Input Validation: Checking Before Trusting Data
Input validation checks incoming data against explicit acceptance rules before the program uses it.
Summary
Input validation checks incoming data against explicit acceptance rules before the program uses it. External input remains untrusted until it passes checks appropriate to the intended operation. Successful parsing establishes a data type but does not guarantee an acceptable application-specific value.
| Check | Purpose | Example |
|---|---|---|
| Syntactic validation | Verify representation | Check whether a string parses as an integer |
| Semantic validation | Verify application rules | Check whether the integer falls within an allowed range |
Validation must occur before operations that depend on the checked property, including array access and memory allocation. Allowlist validation specifies permitted values, formats, or ranges. Invalid input should follow a defined error path, such as rejection with a clear message or a controlled retry, without exposing internal program details.
Authoritative validation belongs at the receiving trust boundary, such as a server endpoint, even when interface checks exist. Boundary testing examines values just below, at, and just above acceptance limits. Separating validation from core processing makes assumptions explicit and easier to test.
Common Misconceptions: Successful parsing does not guarantee semantic validity. Interface checks do not replace receiving-boundary validation. Sanitization transforms input but does not establish validity by itself.
🧠 Key Concepts
- Syntactic validation
- Semantic validation
- Allowlist validation
- Trust boundaries
- Validation before use
- Defined error paths
- Boundary testing
- Validation versus sanitization
🧠 Quick Check
See what you remember from the summary.
An input string successfully parses as an integer. What additional check determines whether it satisfies an application's allowed range?
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Input Validation: Checking Before Trusting Data
📘 Overview Input validation checks whether incoming data satisfies a program's requirements before that data is used. Reliable validation checks both representation and meaning, with explicit handling for invalid input.
🧠 Key Idea Treat external input as untrusted until it passes explicit checks for the intended operation. Successful parsing establishes a data type, not necessarily a valid value.
⚔️ Core Details: - Syntactic validation checks representation, such as whether an input string can be parsed as an integer. - Semantic validation checks application rules, such as whether an integer is within an allowed range. - Validation must precede operations that depend on the checked property, such as array access or memory allocation. - Allowlist validation accepts explicitly permitted values or patterns rather than trying to enumerate every forbidden input. - Invalid input follows a defined error path, such as rejection with a message or a controlled retry. - Authoritative validation belongs at the receiving trust boundary, such as a server endpoint, even when interface checks already exist.
🎯 Why It Matters: - Early rejection prevents malformed input from causing crashes, inconsistent state, or incorrect results. - Clear validation errors help users correct mistakes without exposing internal program details. - Boundary checks reduce security risks from oversized values, unexpected formats, and manipulated requests. - Separating validation from core processing makes program assumptions explicit and easier to test.
🧠 Quick Recall: - Input validation - checking incoming data against explicit acceptance rules before use. - Syntactic versus semantic validation - correct representation versus an acceptable application-specific value. - Allowlist - an explicit set of permitted values, formats, or ranges. - Boundary testing - testing values just below, at, and just above each acceptance limit. - Validation versus sanitization - validation accepts or rejects input; sanitization transforms input and does not establish validity by itself.
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