CORS with JWT APIs
Cross-Origin Resource Sharing controls whether browsers allow frontends on one origin to call APIs on another origin with custom headers or cookies. JWT APIs that expect an Authorization Bearer header must explicitly allow that header in Access-Control-Allow-Headers and typically cannot combine Access-Control-Allow-Origin wildcarding with credentialed modes incorrectly. Cookie-based JWT sessions require Access-Control-Allow-Credentials and a specific allowed origin rather than a wildcard. Misconfigured CORS either blocks legitimate single-page apps or over-permits dangerous cross-origin calls. CORS is not an authentication mechanism — it only constrains browser behavior and does not replace server-side JWT verification for non-browser clients. Document allowed origins beside your JWT verification settings so frontend deploys and API gateways stay aligned when authentication headers change.
Why It Matters
SPA architectures dominate modern JWT usage in browsers. Incorrect CORS leads to hours of debugging opaque network failures or, worse, overly permissive origins that expand abuse. Pair strict CORS allowlists with solid JWT verification, HTTPS, and CSRF controls when cookies carry sessions. Review preflight OPTIONS responses whenever authentication headers or cookie policies change in production. Allowlist exact SPA origins, expose only required headers, and never use Access-Control-Allow-Origin: * with Authorization bearer flows. CORS failures are configuration bugs; successful CORS never replaces signature and claim verification.
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Frequently Asked Questions
Why is my Authorization header blocked by CORS?
Browsers require Access-Control-Allow-Headers to include Authorization (and a non-wildcard Allow-Origin for credentialed patterns). Fix the preflight response; do not disable JWT verification.
Can CORS replace JWT signature verification?
Never. CORS only constrains browsers. Non-browser clients can call your API directly, so signatures, audience, issuer, and expiry checks remain mandatory on every request.
Should I allow origin wildcard with bearer tokens?
Avoid * for credentialed or sensitive APIs. Allowlist exact frontend origins so stolen tokens are harder to use from arbitrary malicious sites via browsers.