diff --git a/.changeset/polite-balloons-rhyme.md b/.changeset/polite-balloons-rhyme.md
new file mode 100644
index 000000000000..319f9a7467ba
--- /dev/null
+++ b/.changeset/polite-balloons-rhyme.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Ensures that URLs with multiple leading slashes (e.g. `//admin`) are normalized to a single slash before reaching middleware, so that pathname checks like `context.url.pathname.startsWith('/admin')` work consistently regardless of the request URL format
diff --git a/packages/astro/src/core/app/base.ts b/packages/astro/src/core/app/base.ts
index bb263aa19407..bc4b299b7005 100644
--- a/packages/astro/src/core/app/base.ts
+++ b/packages/astro/src/core/app/base.ts
@@ -1,5 +1,6 @@
 import {
 	appendForwardSlash,
+	collapseDuplicateLeadingSlashes,
 	collapseDuplicateTrailingSlashes,
 	hasFileExtension,
 	isInternalPath,
@@ -187,6 +188,10 @@ export abstract class BaseApp<P extends Pipeline = AppPipeline> {
 	}
 
 	public removeBase(pathname: string) {
+		// Collapse multiple leading slashes to prevent middleware authorization bypass.
+		// Without this, `//admin` would be treated as starting with base `/` and sliced
+		// to `/admin` for routing, while middleware still sees `//admin` in the URL.
+		pathname = collapseDuplicateLeadingSlashes(pathname);
 		if (pathname.startsWith(this.manifest.base)) {
 			return pathname.slice(this.baseWithoutTrailingSlash.length + 1);
 		}
diff --git a/packages/astro/src/core/render-context.ts b/packages/astro/src/core/render-context.ts
index ca52a80bf4b9..aaa4071b86df 100644
--- a/packages/astro/src/core/render-context.ts
+++ b/packages/astro/src/core/render-context.ts
@@ -37,6 +37,7 @@ import { getParams, getProps, type Pipeline, Slots } from './render/index.js';
 import { isRoute404or500, isRouteExternalRedirect, isRouteServerIsland } from './routing/match.js';
 import { copyRequest, getOriginPathname, setOriginPathname } from './routing/rewrite.js';
 import { AstroSession } from './session/runtime.js';
+import { collapseDuplicateLeadingSlashes } from '@astrojs/internal-helpers/path';
 import { validateAndDecodePathname } from './util/pathname.js';
 
 /**
@@ -86,6 +87,10 @@ export class RenderContext {
 
 	static #createNormalizedUrl(requestUrl: string): URL {
 		const url = new URL(requestUrl);
+		// Collapse multiple leading slashes so middleware sees the canonical pathname.
+		// Without this, a request to `//admin` would preserve `//admin` in context.url.pathname,
+		// bypassing middleware checks like `pathname.startsWith('/admin')`.
+		url.pathname = collapseDuplicateLeadingSlashes(url.pathname);
 		try {
 			// Decode and validate pathname to prevent multi-level encoding bypass attacks
 			url.pathname = validateAndDecodePathname(url.pathname);
diff --git a/packages/astro/test/units/app/double-slash-bypass.test.js b/packages/astro/test/units/app/double-slash-bypass.test.js
new file mode 100644
index 000000000000..f5defd12b491
--- /dev/null
+++ b/packages/astro/test/units/app/double-slash-bypass.test.js
@@ -0,0 +1,162 @@
+// @ts-check
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { App } from '../../../dist/core/app/app.js';
+import { parseRoute } from '../../../dist/core/routing/parse-route.js';
+import { createComponent, render } from '../../../dist/runtime/server/index.js';
+import { createManifest } from './test-helpers.js';
+
+/**
+ * Security tests for double-slash URL prefix middleware authorization bypass.
+ *
+ * Vulnerability: A normalization inconsistency between route matching and middleware
+ * URL construction allows bypassing middleware-based authorization by prepending an
+ * extra `/` to the URL path (e.g., `//admin` instead of `/admin`).
+ *
+ * - `removeBase("//admin")` strips one slash → router matches `/admin`
+ * - `context.url.pathname` preserves `//admin` → middleware `startsWith("/admin")` fails
+ *
+ * See: withastro/astro-security#5
+ * CWE-647: Use of Non-Canonical URL Paths for Authorization Decisions
+ * CWE-285: Improper Authorization
+ */
+
+const routeOptions = /** @type {Parameters<typeof parseRoute>[1]} */ (
+	/** @type {any} */ ({
+		config: { base: '/', trailingSlash: 'ignore' },
+		pageExtensions: [],
+	})
+);
+
+const adminRouteData = parseRoute('admin', routeOptions, {
+	component: 'src/pages/admin.astro',
+});
+
+const dashboardRouteData = parseRoute('dashboard', routeOptions, {
+	component: 'src/pages/dashboard.astro',
+});
+
+const publicRouteData = parseRoute('index.astro', routeOptions, {
+	component: 'src/pages/index.astro',
+});
+
+const adminPage = createComponent(() => {
+	return render`<h1>Admin Panel</h1>`;
+});
+
+const dashboardPage = createComponent(() => {
+	return render`<h1>Dashboard</h1>`;
+});
+
+const publicPage = createComponent(() => {
+	return render`<h1>Public</h1>`;
+});
+
+const pageMap = new Map([
+	[
+		adminRouteData.component,
+		async () => ({
+			page: async () => ({
+				default: adminPage,
+			}),
+		}),
+	],
+	[
+		dashboardRouteData.component,
+		async () => ({
+			page: async () => ({
+				default: dashboardPage,
+			}),
+		}),
+	],
+	[
+		publicRouteData.component,
+		async () => ({
+			page: async () => ({
+				default: publicPage,
+			}),
+		}),
+	],
+]);
+
+/**
+ * Middleware that blocks access to /admin and /dashboard routes,
+ * as recommended in the official Astro authentication docs.
+ * @returns {() => Promise<{onRequest: import('../../../dist/types/public/common.js').MiddlewareHandler}>}
+ */
+function createAuthMiddleware() {
+	return async () => ({
+		onRequest: /** @type {import('../../../dist/types/public/common.js').MiddlewareHandler} */ (
+			async (context, next) => {
+				const protectedPaths = ['/admin', '/dashboard'];
+				if (protectedPaths.some((p) => context.url.pathname.startsWith(p))) {
+					return new Response('Forbidden', { status: 403 });
+				}
+				return next();
+			}
+		),
+	});
+}
+
+/**
+ * @param {ReturnType<typeof createAuthMiddleware>} middleware
+ */
+function createApp(middleware) {
+	return new App(
+		createManifest({
+			routes: [
+				{ routeData: adminRouteData },
+				{ routeData: dashboardRouteData },
+				{ routeData: publicRouteData },
+			],
+			pageMap,
+			middleware,
+		}),
+	);
+}
+
+describe('Security: double-slash URL prefix middleware bypass', () => {
+	it('middleware blocks /admin with normal request', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/admin');
+		const response = await app.render(request);
+		assert.equal(response.status, 403, '/admin should be blocked by middleware');
+	});
+
+	it('middleware blocks //admin (double-slash bypass attempt)', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com//admin');
+		const response = await app.render(request);
+		assert.equal(response.status, 403, '//admin should also be blocked by middleware');
+	});
+
+	it('middleware blocks ///admin (triple-slash bypass attempt)', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com///admin');
+		const response = await app.render(request);
+		assert.equal(response.status, 403, '///admin should also be blocked by middleware');
+	});
+
+	it('middleware blocks //dashboard (double-slash on another protected route)', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com//dashboard');
+		const response = await app.render(request);
+		assert.equal(response.status, 403, '//dashboard should also be blocked by middleware');
+	});
+
+	it('middleware blocks //admin/ (double-slash with trailing slash)', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com//admin/');
+		const response = await app.render(request);
+		assert.equal(response.status, 403, '//admin/ should also be blocked by middleware');
+	});
+
+	it('public route is still accessible', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/');
+		const response = await app.render(request);
+		assert.equal(response.status, 200, '/ should be accessible');
+		const html = await response.text();
+		assert.match(html, /Public/);
+	});
+});
diff --git a/packages/astro/test/units/app/test-helpers.js b/packages/astro/test/units/app/test-helpers.js
index 1a7cb3c89997..9a48dd887dd7 100644
--- a/packages/astro/test/units/app/test-helpers.js
+++ b/packages/astro/test/units/app/test-helpers.js
@@ -1,10 +1,24 @@
 // @ts-check
 
-export function createManifest({ routes, pageMap, base = '/', trailingSlash = 'ignore' } = {}) {
+/**
+ * @param {object} [options]
+ * @param {any[]} [options.routes]
+ * @param {Map<string, Function>} [options.pageMap]
+ * @param {string} [options.base]
+ * @param {string} [options.trailingSlash]
+ * @param {Function} [options.middleware]
+ */
+export function createManifest({
+	routes,
+	pageMap,
+	base = '/',
+	trailingSlash = 'ignore',
+	middleware = undefined,
+} = {}) {
 	const rootDir = new URL('file:///astro-test/');
 	const buildDir = new URL('file:///astro-test/dist/');
 
-	return {
+	return /** @type {import('../../../dist/core/app/types.js').SSRManifest} */ ({
 		adapterName: 'test-adapter',
 		routes,
 		site: undefined,
@@ -16,6 +30,7 @@ export function createManifest({ routes, pageMap, base = '/', trailingSlash = 'i
 		assetsPrefix: undefined,
 		renderers: [],
 		serverLike: true,
+		middlewareMode: /** @type {'classic'} */ ('classic'),
 		clientDirectives: new Map(),
 		entryModules: {},
 		inlinedScripts: new Map(),
@@ -26,11 +41,12 @@ export function createManifest({ routes, pageMap, base = '/', trailingSlash = 'i
 		serverIslandMappings: undefined,
 		key: Promise.resolve(/** @type {CryptoKey} */ ({})),
 		i18n: undefined,
-		middleware: undefined,
+		middleware,
 		actions: undefined,
 		sessionDriver: undefined,
 		checkOrigin: false,
 		allowedDomains: undefined,
+		actionBodySizeLimit: 0,
 		sessionConfig: undefined,
 		cacheDir: rootDir,
 		srcDir: rootDir,
@@ -54,7 +70,7 @@ export function createManifest({ routes, pageMap, base = '/', trailingSlash = 'i
 		experimentalQueuedRendering: {
 			enabled: false,
 		},
-	};
+	});
 }
 
 export function createRouteInfo(routeData) {
diff --git a/packages/internal-helpers/src/path.ts b/packages/internal-helpers/src/path.ts
index 3f551ab433ac..060c8ca4e4d5 100644
--- a/packages/internal-helpers/src/path.ts
+++ b/packages/internal-helpers/src/path.ts
@@ -15,6 +15,15 @@ export function prependForwardSlash(path: string) {
 	return path[0] === '/' ? path : '/' + path;
 }
 
+export const MANY_LEADING_SLASHES = /^\/{2,}/;
+
+export function collapseDuplicateLeadingSlashes(path: string) {
+	if (!path) {
+		return path;
+	}
+	return path.replace(MANY_LEADING_SLASHES, '/');
+}
+
 export const MANY_TRAILING_SLASHES = /\/{2,}$/g;
 
 export function collapseDuplicateTrailingSlashes(path: string, trailingSlash: boolean) {