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Server Action POSTs Are Retargeted By Non-Next History.PushState/ReplaceState Calls

Next.js patches window.history.pushState/replaceState and treats any external call that lacks the internal __NA/_N state as a navigation event. The provided URL is passed unfiltered to ACTION_RESTORE, which overwrites state.canonicalUrl. Server Action dispatches then POST to this corrupted canonical URL instead of the original page route, causing silent submission loss, execution with corrupted dynamic params, or unexpected redirects.

highConfidence 92%Next.js Affected V16.2.12

Origin Analysis

In next/dist/client/components/app-router.js, applyUrlFromHistoryPushReplace forwards the caller-supplied URL directly to dispatchAppRouterAction({ type: ACTION_RESTORE, url: new URL(url ?? href, href) }) without validating whether the URL is a legitimate app route or differs only in query string. The server-action-reducer subsequently uses state.canonicalUrl (now set from this external URL) as the fetch endpoint for Server Actions. This design flaw allows any script that calls history.replaceState with a pathname to silently change where Server Action POSTs are sent.

1. Clone https://github.com/FahadAlazemi/next-server-action-history-retarget 2. Run npm install && npm run build && npm start 3. Open http://localhost:3000/ar/signup (route app/[lang]/signup/page.tsx) 4. In devtools console run: history.replaceState({}, '', '/[lang]/signup') 5. Click Sign up and observe the POST goes to /[lang]/signup instead of /ar/signup, resulting in 404 or an unexpected response error.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Applications that intentionally rely on external pathname history updates to sync usePathname or trigger shallow navigation will no longer work as expected. Before deploying, monitor any analytics, A/B testing, or third-party scripts that might call replaceState with a different pathname. Rollback recommendation: if this breaks legitimate flows, revert this patch and instead modify server-action-reducer to derive the POST endpoint from the route tree or action metadata rather than state.canonicalUrl. Validate that url is same-origin and avoid unescaped dynamic segment placeholders.