Fix Improper Error Handling in Fresh
Improper error handling in Fresh (Deno) frameworks often leads to CWE-209: Information Exposure Through an Error Message. By default, unhandled exceptions can leak stack traces, local file paths, and runtime versions to the client. An attacker uses this metadata to map the server's internal structure and identify vulnerable dependency versions. Secure Fresh apps must implement global error boundaries and sanitize API responses.
The Vulnerable Pattern
// routes/api/user/[id].ts import { Handlers } from "$fresh/server.ts";
export const handler: Handlers = { async GET(_req, ctx) { try { const user = await Deno.readTextFile(./db/${ctx.params.id}.json); return new Response(user); } catch (err) { // VULNERABLE: Leaking raw error details and stack traces to the client return new Response(JSON.stringify({ status: “error”, message: err.message, stack: err.stack }), { status: 500, headers: { “Content-Type”: “application/json” } }); } } };
The Secure Implementation
The fix involves two layers: 1. Implementing a custom '_500.tsx' route to act as a global catch-all for UI rendering errors, ensuring users never see the default Deno/Fresh stack trace. 2. In API handlers, explicitly catching exceptions and returning a generic JSON object. By generating a 'traceId' (UUID), you allow developers to correlate client-side reports with server-side logs without exposing sensitive system internals to potential attackers.
// routes/_500.tsx (Global Error Boundary) import { ErrorPageProps } from "$fresh/server.ts";export default function Error500Page({ error }: ErrorPageProps) { console.error(“[Server Error]:”, error); // Log full error server-side only return (
); }500 - Internal Server Error
The incident has been logged. Please contact support if the issue persists.
// routes/api/user/[id].ts (Sanitized Handler) export const handler: Handlers = { async GET(_req, ctx) { try { const user = await Deno.readTextFile(./db/${ctx.params.id}.json); return new Response(user); } catch (err) { const traceId = crypto.randomUUID(); console.error([${traceId}] Database access error:, err); // SECURE: Generic message and unique reference ID for debugging logs return new Response(JSON.stringify({ error: “Internal Server Error”, ref: traceId }), { status: 500, headers: { “Content-Type”: “application/json” } }); } } };
Your Fresh API
might be exposed to Improper Error Handling
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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