Astro's middleware authentication checks based on url.pathname can be bypassed via url encoded values

/** The main logic to route dev server requests to pages in Astro. */
export async function handleRequest({
    pipeline,
    routesList,
    controller,
    incomingRequest,
    incomingResponse,
}: HandleRequest) {
    const { config, loader } = pipeline;
    const origin = `${loader.isHttps() ? 'https' : 'http'}://${
        incomingRequest.headers[':authority'] ?? incomingRequest.headers.host
    }`;

    const url = new URL(origin + incomingRequest.url);
    let pathname: string;
    if (config.trailingSlash === 'never' && !incomingRequest.url) {
        pathname = '';
    } else {
        // We already have a middleware that checks if there's an incoming URL that has invalid URI, so it's safe
        // to not handle the error: packages/astro/src/vite-plugin-astro-server/base.ts
        pathname = decodeURI(url.pathname); // here this url is for routing/rendering
    }

    // Add config.base back to url before passing it to SSR
    url.pathname = removeTrailingForwardSlash(config.base) + url.pathname; // this is used for middleware context

import { defineMiddleware } from "astro/middleware";

export const onRequest = defineMiddleware(async (context, next) => {
  const isAuthed = false;  // simulate no auth
  if (context.url.pathname === "/admin" && !isAuthed) {
    return context.redirect("/");
  }
  return next();
});