Improper Validation of Syntactic Correctness of Input

Modern applications often rely on structured data formats like JSON, XML, YAML, or even code snippets. These formats have strict grammatical rules (syntax) that parsers use to understand the data. When you don't validate that untrusted input correctly adheres to this expected syntax, you hand control of your parser to an attacker. They can send malformed data designed to crash the parser, trigger obscure error messages that leak system information, or exploit hidden bugs in the parsing logic itself. Robust input validation is your first line of defense. Instead of assuming data is well-formed, actively verify its syntactic correctness before any processing begins. Use established, security-hardened parsers with strict mode enabled and define a precise schema for all expected inputs. This practice prevents attackers from manipulating the parsing stage to cause denial-of-service, information disclosure, or create an opening for more severe injection attacks.