Macro viruses add their code to the macros associated with documents, spreadsheets and other data files. The first macro virus, called Concept, appeared in July 1995 and macro viruses (mostly infecting Word documents) subsequently became the dominant type of virus until the turn of the century, when Microsoft disabled macros by default in Office (versions since Office 2000): since then, cybercriminals have had to try and trick their victims into enabling macros before their infected macro is able to run.
Macro viruses are most commonly found embedded in documents or inserted as malicious code into word-processing programs. They may come from documents attached to emails, or the code may be downloaded after clicking on "phishing" links in banner ads or URLs. They are difficult to detect, as they do not operate until an infected macro is run, at which time they perform a series of commands. A macro virus is similar to a Trojan virus, since it may appear benign and users may not immediately notice any ill effects. Unlike Trojans, however, macro viruses can replicate themselves and infect other computers.
The main risk of macro viruses is their ability to spread quickly. Once an infected macro is run, all other documents on a user's computer become infected. Some of these viruses cause abnormalities in text documents, such as missing or inserted words, while others access email accounts and send out copies of infected files to all of a user's contacts, who in turn open and access these files because they come from a trusted source.
These viruses can also be designed to erase or compromise stored data. In addition, it's important to note that macro viruses are cross-platform; they can infect both Windows and Mac computers using the same code. Any program that uses macros can operate as a host, and any copy of an infected program — sent via email, stored on disk or on a USB drive — will contain the virus.
To remove these viruses, users should rely on security software that provides specific macro virus detection and removal tools. Regular scans will clean any infected documents and ensure no new computer viruses are downloaded.
Macro viruses can be found in several different forms. Although some people consider them to be a relic of the late 1990s, they have in fact made a resurgence in recent years, forcing users to be extra vigilant.