A security incident is an event that may indicate that an organization's systems or data have been compromised or that measures put in place to protect them have failed.
In IT, an event is anything that has significance for system hardware or software and an incident is an event that disrupts normal operations. Security events are usually distinguished from security incidents by the degree of severity and the associated potential risk to the organization.
If a single user is denied access to a requested service, for example, that can be considered a security event because there is some possibility that it indicates a compromised system but the access failure could also be caused by a lot of other things and that single event does not typically have a severe impact on the organization. However, if large numbers of users are denied access, it is likely to mean that there is a more serious problem, such as a denial of service (DoS) attack, so that event may be classified as a security incident.
According to NIST Special Publication 800-61, a security incident is the violation of an explicit or implied security policy.
- Attempts from unauthorized sources to access systems or data.
- Unplanned disruption to a service or denial of a service.
- Unauthorized processing or storage of data.
- Unauthorized changes to system hardware, firmware, or software.