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.
This entry needs more investigation. Public vulnerability research generally focuses on the manipulations that generate invalid structure, instead of the weaknesses that are exploited by those manipulations. For example, a common attack involves making a request that omits a required field, which can trigger a crash in some cases. The crash could be due to a named chain such as CWE-690 (Unchecked Return Value to NULL Pointer Dereference), but public reports rarely cover this aspect of a vulnerability.
The validity of input could be roughly classified along "syntactic",
"semantic", and "lexical" dimensions. If the specification requires that an
input value should be delimited with the "[" and "]" square brackets, then
any input that does not follow this specification would be syntactically
invalid. If the input between the brackets is expected to be a number, but
the letters "aaa" are provided, then the input is syntactically invalid. If
the input is a number and enclosed in brackets, but the number is outside of
the allowable range, then it is semantically invalid. The
inter-relationships between these properties - and their associated
weaknesses- need further exploration.