--- name: implementing-api-schema-validation-security-83a215f9ef49 description: Implement API schema validation using OpenAPI specifications and JSON Schema to enforce input/output contracts and prevent injection, data exposure, and mass assignment attacks. domain: cybersecurity subdomain: api-security tags: - api-security - schema-validation - openapi - json-schema - input-validation - data-leakage-prevention - mass-assignment - api-gateway version: '1.0' author: mahipal license: Apache-2.0 nist_csf: - PR.PS-01 - ID.RA-01 - PR.DS-10 - DE.CM-01 mitre_attack: - T1190 - T1059.007 - T1552.001 - T1055 - T1059 --- # Implementing API Schema Validation Security ## Overview API schema validation enforces that all data exchanged through APIs conforms to a predefined structure defined in OpenAPI Specification (OAS) or JSON Schema documents. This prevents injection attacks (SQLi, XSS, XXE), blocks mass assignment by rejecting unknown properties, prevents data leakage by validating response schemas, and ensures type safety across all API interactions. Schema validation operates at both the API gateway level (runtime enforcement) and during development (shift-left security). ## When to Use - When deploying or configuring implementing api schema validation security capabilities in your environment - When establishing security controls aligned to compliance requirements - When building or improving security architecture for this domain - When conducting security assessments that require this implementation ## Prerequisites - OpenAPI Specification v3.0 or v3.1 for all API endpoints - API gateway with schema validation support (Cloudflare API Shield, Kong, AWS API Gateway) - JSON Schema draft-07 or later understanding - Development environment with OpenAPI validation libraries - CI/CD pipeline for automated schema compliance testing ## Core Implementation ### OpenAPI Schema with Security Constraints ```yaml openapi: 3.1.0 info: title: Secure E-Commerce API version: 2.0.0 servers: - url: https://api.example.com/v2 description: Production (HTTPS enforced) security: - OAuth2: - read:products - write:orders paths: /products: post: operationId: createProduct security: - OAuth2: [write:products] requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ProductCreate' responses: '201': description: Product created content: application/json: schema: $ref: '#/components/schemas/Product' '400': $ref: '#/components/responses/ValidationError' '401': $ref: '#/components/responses/Unauthorized' /products/{productId}: get: operationId: getProduct parameters: - name: productId in: path required: true schema: type: string format: uuid pattern: '^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$' responses: '200': content: application/json: schema: $ref: '#/components/schemas/Product' components: schemas: ProductCreate: type: object required: [name, price, category] properties: name: type: string minLength: 1 maxLength: 200 pattern: '^[a-zA-Z0-9\s\-\.]+$' # No special chars for injection prevention description: type: string maxLength: 2000 # Sanitize HTML entities price: type: number format: float minimum: 0.01 maximum: 999999.99 exclusiveMinimum: 0 category: type: string enum: [electronics, clothing, food, furniture, other] tags: type: array items: type: string maxLength: 50 pattern: '^[a-zA-Z0-9\-]+$' maxItems: 10 uniqueItems: true additionalProperties: false # CRITICAL: Prevents mass assignment Product: type: object required: [id, name, price] properties: id: type: string format: uuid readOnly: true name: type: string price: type: number category: type: string tags: type: array items: type: string createdAt: type: string format: date-time readOnly: true additionalProperties: false # Prevents data leakage of internal fields ValidationErrorResponse: type: object required: [code, message] properties: code: type: string enum: [VALIDATION_ERROR] message: type: string maxLength: 500 details: type: array items: type: object properties: field: type: string error: type: string additionalProperties: false maxItems: 50 additionalProperties: false responses: ValidationError: description: Request validation failed content: application/json: schema: $ref: '#/components/schemas/ValidationErrorResponse' Unauthorized: description: Authentication required securitySchemes: OAuth2: type: oauth2 flows: authorizationCode: authorizationUrl: https://auth.example.com/authorize tokenUrl: https://auth.example.com/token scopes: read:products: Read product data write:products: Create and update products write:orders: Create orders ``` ### Server-Side Schema Validation (Python/FastAPI) ```python """API Schema Validation Middleware for FastAPI Enforces strict schema validation on all request and response payloads to prevent injection, mass assignment, and data leakage attacks. """ from fastapi import FastAPI, Request, Response, HTTPException from fastapi.middleware import Middleware from pydantic import BaseModel, Field, field_validator, ConfigDict from typing import List, Optional import re import json from starlette.middleware.base import BaseHTTPMiddleware app = FastAPI() # Strict Pydantic models with security constraints class ProductCreate(BaseModel): model_config = ConfigDict(extra='forbid') # Reject unknown fields (mass assignment) name: str = Field(min_length=1, max_length=200, pattern=r'^[a-zA-Z0-9\s\-\.]+$') description: Optional[str] = Field(default=None, max_length=2000) price: float = Field(gt=0, le=999999.99) category: str = Field(pattern=r'^(electronics|clothing|food|furniture|other)$') tags: Optional[List[str]] = Field(default=None, max_length=10) @field_validator('name') @classmethod def sanitize_name(cls, v): # Prevent XSS via HTML entities dangerous_patterns = [' 10: raise ValueError('Maximum 10 tags allowed') for tag in v: if not re.match(r'^[a-zA-Z0-9\-]+$', tag) or len(tag) > 50: raise ValueError(f'Invalid tag format: {tag}') return v class ProductResponse(BaseModel): """Response model that explicitly defines allowed output fields. Prevents leakage of internal fields like internal_notes, cost_price, etc.""" model_config = ConfigDict(extra='forbid') id: str name: str price: float category: str tags: List[str] = [] created_at: str class ResponseValidationMiddleware(BaseHTTPMiddleware): """Middleware to validate response payloads against schema. Prevents accidental data leakage by checking response content.""" SCHEMA_MAP = { '/api/v2/products': { 'POST': {'response_model': ProductResponse}, 'GET': {'response_model': ProductResponse}, } } async def dispatch(self, request: Request, call_next): response = await call_next(request) # Only validate JSON responses content_type = response.headers.get('content-type', '') if 'application/json' not in content_type: return response # Check if endpoint has a registered response schema path = request.url.path method = request.method route_config = self.SCHEMA_MAP.get(path, {}).get(method) if not route_config: return response # Read and validate response body body = b"" async for chunk in response.body_iterator: body += chunk try: data = json.loads(body) model = route_config['response_model'] if isinstance(data, list): for item in data: model.model_validate(item) else: model.model_validate(data) except Exception as e: # Log the validation failure for security monitoring print(f"SECURITY: Response schema violation on {method} {path}: {e}") # Return a safe error instead of potentially leaked data return Response( content=json.dumps({"error": "Internal server error"}), status_code=500, media_type="application/json" ) return Response( content=body, status_code=response.status_code, headers=dict(response.headers), media_type=response.media_type ) app.add_middleware(ResponseValidationMiddleware) @app.post("/api/v2/products", response_model=ProductResponse, status_code=201) async def create_product(product: ProductCreate): # ProductCreate model with extra='forbid' automatically rejects # any unknown fields, preventing mass assignment attacks # (e.g., attacker trying to set is_admin=true or price=0) pass ``` ### Cloudflare API Shield Schema Validation ```bash # Upload OpenAPI schema to Cloudflare API Shield curl -X POST "https://api.cloudflare.com/client/v4/zones/{zone_id}/api_gateway/user_schemas" \ -H "Authorization: Bearer ${CF_API_TOKEN}" \ -H "Content-Type: multipart/form-data" \ -F "file=@openapi.yaml" \ -F "kind=openapi_v3" # Enable schema validation with blocking mode curl -X PATCH "https://api.cloudflare.com/client/v4/zones/{zone_id}/api_gateway/settings/schema_validation" \ -H "Authorization: Bearer ${CF_API_TOKEN}" \ -H "Content-Type: application/json" \ -d '{ "validation_default_mitigation_action": "block", "validation_override_mitigation_action": null }' ``` ### CI/CD Schema Compliance Testing ```yaml # GitHub Actions workflow for schema validation in CI name: API Schema Security Check on: pull_request: paths: ['api/**', 'openapi/**'] jobs: schema-security: runs-on: ubuntu-latest steps: - uses: actions/checkout@v4 - name: Validate OpenAPI Schema run: | npm install -g @stoplight/spectral-cli spectral lint openapi.yaml --ruleset .spectral-security.yaml - name: Check for Security Anti-Patterns run: | python3 scripts/schema_security_check.py openapi.yaml - name: Run Contract Tests run: | npm install -g dredd dredd openapi.yaml http://localhost:3000 --hookfiles=./test/hooks.js ``` ## Security Anti-Patterns to Detect | Anti-Pattern | Risk | Fix | |---|---|---| | `additionalProperties: true` or missing | Mass assignment | Set `additionalProperties: false` | | No `maxLength` on strings | Buffer overflow, DoS | Add appropriate `maxLength` constraints | | No `pattern` on string fields | Injection attacks | Add regex patterns to restrict input | | No `enum` for fixed-value fields | Unexpected input processing | Use `enum` for fields with known values | | `format: password` without TLS | Credential exposure | Enforce HTTPS-only server URLs | | Missing error response schemas | Information leakage | Define all 4xx/5xx response schemas | | `readOnly` fields in request body | Data manipulation | Enforce `readOnly` server-side | ## References - OpenAPI Specification v3.1: https://spec.openapis.org/oas/v3.1.0 - Cloudflare API Shield Schema Validation: https://developers.cloudflare.com/api-shield/security/schema-validation/ - Redocly API Security by Design: https://redocly.com/learn/security - Impart Security API Validation: https://www.impart.ai/blog/detect-and-fix-api-vulnerabilities-using-validation-secure-principles-and-real-time-response - OWASP API Security Top 10 2023: https://owasp.org/API-Security/editions/2023/en/0x00-header/ --- Source attribution: 26zl Original project: https://github.com/26zl/cybersec-toolkit/blob/2ad2519d437cda1f8a822ef7adc336993601a5ac/.claude/skills/implementing-api-schema-validation-security/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: 26zl Original project: https://github.com/26zl/cybersec-toolkit/blob/2ad2519d437cda1f8a822ef7adc336993601a5ac/.claude/skills/implementing-api-schema-validation-security/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Product Management Roles: Backend Developer, Data Engineer, ML / AI Engineer, Product Manager, Security Engineer Specialties: API Tools: Aws Api Gateway, Cloudflare Api Shield, Kong Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 45907651ec8df0ce8407b58f9090b9329e77badf7d03767512ddba5569b5520c