Fix Security Misconfiguration in Fresh
Fresh's zero-config approach is optimized for speed, but it leaves security headers to the developer's discretion. A default Fresh setup is a playground for XSS, clickjacking, and MIME-sniffing because it lacks a hardened middleware configuration. To secure a Fresh app, you must intercept the response lifecycle and inject mandatory security headers.
The Vulnerable Pattern
// routes/_middleware.ts import { FreshContext } from "$fresh/server.ts";
export const handler = [ async function middleware(_req: Request, ctx: FreshContext) { // VULNERABLE: Standard middleware that passes the request through. // No security headers are set, leaving the app exposed to protocol-level attacks. return await ctx.next(); }, ];
The Secure Implementation
The vulnerability lies in 'Implicit Trust'—assuming the framework handles the HTTP security layer. In Fresh, if you don't define a global middleware to set security headers, the browser operates under the most permissive defaults. The fix requires capturing the Response object from 'ctx.next()' and manually appending headers. Specifically, CSP mitigates XSS by whitelisting trusted scripts, 'nosniff' prevents browsers from executing non-executable MIME types, and HSTS ensures that the 'Secure' flag on your cookies isn't bypassed by a downgrade to HTTP.
// routes/_middleware.ts import { FreshContext } from "$fresh/server.ts";export const handler = [ async function middleware(_req: Request, ctx: FreshContext) { const resp = await ctx.next();
// SECURE: Implementing a defense-in-depth header strategy const headers = resp.headers; // Prevent XSS by restricting source origins headers.set("Content-Security-Policy", "default-src 'self'; script-src 'self' 'unsafe-inline'; object-src 'none';"); // Stop MIME-type sniffing headers.set("X-Content-Type-Options", "nosniff"); // Prevent Clickjacking headers.set("X-Frame-Options", "DENY"); // Enforce HTTPS headers.set("Strict-Transport-Security", "max-age=31536000; includeSubDomains; preload"); // Control referrer information leakage headers.set("Referrer-Policy", "strict-origin-when-cross-origin"); return resp;
}, ];
Your Fresh API
might be exposed to Security Misconfiguration
74% of Fresh apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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