next/navigation useRouter returned the module-level app router singleton even when no AppRouterContext provider was mounted. That masked components rendered outside the App Router boundary, diverging from Next.js and hiding integration bugs.
The shim now reads the mounted AppRouterContext and throws the Next.js invariant when it is absent. The App Router browser and SSR roots provide the existing appRouterInstance so legitimate app renders keep the same public router surface.
* fix(app-router): block javascript: URLs in router.push/replace/prefetch
router.push("javascript:alert(1)") currently runs the script. _appRouter.push,
.replace, and .prefetch in shims/navigation.ts forward the href straight to
navigateClientSide, where isExternalUrl matches the javascript: scheme and
falls through to window.location.assign, which executes the URL. router-derived
hrefs (URL params, user content, query state) become an XSS vector.
Next.js blocks dangerous URI schemes at the router boundary and throws
"Next.js has blocked a javascript: URL as a security precaution." See
packages/next/src/client/components/app-router-instance.ts:343,400,440,458 in
the canary tree. Vinext's url-safety.isDangerousScheme already covers
javascript:/data:/vbscript: with the same obfuscation handling Next.js uses,
and Link and Form already gate on it; the App Router programmatic surface was
the remaining gap.
Add assertSafeNavigationHref at the top of push/replace/prefetch, before the
isServer short-circuit so SSR callers fail loudly too. Throw an Error with
the verbatim Next.js message so consumer error tooling can match across both
frameworks.
Test: tests/router-javascript-urls.test.ts ports the assertions from
.nextjs-ref/test/e2e/app-dir/javascript-urls/javascript-urls.test.ts down to
unit scope, covering javascript:/data:/vbscript:, uppercase, embedded tabs,
leading whitespace, and a safe-URL no-throw guard for each method.
* test(router): split ported vs vinext-only dangerous-scheme cases
The header said "Ported from Next.js" but data: and vbscript: are vinext
extensions, not in the upstream e2e suite. Add a Coverage split note that
attributes each scheme: javascript: + obfuscation variants come from
test/e2e/app-dir/javascript-urls/javascript-urls.test.ts; data: and vbscript:
ride on top via shims/url-safety.isDangerousScheme.
* fix app router dangerous redirect gaps
* test address dangerous URL review nits
Nathan Nguyen