fix(next): block encoded and escaped open redirects in getSafeRedirect (#11907)

### What

This PR improves the `getSafeRedirect` utility to improve security
around open redirect handling.

### How

- Normalizes and decodes the redirect path using `decodeURIComponent`
- Catches malformed encodings with a try/catch fallback
- Blocks open redirects

Author: PatrikKozak

packages/next/src/utilities/getSafeRedirect.spec.ts

@@ -0,0 +1,55 @@
+import { getSafeRedirect } from './getSafeRedirect'
+
+const fallback = '/admin' // default fallback if the input is unsafe or invalid
+
+describe('getSafeRedirect', () => {
+  // Valid - safe redirect paths
+  it.each([['/dashboard'], ['/admin/settings'], ['/projects?id=123'], ['/hello-world']])(
+    'should allow safe relative path: %s',
+    (input) => {
+      // If the input is a clean relative path, it should be returned as-is
+      expect(getSafeRedirect(input, fallback)).toBe(input)
+    },
+  )
+
+  // Invalid types or empty inputs
+  it.each(['', null, undefined, 123, {}, []])(
+    'should fallback on invalid or non-string input: %s',
+    (input) => {
+      // If the input is not a valid string, it should return the fallback
+      expect(getSafeRedirect(input as any, fallback)).toBe(fallback)
+    },
+  )
+
+  // Unsafe redirect patterns
+  it.each([
+    '//example.com', // protocol-relative URL
+    '/javascript:alert(1)', // JavaScript scheme
+    '/JavaScript:alert(1)', // case-insensitive JavaScript
+    '/http://unknown.com', // disguised external redirect
+    '/https://unknown.com', // disguised external redirect
+    '/%2Funknown.com', // encoded slash — could resolve to //
+    '/\\/unknown.com', // escaped slash
+    '/\\\\unknown.com', // double escaped slashes
+    '/\\unknown.com', // single escaped slash
+    '%2F%2Funknown.com', // fully encoded protocol-relative path
+    '%2Fjavascript:alert(1)', // encoded JavaScript scheme
+  ])('should block unsafe redirect: %s', (input) => {
+    // All of these should return the fallback because they’re unsafe
+    expect(getSafeRedirect(input, fallback)).toBe(fallback)
+  })
+
+  // Input with extra spaces should still be properly handled
+  it('should trim whitespace before evaluating', () => {
+    // A valid path with surrounding spaces should still be accepted
+    expect(getSafeRedirect('   /dashboard   ', fallback)).toBe('/dashboard')
+
+    // An unsafe path with spaces should still be rejected
+    expect(getSafeRedirect('   //example.com   ', fallback)).toBe(fallback)
+  })
+
+  // If decoding the input fails (e.g., invalid percent encoding), it should not crash
+  it('should return fallback on invalid encoding', () => {
+    expect(getSafeRedirect('%E0%A4%A', fallback)).toBe(fallback)
+  })
+})

packages/next/src/utilities/getSafeRedirect.ts

@@ -6,14 +6,25 @@ export const getSafeRedirect = (
     return fallback
   }
 
-  // Ensures that any leading or trailing whitespace doesn’t affect the checks
-  const redirectPath = redirectParam.trim()
+  // Normalize and decode the path
+  let redirectPath: string
+  try {
+    redirectPath = decodeURIComponent(redirectParam.trim())
+  } catch {
+    return fallback // invalid encoding
+  }
 
   const isSafeRedirect =
     // Must start with a single forward slash (e.g., "/admin")
     redirectPath.startsWith('/') &&
-    // Prevent protocol-relative URLs (e.g., "//evil.com")
+    // Prevent protocol-relative URLs (e.g., "//example.com")
     !redirectPath.startsWith('//') &&
+    // Prevent encoded slashes that could resolve to protocol-relative
+    !redirectPath.startsWith('/%2F') &&
+    // Prevent backslash-based escape attempts (e.g., "/\\/example.com", "/\\\\example.com", "/\\example.com")
+    !redirectPath.startsWith('/\\/') &&
+    !redirectPath.startsWith('/\\\\') &&
+    !redirectPath.startsWith('/\\') &&
     // Prevent javascript-based schemes (e.g., "/javascript:alert(1)")
     !redirectPath.toLowerCase().startsWith('/javascript:') &&
     // Prevent attempts to redirect to full URLs using "/http:" or "/https:"

test/admin-root/payload-types.ts

@@ -54,6 +54,7 @@ export type SupportedTimezones =
   | 'Asia/Singapore'
   | 'Asia/Tokyo'
   | 'Asia/Seoul'
+  | 'Australia/Brisbane'
   | 'Australia/Sydney'
   | 'Pacific/Guam'
   | 'Pacific/Noumea'