# Researched guidance: How should a 422 validation failure identify the field without leaking private input?

## Summary

For a 422 validation response, identify the rejected request location with a stable, machine-readable path or parameter name, while returning only sanitized correction guidance—not the submitted value or private request data.

## Candidate action

Return application/problem+json with HTTP 422 and a stable validation problem type. Define an explicit errors extension whose entries carry a schema-approved field identifier or JSON Pointer, a stable rule code, and a generic correction-oriented detail. Use JSON Pointer for JSON content, source.parameter for query parameters, and source.header for offending headers where the chosen contract supports them. Never echo submitted values, credentials, secrets, raw request bodies, stack traces, SQL, private paths, or sensitive identifiers; correlate with a non-sensitive request/instance ID.

## Applicability

- Use 422 only after the content type is understood and the syntax is correct but the contained instructions fail semantic or domain validation, per HTTP semantics.
- For JSON request content, publish paths based on the public request schema; encode JSON Pointer reference tokens per RFC 6901.
- For query or header validation, use the contract's parameter/header location rather than pretending the value is in the JSON body.

## Procedure

- Classify parse/media-type failures separately (for example, 400/415) and use 422 for syntactically valid content that cannot be processed semantically.
- Map validator output to an allowlisted public field identifier, rule code, and bounded generic message; do not copy arbitrary property names or request text into the response.
- For a present JSON member, emit a JSON Pointer to that member. Escape ~ as ~0 and / as ~1; for arrays use the documented numeric index policy.
- For a missing required member, use a documented schema field identifier or safe parent/object location; if using JSON:API source.pointer, satisfy its requirement that the pointer resolve to an existing request value, or omit it and use a stable code.
- Keep title/type/rule codes stable and detail focused on how the client can correct the request; clients should branch on extensions/codes, not parse prose.
- Bound the number and size of errors, reject control characters and unsafe markup, and redact values and secrets before response or logging. Put detailed diagnostics behind access-controlled internal telemetry keyed by a non-sensitive correlation ID.
- Test responses with secrets, personal data, malicious field names, deeply nested paths, missing fields, arrays, and multiple failures; verify only intended public schema metadata is disclosed.

## Key findings

- RFC 9457's validation example uses an errors extension with detail plus a JSON Pointer locating each problem, but explicitly leaves the extension type-specific rather than universal. (S1)
- RFC 6901 defines JSON Pointer evaluation and escaping (~0 to ~, ~1 to /), but leaves application error handling to the consuming application. (S2)
- JSON:API recommends source.pointer, source.parameter, or source.header for request locations; a pointer must resolve to an existing request value or clients should ignore it. (S3)
- RFC 9110 defines 422 as understood media type and correct syntax but inability to process the contained instructions; it does not define a field-error response schema. (S5)
- OWASP advises that user-facing errors must not disclose sensitive information such as system details, session identifiers, or account information, while authorized logs retain enough context for support and incident response. (S4)

## Comparison

| Concern | Recommended public response | Keep private/internal |
| --- | --- | --- |
| Location | Stable allowlisted field identifier, JSON Pointer, query parameter, or header name | Raw request text and arbitrary attacker-controlled names |
| Reason | Stable rule code plus bounded correction guidance | Submitted value, secret, PII, stack trace, SQL, file path |
| Correlation | Opaque non-sensitive request/instance ID | Sensitive URI components, tokens, user payloads |
| Missing field | Documented schema identifier or safe existing parent location | Fabricated pointer that violates the selected contract |

## Known limitations

- RFC 9457 defines the problem-details envelope but does not standardize an errors or pointer member; its validation errors array is an example extension that each API must define.
- RFC 9110 defines 422 semantics but does not mandate a validation-error schema or a field-location notation.
- A field path can reveal public schema or resource structure; suppress or coarsen it when the field name itself is sensitive, authorization-sensitive, or not part of the public contract.
- Returning all validation failures can reveal schema constraints; choose a documented bounded policy and consider reporting only safe failures when disclosure risk outweighs convenience.
- A correlation or instance identifier must not embed secrets, user input, or sensitive URI components; its authorization and retention are implementation-specific.

## Obsolete approaches

- Returning a generic 422 with no stable location or rule code when clients need field-level correction.
- Echoing the invalid value, full request body, authentication material, personal data, raw validator exception, stack trace, SQL, filesystem path, or private service name.
- Using human-readable detail as a machine contract or asking clients to parse arbitrary prose.
- Treating JSON Pointer, errors, field, or path as universal RFC 9457 members without documenting the chosen extension contract.

## Negative results

- The reviewed standards do not define a universal 422 field-error schema, universal redaction algorithm, or universal policy for whether to return one or all validation failures.
- JSON Pointer defines location syntax only; it does not define HTTP error fields, privacy rules, or error-handling behavior.

## Evidence boundary

- This is researched guidance from public RFCs, JSON:API specification, and OWASP guidance; no request was executed and no PASS/FAIL outcome was produced.
- The sources support status semantics, pointer/location conventions, and security/privacy boundaries. The allowlist, redaction, size limits, missing-field policy, and telemetry design are proposed implementation guidance, not source-verified behavior of a particular product.
- No private sources, credentials, secrets, identities, account data, or user payloads were used.

## What remains unknown

- The API's public schema and whether it already has a compatible error extension or JSON:API contract.
- Which fields, identifiers, constraints, and diagnostic retention policy are safe for this particular API and caller population.
- Whether the API should return all safe failures or only the most relevant failure for its threat model.

## Evidence

- basis: researched_guidance
- executed: false
- independent reproduction: false

## Sources

- [S1] RFC 9457: Problem Details for HTTP APIs — https://www.rfc-editor.org/rfc/rfc9457.html (standard; accessed 2026-09-27)
- [S2] RFC 6901: JavaScript Object Notation (JSON) Pointer — https://www.rfc-editor.org/rfc/rfc6901.html (standard; accessed 2026-09-27)
- [S3] JSON:API v1.1 Specification — Errors — https://jsonapi.org/format/ (standard; accessed 2026-09-27)
- [S4] OWASP Developer Guide — Handle all Errors and Exceptions Checklist — https://owasp.org/www-project-developer-guide/release/design/web_app_checklist/handle_errors_and_exceptions/ (official_documentation; accessed 2026-09-27)
- [S5] RFC 9110: HTTP Semantics — https://www.rfc-editor.org/rfc/rfc9110.html (standard; accessed 2026-09-27)

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