diff --git a/.changeset/reject-double-encoded-paths.md b/.changeset/reject-double-encoded-paths.md
new file mode 100644
index 000000000000..d904898ec86a
--- /dev/null
+++ b/.changeset/reject-double-encoded-paths.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Rejects double-encoded URL paths with a 400 response instead of silently falling back to partial decoding
diff --git a/packages/astro/src/core/app/base.ts b/packages/astro/src/core/app/base.ts
index f5fc8166ea91..528d9d22f0dc 100644
--- a/packages/astro/src/core/app/base.ts
+++ b/packages/astro/src/core/app/base.ts
@@ -22,6 +22,7 @@ import { appSymbol } from '../constants.js';
 import { DefaultErrorHandler } from '../errors/default-handler.js';
 import type { ErrorHandler } from '../errors/handler.js';
 import { setRenderOptions } from './render-options.js';
+import { MultiLevelEncodingError } from '../util/pathname.js';
 import type { WaitUntilHook } from '../wait-until.js';
 import type { AppPipeline } from './pipeline.js';
 import type { SSRManifest } from './types.js';
@@ -448,15 +449,24 @@ export abstract class BaseApp<P extends Pipeline = AppPipeline> {
 		};
 
 		let response: Response;
-		if (this.#fetchHandler instanceof DefaultFetchHandler) {
-			// Fast path: pass options directly, skip Reflect.set/get round-trip
-			Reflect.set(request, appSymbol, this);
-			response = await this.#fetchHandler.renderWithOptions(request, resolvedOptions);
-		} else {
-			// User-provided fetch handler: stamp options + app on the request
-			setRenderOptions(request, resolvedOptions);
-			Reflect.set(request, appSymbol, this);
-			response = await this.#fetchHandler.fetch(request);
+		try {
+			if (this.#fetchHandler instanceof DefaultFetchHandler) {
+				// Fast path: pass options directly, skip Reflect.set/get round-trip
+				Reflect.set(request, appSymbol, this);
+				response = await this.#fetchHandler.renderWithOptions(request, resolvedOptions);
+			} else {
+				// User-provided fetch handler: stamp options + app on the request
+				setRenderOptions(request, resolvedOptions);
+				Reflect.set(request, appSymbol, this);
+				response = await this.#fetchHandler.fetch(request);
+			}
+		} catch (err: any) {
+			// Multi-level encoding (e.g., %2561 → %61) is rejected during URL
+			// normalization in FetchState. Return 400 without rendering an error page.
+			if (err instanceof MultiLevelEncodingError) {
+				return new Response('Bad Request', { status: 400 });
+			}
+			throw err;
 		}
 		this.#warnMissingFeatures();
 		if (response.headers.get(ASTRO_ERROR_HEADER)) {
diff --git a/packages/astro/src/core/util/normalized-url.ts b/packages/astro/src/core/util/normalized-url.ts
index 49f32a2c344c..455d7adb7490 100644
--- a/packages/astro/src/core/util/normalized-url.ts
+++ b/packages/astro/src/core/util/normalized-url.ts
@@ -1,5 +1,5 @@
 import { collapseDuplicateSlashes } from '@astrojs/internal-helpers/path';
-import { validateAndDecodePathname } from './pathname.js';
+import { MultiLevelEncodingError, validateAndDecodePathname } from './pathname.js';
 
 /**
  * Creates a normalized URL from a request URL string.
@@ -16,7 +16,13 @@ export function createNormalizedUrl(requestUrl: string): URL {
 export function normalizeUrl(url: URL): URL {
 	try {
 		url.pathname = validateAndDecodePathname(url.pathname);
-	} catch {
+	} catch (e) {
+		// Multi-level encoding (e.g., %2561 → %61) must be rejected, not silently decoded.
+		// Let this error propagate so the caller can return a 400 response.
+		if (e instanceof MultiLevelEncodingError) {
+			throw e;
+		}
+		// For other decoding failures (truly malformed URLs), fall back gracefully.
 		try {
 			url.pathname = decodeURI(url.pathname);
 		} catch {
diff --git a/packages/astro/src/core/util/pathname.ts b/packages/astro/src/core/util/pathname.ts
index ed79de2c1a9a..f8ebf98ffee7 100644
--- a/packages/astro/src/core/util/pathname.ts
+++ b/packages/astro/src/core/util/pathname.ts
@@ -1,3 +1,15 @@
+/**
+ * Error thrown when multi-level URL encoding is detected in a pathname.
+ * This is a distinct error type so callers can handle it specifically
+ * (e.g., returning a 400 response) rather than falling back to partial decoding.
+ */
+export class MultiLevelEncodingError extends Error {
+	constructor() {
+		super('Multi-level URL encoding is not allowed');
+		this.name = 'MultiLevelEncodingError';
+	}
+}
+
 /**
  * Validates that a pathname is not multi-level encoded.
  * Detects if a pathname contains encoding that was encoded again (e.g., %2561dmin where %25 decodes to %).
@@ -5,7 +17,8 @@
  *
  * @param pathname - The pathname to validate
  * @returns The decoded pathname if valid
- * @throws Error if multi-level encoding is detected
+ * @throws MultiLevelEncodingError if multi-level encoding is detected
+ * @throws Error if the pathname contains invalid URL encoding
  */
 export function validateAndDecodePathname(pathname: string): string {
 	let decoded: string;
@@ -24,7 +37,7 @@ export function validateAndDecodePathname(pathname: string): string {
 	const decodedStillHasEncoding = /%[0-9a-fA-F]{2}/.test(decoded);
 
 	if (hasDecoding && decodedStillHasEncoding) {
-		throw new Error('Multi-level URL encoding is not allowed');
+		throw new MultiLevelEncodingError();
 	}
 
 	return decoded;
diff --git a/packages/astro/test/middleware.test.ts b/packages/astro/test/middleware.test.ts
index e391938e03d1..e6919fca6967 100644
--- a/packages/astro/test/middleware.test.ts
+++ b/packages/astro/test/middleware.test.ts
@@ -23,14 +23,14 @@ describe('Middleware in DEV mode', () => {
 	});
 
 	describe('Path encoding in middleware', () => {
-		it('should reject double-encoded paths with 404', async () => {
+		it('should reject double-encoded paths with 400', async () => {
 			const res = await fixture.fetch('/%2561dmin', { redirect: 'manual' });
-			assert.equal(res.status, 404);
+			assert.equal(res.status, 400);
 		});
 
-		it('should reject triple-encoded paths with 404', async () => {
+		it('should reject triple-encoded paths with 400', async () => {
 			const res = await fixture.fetch('/%252561dmin', { redirect: 'manual' });
-			assert.equal(res.status, 404);
+			assert.equal(res.status, 400);
 		});
 	});
 
@@ -132,16 +132,16 @@ describe('Middleware API in PROD mode, SSR', () => {
 	});
 
 	describe('Path encoding in middleware', () => {
-		it('should reject double-encoded paths with 404', async () => {
+		it('should reject double-encoded paths with 400', async () => {
 			const request = new Request('http://example.com/%2561dmin');
 			const response = await app.render(request);
-			assert.equal(response.status, 404);
+			assert.equal(response.status, 400);
 		});
 
-		it('should reject triple-encoded paths with 404', async () => {
+		it('should reject triple-encoded paths with 400', async () => {
 			const request = new Request('http://example.com/%252561dmin');
 			const response = await app.render(request);
-			assert.equal(response.status, 404);
+			assert.equal(response.status, 400);
 		});
 	});
 
diff --git a/packages/astro/test/units/app/double-encoding-bypass.test.ts b/packages/astro/test/units/app/double-encoding-bypass.test.ts
new file mode 100644
index 000000000000..ab61e300f5f3
--- /dev/null
+++ b/packages/astro/test/units/app/double-encoding-bypass.test.ts
@@ -0,0 +1,160 @@
+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 type { MiddlewareHandler } from '../../../dist/types/public/common.js';
+import { createManifest, createRouteInfo } from './test-helpers.ts';
+
+/**
+ * Tests that double-URL-encoded paths do not bypass middleware authorization.
+ *
+ * When a path like /api/%2561dmin/users is received, validateAndDecodePathname
+ * detects the multi-level encoding and must reject the request rather than
+ * silently falling back to a single decodeURI() that leaves middleware
+ * seeing a half-decoded pathname (/api/%61dmin/users) that doesn't match
+ * its authorization checks.
+ */
+
+const routeOptions: Parameters<typeof parseRoute>[1] = {
+	config: { base: '/', trailingSlash: 'ignore' },
+	pageExtensions: [],
+} as any;
+
+// Catch-all API endpoint: /api/[...path]
+// Represents a typical BFF or API proxy behind middleware auth.
+const apiCatchAllRouteData = parseRoute('api/[...path].ts', routeOptions, {
+	component: 'src/pages/api/[...path].ts',
+	type: 'endpoint',
+});
+
+const publicRouteData = parseRoute('index.astro', routeOptions, {
+	component: 'src/pages/index.astro',
+});
+
+const publicPage = createComponent((_result: any, _props: any, _slots: any) => {
+	return render`<h1>Public</h1>`;
+});
+
+const pageMap = new Map<string, any>([
+	[
+		apiCatchAllRouteData.component,
+		async () => ({
+			page: async () => ({
+				GET: async ({ params, url }: { params: Record<string, any>; url: URL }) =>
+					new Response(
+						JSON.stringify({
+							path: params.path,
+							url_pathname: url.pathname,
+						}),
+						{ headers: { 'Content-Type': 'application/json' } },
+					),
+			}),
+		}),
+	],
+	[
+		publicRouteData.component,
+		async () => ({
+			page: async () => ({
+				default: publicPage,
+			}),
+		}),
+	],
+]);
+
+/**
+ * Middleware that blocks access to /api/admin, as documented in Astro's
+ * middleware guide for path-based authorization.
+ */
+function createAuthMiddleware() {
+	return (async () => ({
+		onRequest: (async (context, next) => {
+			if (context.url.pathname.startsWith('/api/admin')) {
+				return new Response(JSON.stringify({ error: 'Unauthorized' }), {
+					status: 401,
+					headers: { 'Content-Type': 'application/json' },
+				});
+			}
+			return next();
+		}) satisfies MiddlewareHandler,
+	})) as () => Promise<{ onRequest: MiddlewareHandler }>;
+}
+
+function createApp(middleware: ReturnType<typeof createAuthMiddleware>) {
+	return new App(
+		createManifest({
+			routes: [createRouteInfo(apiCatchAllRouteData), createRouteInfo(publicRouteData)],
+			pageMap: pageMap as any,
+			middleware: middleware as any,
+		}) as any,
+	);
+}
+
+describe('URL normalization: double-encoding middleware bypass', () => {
+	it('middleware blocks /api/admin/users', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/admin/users');
+		const response = await app.render(request);
+		assert.equal(response.status, 401, '/api/admin/users should be blocked by middleware');
+	});
+
+	it('middleware blocks single-encoded /api/%61dmin/users', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/%61dmin/users');
+		const response = await app.render(request);
+		assert.equal(
+			response.status,
+			401,
+			'/api/%61dmin/users should be blocked (single encoding decodes to /api/admin/users)',
+		);
+	});
+
+	it('rejects double-encoded /api/%2561dmin/users with 400', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/%2561dmin/users');
+		const response = await app.render(request);
+		assert.equal(
+			response.status,
+			400,
+			'/api/%2561dmin/users should be rejected as a bad request, not served',
+		);
+	});
+
+	it('rejects double-encoded paths with multiple encoded segments', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/%2561dmin/%75sers');
+		const response = await app.render(request);
+		assert.equal(
+			response.status,
+			400,
+			'/api/%2561dmin/%75sers should be rejected as a bad request',
+		);
+	});
+
+	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/);
+	});
+
+	it('non-protected API routes are still accessible', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/public/data');
+		const response = await app.render(request);
+		assert.equal(response.status, 200, '/api/public/data should be accessible');
+		const body = await response.json();
+		assert.equal(body.path, 'public/data');
+	});
+
+	it('single-encoded non-admin API routes still work', async () => {
+		const app = createApp(createAuthMiddleware());
+		const request = new Request('http://example.com/api/us%65rs/list');
+		const response = await app.render(request);
+		assert.equal(response.status, 200, '/api/us%65rs/list should be accessible');
+		const body = await response.json();
+		assert.equal(body.path, 'users/list');
+	});
+});