NIST 800-53 SI-10 Information Input Validation: What to Show

SI-10 asks you to check the validity of the information inputs you define. It is the control behind most of the secure coding questions customers ask, from SQL injection to file upload handling.

800-53 binds federal systems and contractors. For a software company it is a reference that turns "do you validate input?" into something an auditor can test.

What it means in practice

NIST describes the check as verifying valid syntax and semantics, including character set, length, numerical range and acceptable values. Its example is a field that accepts whole numbers from 1 to 100. Inputs like 387, abc or %K% are invalid and are not accepted.

Valid inputs vary by field, so rules must be per field and not one global filter. The guidance also points at structured messages. Commands and queries mix data with control information. If an application builds them from attacker supplied input without proper handling, the attacker can change the meaning. That is the root of injection attacks.

The control does not say "sanitize everything." It says define what is valid and check against it.

The enhancements worth knowing

• SI-10(1) Manual Override Capability, which controls who can override validation.
• SI-10(2) Review and Resolve Errors, which asks you to look at validation failures and act on them.
• SI-10(3) Predictable Behavior, so the system behaves in a known way on invalid input.
• SI-10(4) Timing Interactions.
• SI-10(5) Restrict Inputs to Trusted Sources and Approved Formats.
• SI-10(6) Injection Prevention, the one most application reviews map to.

Evidence to collect

• A secure coding standard that states validation rules: allow lists over block lists, length and type checks, server side enforcement.
• API schemas such as OpenAPI or JSON Schema, with validation turned on at the gateway or in the framework.
• Code samples or a framework configuration showing parameterized queries and output encoding.
• Static analysis and dynamic scan reports, with findings tracked to closure.
• Test cases for invalid input, including boundaries and oversized payloads.
• Logs or metrics of rejected input, and a record that someone reviews them.
• For file uploads, the type, size and content checks.

Common gaps

Validation only in the browser. Schemas that exist but are not enforced. Different services validating the same field differently. String building for queries in one old module. Free text fields with no length limit. Deserialization of data from outside the trust boundary. Validation errors that are dropped silently, so nobody sees an attack in progress.

Dependencies add a second path. A parsing or templating library with a known flaw can break an otherwise careful validation scheme.

How to implement it

1. Define a schema for every external input: API bodies, query strings, headers, files, queue messages.
2. Enforce the schema server side, as early as possible, and reject on failure.
3. Use parameterized queries and safe APIs so data cannot become code.
4. Encode output for its context, which also supports SI-15.
5. Log rejected input without logging secrets, and alert on spikes.
6. Add invalid input cases to your test suite and run them on every build.
7. Scan the application and its dependencies before release.

Try it yourself

Check the libraries behind your parsers and frameworks. Paste a dependency manifest into the free in-browser scan at gold.safeguard.sh/scan and look for injection and parsing vulnerabilities in what you ship.

Quick check

Sources

• NIST SP 800-53 Rev 5, SI-10 in the CPRT catalog
• NIST OSCAL catalog for Rev 5