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Reject invalid data at a trusted server or receiving service before business processing and before issuing a database command. Then use database constraints to protect durable rules at the point of storage. Browser checks can make forms easier to use, but they are not authoritative: requests from browsers, internal APIs, partner feeds, queues, and files must all be checked by the component that receives them.
Why validate data before a database write?
Validation checks whether incoming data meets the requirements for the application and the operation that will use it. Done at a trusted intake boundary, it can stop malformed or semantically invalid data before it enters further processing or storage. OWASP recommends not issuing a database command when validation fails: OWASP Secure Database Access.
This also gives the receiving application a clear point to reject a request and return a useful error, rather than leaving a failed write or a later consumer to discover the problem. Data does not become trustworthy just because it came from an internal service or transport; each receiving component should apply the rules relevant to its role.
What should you validate?
Define checks for each field and operation. OWASP distinguishes syntax—whether data has the expected shape—from semantics—whether it makes sense for the intended use. Depending on the field, checks may include:
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- Type and format: confirm that values can be parsed into the expected representation and match the required format.
- Allowed values: accept only the permitted options where the set is known.
- Length and structure: enforce string limits and validate the structure and contents of nested objects or arrays.
- Missing and null behavior: decide whether a value is required, optional, or allowed to be null.
- Ranges and relationships: check permitted numeric or date ranges and relationships between fields, such as requiring a booking end date to follow its start date.
Prefer allowlists that describe acceptable input over attempts to block every suspicious string. Rejecting apostrophes, for example, can exclude legitimate names and does not make SQL queries safe. Parse input safely, set request-size and parser limits before buffering or parsing, and validate the representation the application will actually use. If validation fails, stop the write rather than letting partially checked data continue. Return a clear error without exposing sensitive implementation details.
Which layer should validate the data?
Client-side checks, trusted server-side validation, and database constraints serve different purposes; they are complementary rather than alternatives.
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| Layer | Role and timing | What it is suited to | Limit |
|---|---|---|---|
| Browser or client | Provides feedback before submission | Convenient checks that help a user correct a form | A caller can bypass or alter client-side checks |
| Trusted server or receiving service | Checks data at intake, before business processing and database writes | Authoritative, operation-specific validation and helpful error responses | Does not by itself guarantee that every write path preserves durable invariants |
| Database | Checks constraints when data is written | Structural rules that should hold regardless of which application path writes the data | Does not replace context-aware validation or business decisions in the application |
Microsoft recommends validating data before it enters a trusted tier and again when it crosses trust boundaries: SQL Server security best practices. In practice, apply authoritative validation in the server or receiving component even if the browser already checked the input. At persistence, keep database constraints for invariants that must hold across write paths.
PostgreSQL 18 documents CHECK, NOT NULL, UNIQUE, primary key, and foreign key constraints; a write that violates a constraint raises an error: PostgreSQL 18 constraints. The application can check context and provide an understandable explanation, while the database enforces structural integrity. Keep these rules aligned so an application-level check can explain a failure that the database would otherwise reject.
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What validation does not replace
Parameterized queries
A validated string is not automatically safe to concatenate into SQL. OWASP recommends parameterized queries as the primary SQL injection defense. Validation can be an additional check, especially for query components such as identifiers that cannot be bound as values: OWASP SQL Injection Prevention Cheat Sheet.
Authorization
A well-formed account ID does not prove that the caller is allowed to view or change that account. Check access rights separately for the requested resource and operation.
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Output encoding
Data that passed input validation may still need context-appropriate output encoding when rendered. Validation at intake is not a substitute for safe handling at the point of output.
Business rules
Correct format does not make an action valid in a workflow. An application still needs to verify, for example, that a client-submitted price is authoritative or that a transaction has not skipped a required step. OWASP’s input validation guidance explains these limits: OWASP Input Validation Cheat Sheet.
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A practical pre-write checklist
- Validate data at every trusted intake boundary, regardless of whether it came from a browser, internal API, partner, queue, or file.
- Check syntax, semantics, missing or null values, allowed choices, limits, and relationships relevant to the operation.
- Apply size and parser limits before accepting large or complex input.
- Stop processing and do not issue the database command when validation fails; return a clear, appropriately limited error.
- Use database constraints to preserve structural invariants across write paths.
- Use parameterized SQL, authorization checks, output encoding, and business-rule enforcement for their separate purposes.
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