The product does not handle or incorrectly handles input that is not syntactically well-formed with respect to the associated specification.
If an input is syntactically invalid, then processing the input could place the system in an unexpected state that could lead to a crash, consume available system resources or other unintended behaviors.
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)