The rising dependence on increasingly complex and interconnected computer systems in most domains of life is the main factor that causes vulnerability to cyber attacks, since virtually all computer systems have bugs that can be exploited by attackers. Although it is impossible or impractical to create a perfectly secure system, there are many defense mechanisms that can make a system more difficult to attack. Perpetrators of a cyber attack can be criminals, hack activists, or states. They attempt to find weaknesses in a system, exploit them and create malware to carry out their goals, and deliver it to the targeted system. Once installed, the malware can have a variety of effects depending on its purpose. Detection of cyber attacks is often absent or delayed, especially when the malware attempts to spy on the system while remaining undiscovered. If it is discovered, the targeted organization may attempt to collect evidence about the attack, remove malware from its systems, and close the vulnerability that enabled the attack. Cyber attacks can cause a variety of harms to targeted individuals, organizations, and governments, including significant financial losses and identity theft. They are usually illegal both as a method of crime and warfare, although correctly attributing the attack is difficult and perpetrators are rarely prosecuted.