time.

Behavioural measures

Whether the attack is targeted or opportunistic, a large number of malicious codes infiltrate their victims’ devices via infection vectors that play on human vulnerabilities.

Before publishing malicious software, its authors check to see if it can be detected via websites such as

The codes then often remain invisible to antivirus software for at least the first few days of the attack, which is enough time for the antivirus to collect the updated signatures. 

It is therefore vital for any organisation to train their employees and make them aware of the risks incurred by malicious codes and of commonly used infection vectors.

Preventing infection

Infection with malicious codes can be prevented through responsible and careful behaviour when using email, removable devices and when surfing the Internet.

Make sure you:

  • observe best practice relating to the use of email
  • avoid the use of unknown removable devices.
    • If you find any such devices, send them to your IT department for analysis. 
    • If you are given removable devices at conferences, have them analysed by experts before you use them.
    • Be very careful if anyone sends you a removable device. If you do not know the person who sent it to you, don’t use it. If it is someone you know, check with them to make sure they were the ones who sent it and remember to get the devices analysed by experts before you use them.

When surfing the Internet: 

  • only open links on websites you trust entirely. Otherwise, you might be taken to a malicious website.
  • pay attention to warnings from your web browser (“safe browsing” on IE, Firefox, Chrome), or add-ons such as WOT.
  • be wary of adverts that appear on websites that are not completely trustworthy. They may contain malicious codes or take you to malicious websites.

When travelling or on business trips, make sure you: 

  • keep an eye on your IT equipment. Ill-intentioned persons could try to ambush it or install malicious codes. (Threats: see equipment traps, software traps). 

You should also remember that even equipment located within the organisation can be easily infected if ill-intentioned people gain access to it. (SMEs see: Infiltrating the premises and Insertion or removal of hardware and Use of unapproved software).

Minimise the impacts

Each employee should know how to act in case of infection:  

  • the infected device must be isolated and removed from the network;
  • it must not be used again until the virus has been removed;
  • it must be reported to the managers responsible for this type of incident immediately.

Organisational practices

To prevent infection by malicious codes, organisational measures must be implemented. Draft and enforce the following sectoral policies:

The installation of the following technical measures is recommended:

  • antivirus
    Antiviruses can detect a large number of malicious codes and prevent infection. Each computer within your organisation, as well as the file servers and email servers should have an antivirus installed. If possible, use a different antivirus for computers and for servers to increase the likelihood of malicious codes being detected. Update these antiviruses on a regular basis. Make sure users cannot deactivate the antivirus.
  • firewall
    A firewall set up on a device can both prevent the device being infected as a result of technical vulnerabilities and help detect a possible infection.
  • network segmentation
    A ‘company firewall’ may help prevent a virus from spreading throughout the whole company network. It can also prevent the exploitation of certain technical vulnerabilities thanks to the filtering of connection attempts via specific ports.
  • Web filter
    Web filters can prevent computer infections by blocking malicious websites or those with a bad reputation.
  • patches
    Some malicious software tries to exploit technical vulnerabilities. These vulnerabilities can be corrected by patches. However, no system is ever really safe from technical vulnerabilities.
  • backups
    Backups are an effective way of preventing loss of availability and integrity. Unfortunately, they cannot prevent loss of confidentiality. Backups can also help to reduce the impact caused by the system being compromised by a destructive malicious code.
  • encryption
    Encryption can help reduce the likelihood of loss of confidentiality for highly confidential information in the event of infection.

Virus removal

See: SOS – I think my computer is infected