--- name: validating-cors-policies description: Validate CORS policies for security issues and misconfigurations. Use when reviewing cross-origin resource sharing. Trigger with 'validate CORS', 'check CORS policy', or 'review cross-origin'. version: 1.24.0 allowed-tools: Read, WebFetch, WebSearch, Grep license: MIT author: Jeremy Longshore tags: - security - validating-cors compatibility: Designed for Claude Code --- # Validating CORS Policies ## Overview Validate Cross-Origin Resource Sharing configurations in web applications and APIs for security misconfigurations that enable unauthorized cross-origin access. This skill analyzes CORS headers, middleware configurations, and server response behavior to detect wildcard origins, reflected origins, credential leakage, and overly permissive header/method exposure. ## Prerequisites - Access to the target codebase and configuration files in `${CLAUDE_SKILL_DIR}/` - For live endpoint testing: WebFetch tool available and target URLs accessible - Familiarity with the web framework in use (Express, Django, Flask, Spring, ASP.NET, etc.) - Reference: `${CLAUDE_SKILL_DIR}/references/README.md` for CORS specification details, common vulnerability patterns, and example policies ## Instructions 1. Locate all CORS configuration points by scanning for `Access-Control-Allow-Origin`, `cors()` middleware, `@CrossOrigin` annotations, CORS policy builders, and server config directives (nginx `add_header`, Apache `Header set`) using Grep. 2. Check for wildcard origin (`Access-Control-Allow-Origin: *`) -- flag as severity high when combined with `Access-Control-Allow-Credentials: true`, which browsers reject but indicates a misunderstanding of the security model. 3. Detect origin reflection patterns where the server echoes back the `Origin` request header without validation -- search for code that reads the `Origin` header and sets it directly in the response. Flag as CWE-942 (Permissive Cross-domain Policy), severity critical. 4. Validate the origin allowlist: check that allowed origins use exact string matching rather than substring or regex patterns vulnerable to bypass (e.g., `example.com.evil.com` matching a check for `example.com`). 5. Assess `Access-Control-Allow-Methods` -- flag if dangerous methods (`PUT`, `DELETE`, `PATCH`) are exposed without necessity. Verify that preflight (`OPTIONS`) responses include appropriate method restrictions. 6. Evaluate `Access-Control-Allow-Headers` -- flag wildcard header allowance or exposure of sensitive headers like `Authorization`, `Cookie`, or custom auth headers to broader origins than necessary. 7. Check `Access-Control-Expose-Headers` for leakage of internal headers (e.g., `X-Request-Id`, `X-Internal-Trace`) to cross-origin consumers. 8. Verify `Access-Control-Max-Age` is set to a reasonable value (600-86400 seconds) to balance security with performance -- missing or excessively long max-age values deserve a low-severity note. 9. For live endpoints, issue preflight requests via WebFetch with various `Origin` values (legitimate, malicious, null) and analyze the response headers to confirm server behavior matches the codebase configuration. 10. Compile findings with severity ratings, map to OWASP Testing Guide OTG-CLIENT-007, and provide remediation with correct CORS middleware configuration examples. ## Output - **CORS configuration inventory**: Table of all CORS-enabled endpoints, their allowed origins, methods, headers, and credentials settings - **Findings report**: Each finding includes severity, affected endpoint/file, CWE reference (CWE-942, CWE-346), observed behavior, and remediation code - **Preflight test results**: For live endpoints, a table of Origin values tested and the corresponding server responses - **Remediation examples**: Framework-specific CORS configuration snippets (Express `cors()`, Django `django-cors-headers`, Spring `@CrossOrigin`, nginx headers) ## Error Handling | Error | Cause | Solution | |-------|-------|----------| | No CORS configuration found | CORS handled at infrastructure layer (CDN, API gateway) | Check CDN/gateway configs (Cloudflare, AWS API Gateway, nginx) for CORS header injection | | WebFetch blocked or timed out | Target endpoint unreachable or rate-limited | Verify URL accessibility; fall back to static codebase analysis of CORS middleware configuration | | Inconsistent CORS behavior across endpoints | Multiple CORS configurations at different layers | Map each layer (application, reverse proxy, CDN) and document the effective policy per endpoint | | Origin reflection false positive | Dynamic origin validation with a secure allowlist | Verify the allowlist logic uses exact matching; mark as informational if the implementation is secure | | Preflight not triggering | Request classified as "simple request" by the browser | Note that simple GET/POST requests bypass preflight; test with custom headers to force preflight | ## Examples ### Express.js CORS Middleware Audit Scan `${CLAUDE_SKILL_DIR}/src/app.js` and `${CLAUDE_SKILL_DIR}/src/middleware/` for `cors()` configuration. Flag `origin: true` (reflects any origin) as CWE-942, severity critical. Recommend replacing with an explicit allowlist: `origin: ['https://app.example.com', 'https://admin.example.com']`. ### Nginx CORS Header Review Grep `${CLAUDE_SKILL_DIR}/nginx/` for `add_header Access-Control-Allow-Origin`. Flag any `$http_origin` variable usage that reflects the origin without validation. Verify that `Access-Control-Allow-Credentials` is only set for origins in the allowlist using an `if` block or `map` directive. ### API Gateway CORS Configuration Review `${CLAUDE_SKILL_DIR}/infra/api-gateway.yaml` or equivalent IaC definitions for CORS settings. Flag wildcard `*` in allowed origins when credentials are enabled. Verify that `Access-Control-Allow-Methods` is scoped to only the HTTP methods each endpoint actually supports. ## Resources - [MDN: Cross-Origin Resource Sharing](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) - [OWASP Testing for CORS (OTG-CLIENT-007)](https://owasp.org/www-project-web-security-testing-guide/latest/4-Web_Application_Security_Testing/11-Client-side_Testing/07-Testing_Cross_Origin_Resource_Sharing) - [CWE-942: Permissive Cross-domain Policy](https://cwe.mitre.org/data/definitions/942.html) - [CWE-346: Origin Validation Error](https://cwe.mitre.org/data/definitions/346.html) - [Fetch Standard: CORS Protocol](https://fetch.spec.whatwg.org/#http-cors-protocol)