critical

A condition of a system resource such that denial of access to, or lack of availability of, that resource would jeopardize a system user's ability to perform a primary function or would result in other serious consequences, such as human injury or loss of life. (See: availability, precedence. Compare: sensitive.)

An indication that an application is not permitted to ignore an extension. [X509]

Tutorial: Each extension of an X.509 certificate or CRL is flagged as either "critical" or "non-critical". In a certificate, if a computer program does not recognize an extension's type (i.e., does not implement its semantics), then if the extension is critical, the program is required to treat the certificate as invalid; but if the extension is non-critical, the program is permitted to ignore the extension.

In a CRL, if a program does not recognize a critical extension that is associated with a specific certificate, the program is required to assume that the listed certificate has been revoked and is no longer valid, and then take whatever action is required by local policy.

When a program does not recognize a critical extension that is associated with the CRL as a whole, the program is required to assume that all listed certificates have been revoked and are no longer valid. However, since failing to process the extension may mean that the list has not been completed, the program cannot assume that other certificates are valid, and the program needs to take whatever action is therefore required by local policy.