Ensure checkout cancel_to param is a relative url (#96514)

* Use a url origin check to confirm path is a relative url

* Add tests around cancel_to

client/my-sites/checkout/src/lib/leave-checkout.ts

@@ -102,7 +102,7 @@ export const leaveCheckout = ( {
 		if ( searchParams.has( 'cancel_to' ) ) {
 			const cancelPath = searchParams.get( 'cancel_to' ) ?? '';
 			// Only allow redirecting to relative paths.
-			if ( cancelPath.match( /^\/(?!\/)/ ) ) {
+			if ( isRelativeUrl( cancelPath ) ) {
 				navigate( cancelPath );
 				return;
 			}
@@ -115,3 +115,11 @@ export const leaveCheckout = ( {
 
 	navigate( closeUrl );
 };
+
+export function isRelativeUrl( url: string ) {
+	try {
+		return new URL( url, window.location.href ).origin === window.location.origin;
+	} catch {
+		return false;
+	}
+}

client/my-sites/checkout/src/test/leave-checkout.ts

@@ -0,0 +1,126 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import { navigate } from 'calypso/lib/navigate';
+import { leaveCheckout, isRelativeUrl } from '../lib/leave-checkout';
+
+// Mock dependencies
+jest.mock( 'calypso/lib/analytics/tracks', () => ( {
+	recordTracksEvent: jest.fn(),
+} ) );
+
+jest.mock( 'calypso/lib/navigate', () => ( {
+	navigate: jest.fn(),
+} ) );
+
+describe( 'leaveCheckout', () => {
+	beforeEach( () => {
+		// Reset all mocks before each test
+		jest.clearAllMocks();
+		// Setup window.location
+		Object.defineProperty( window, 'location', {
+			value: {
+				href: 'https://wordpress.com/checkout',
+				origin: 'https://wordpress.com',
+				search: '',
+			},
+			writable: true,
+		} );
+	} );
+
+	describe( 'cancel_to parameter handling', () => {
+		it( 'should navigate to cancel_to path when it is a relative URL', () => {
+			window.location.search = '?cancel_to=/home';
+
+			leaveCheckout( { tracksEvent: 'checkout_cancel' } );
+
+			expect( navigate ).toHaveBeenCalledWith( '/home' );
+		} );
+
+		it( 'should not navigate to cancel_to path when it is an absolute URL', () => {
+			window.location.search = '?cancel_to=https://example.com';
+
+			leaveCheckout( { tracksEvent: 'checkout_cancel' } );
+
+			expect( navigate ).not.toHaveBeenCalledWith( 'https://example.com' );
+		} );
+
+		it( 'should not navigate to cancel_to path when it is a protocol-relative URL', () => {
+			window.location.search = '?cancel_to=//example.com';
+
+			leaveCheckout( { tracksEvent: 'checkout_cancel' } );
+
+			expect( navigate ).not.toHaveBeenCalledWith( '//example.com' );
+		} );
+
+		it( 'should not navigate to cancel_to path when it is trying to bypass relative path check', () => {
+			window.location.search = '?cancel_to=/\\example.com';
+
+			leaveCheckout( { tracksEvent: 'checkout_cancel' } );
+
+			expect( navigate ).not.toHaveBeenCalledWith( '/\\example.com' );
+		} );
+	} );
+} );
+
+describe( 'isRelativeUrl', () => {
+	beforeEach( () => {
+		Object.defineProperty( window, 'location', {
+			value: {
+				href: 'https://wordpress.com/checkout',
+				origin: 'https://wordpress.com',
+			},
+			writable: true,
+		} );
+	} );
+
+	describe( 'relative paths', () => {
+		const testCases = [
+			'/home',
+			'/path/to/page',
+			'path',
+			'./path',
+			'../path',
+			'https://wordpress.com/path',
+			'wordpress.com/path',
+			'//wordpress.com/path',
+			'/\\wordpress.com/path',
+		];
+
+		testCases.forEach( ( path ) => {
+			it( `should return true for relative path: ${ path }`, () => {
+				expect( isRelativeUrl( path ) ).toBe( true );
+			} );
+		} );
+	} );
+
+	describe( 'absolute URLs', () => {
+		const testCases = [
+			'http://example.com',
+			'//example.com',
+			'https://example.com/path',
+			'/\\example.com',
+		];
+
+		testCases.forEach( ( url ) => {
+			it( `should return false for absolute URL: ${ url }`, () => {
+				expect( isRelativeUrl( url ) ).toBe( false );
+			} );
+		} );
+	} );
+
+	describe( 'invalid URLs', () => {
+		const testCases = [
+			'javascript:alert(1)',
+			'data:text/html,<script>alert(1)</script>',
+			'mailto:[email protected]',
+		];
+
+		testCases.forEach( ( url ) => {
+			it( `should return false for invalid URL: ${ url }`, () => {
+				expect( isRelativeUrl( url ) ).toBe( false );
+			} );
+		} );
+	} );
+} );