Cyber vigilantism is a rough approximation of law enforcement or an attempt at achieving justice or accomplishing something online through unauthorized channels. Acts of cyber vigilantism are conducted by self-appointed individuals or groups online who employ illegal or questionable tactics to achieve a given end. Cyber vigilantes are sometimes referred to as internet vigilantes.
White hat hackers sometimes take it upon themselves to disseminate ethical or friendly viruses and worms. In this case, the vigilante wants to address security vulnerabilities that are not being adequately addressed by conventional security measures. However, the malware they spread may cause as many problems as it ameliorates. For example, a friendly virus may close a security hole in software that was intentionally left open and that might be required for the application to function.
Anonymous is a prime example of a cyber vigilante group. The loosely organized hacktivist collective was created to promote free speech, unimpeded access to information, and transparency in government and corporate activities. To that end, the group often launches DoS attacks on websites, among other activities. In the wake of the 2015 terrorist attacks in Paris, for example, the organization launched a sustained campaign against ISIS and took 5,000 members offline within days. Another cyber vigilante group, WikiLeaks, is dedicated to making information public that would not otherwise be generally available.