A virus signature (also known as a virus definition) is a file or multiple files that are downloaded by a security program to identify a computer virus.
Researchers at antivirus firms study viruses found commonly on the internet, often testing their behavior in a sandbox-type set up. The sandbox is generally a virtual machine-based environment. The behavior, file names used and unique strings of data in the files are all analyzed.
Commercial antivirus utilities have long depended on virus definitions. Security software frequently downloads the files to keep the software able to detect current threats. The files are used to update criteria for detection of files in hard drive scans and checks on running processes. The main weakness of virus signatures is in detection of new viruses.
Another method of virus detection is the use of heuristics. In these systems, antivirus programs use algorithms to detect the behavior patterns that were found when a specific virus was tested by researchers. Strong heuristics are becoming increasingly important because of advanced persistent threats, which conceal themselves through encryption to evade detection by typical scans. Metamorphic and polymophic viruses, which change their code in propagation, also hide malicious software from conventional scanning processes.