Individual skill
cors-security
Strict CORS: no wildcard with credentials, exact-match origin allowlists, sane preflight cache, minimal exposed headers. Use when generating CORS middleware or framework config, setting CORS headers in API Gateway, CloudFront, or Nginx, or reviewing a cross-origin browser-facing endpoint.
dist/agent-skills/.agents/skills/cors-security/SKILL.md
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- Last checked
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- claude-skills Repository topic
- llm Repository topic
- security Repository topic
- skills Repository topic
- security Derived category
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SKILL.mdcore · 5,055 bytesSKILL.mdcore · 5,055 bytesmetadata.jsonasset · 1,040 bytesreferences/verifying-findings.mddoc · 3,778 bytes
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<!-- Native skill bundle for agent-skills (cross-tool convention). Generated by `secure-vibe dev regenerate`. -->
<!-- Do not edit by hand; the source of truth is skills/cors-security/SKILL.md. -->
# CORS Security
Strict CORS: no wildcard with credentials, exact-match origin allowlists, sane preflight cache, minimal exposed headers. Use when generating CORS middleware or framework config, setting CORS headers in API Gateway, CloudFront, or Nginx, or reviewing a cross-origin browser-facing endpoint.
## ALWAYS
- For any non-public endpoint, allow origins from an explicit allowlist. `*` is correct only when the resource is intentionally public *and* cross-origin access never carries credentials.
- When a cross-origin request carries credentials — the browser's credentials mode is `include`, meaning cookies, TLS client certificates, or browser-managed HTTP auth — respond with `Access-Control-Allow-Credentials: true` and a **single explicit origin**. On a credentialed request every CORS header loses its wildcard meaning: `*` in `Allow-Origin`, `Allow-Methods`, `Allow-Headers`, or `Expose-Headers` is read as the literal string `"*"` and the browser blocks the response. That is why "just set it to `*` for now" fails in a way that is hard to debug.
- Compare origins by exact match on the **serialized** origin — scheme, host, and port, all three. Never substring, prefix, or suffix matching: `endsWith("example.com")` accepts `https://example.com.evil.com`, and an unescaped `.` in `^https://api.example.com$` accepts `https://apiXexample.com`.
- Include `Vary: Origin` on any response whose headers depend on the request `Origin`, so a shared cache cannot hand one origin's `Access-Control-Allow-Origin` to another.
[Credential-related source omitted]