nextjs-server-reference-validation/solution/gold.patch

diff --git a/packages/next/src/server/app-render/action-handler.ts b/packages/next/src/server/app-render/action-handler.ts
index 76b065a1fc..cdaf8464f6 100644
--- a/packages/next/src/server/app-render/action-handler.ts
+++ b/packages/next/src/server/app-render/action-handler.ts
@@ -56,10 +56,15 @@ import {
   type ServerModuleMap,
   getServerActionsManifest,
   getServerModuleMap,
+  getActionNotFoundError,
+  getInvalidServerReferenceIdError,
 } from './manifests-singleton'
 import { isNodeNextRequest, isWebNextRequest } from '../base-http/helpers'
 import { normalizeFilePath } from './segment-explorer-path'
-import { extractInfoFromServerReferenceId } from '../../shared/lib/server-reference-info'
+import {
+  extractInfoFromServerReferenceId,
+  mightBeServerReferenceId,
+} from '../../shared/lib/server-reference-info'
 import type { ServerActionLogInfo } from '../dev/server-action-logger'
 import { RedirectStatusCode } from '../../client/components/redirect-status-code'
 import { synchronizeMutableCookies } from '../async-storage/request-store'
@@ -608,7 +613,11 @@ export async function handleAction({
 
   // If the app has no server actions at all, we can 404 early.
   if (!hasServerActions()) {
-    return handleUnrecognizedFetchAction(getActionNotFoundError(actionId))
+    const error =
+      actionId !== null && !mightBeServerReferenceId(actionId)
+        ? getInvalidServerReferenceIdError(actionId)
+        : getActionNotFoundError(actionId)
+    return handleUnrecognizedFetchAction(error)
   }
 
   if (workStore.isStaticGeneration) {
@@ -1369,25 +1378,27 @@ function getActionModIdOrError(
     throw new InvariantError("Missing 'next-action' header.")
   }
 
-  const actionModId = serverModuleMap[actionId]?.id
+  const entry = serverModuleMap[actionId]
 
-  if (!actionModId) {
-    throw getActionNotFoundError(actionId)
+  if (entry == null) {
+    // The proxy throws for malformed IDs and IDs that are missing from the
+    // manifest. It only returns undefined when the ID collides with a
+    // well-known property name (e.g. `toString`) that the proxy excludes from
+    // server reference validation so that framework reflection probes don't
+    // throw. Next.js never produces such an ID, so this is almost certainly a
+    // probe with a known-bad ID. Repeat the manifest's throw logic here so the
+    // caller gets a diagnosable error instead of a `TypeError`.
+    throw mightBeServerReferenceId(actionId)
+      ? getActionNotFoundError(actionId)
+      : getInvalidServerReferenceIdError(actionId)
   }
 
-  return actionModId
-}
-
-function getActionNotFoundError(actionId: string | null): Error {
-  return new Error(
-    `Failed to find Server Action${actionId ? ` "${actionId}"` : ''}. This request might be from an older or newer deployment.\nRead more: https://nextjs.org/docs/messages/failed-to-find-server-action`
-  )
+  return entry.id
 }
 
 const $ACTION_ = '$ACTION_'
 const $ACTION_REF_ = '$ACTION_REF_'
 const $ACTION_ID_ = '$ACTION_ID_'
-const ACTION_ID_EXPECTED_LENGTH = 42
 
 /**
  * This function mirrors logic inside React's decodeAction and should be kept in sync with that.
@@ -1446,14 +1457,14 @@ function isInvalidStringActionDescriptor(
   }
 
   const from = ACTION_DESCRIPTOR_ID_PREFIX.length
-  const to = from + ACTION_ID_EXPECTED_LENGTH
+  const to = actionDescriptor.indexOf('"', from)
+  if (to === -1) {
+    return true
+  }
 
   // We expect actionDescriptor to be '{"id":"<actionId>",...}'
   const actionId = actionDescriptor.slice(from, to)
-  if (
-    actionId.length !== ACTION_ID_EXPECTED_LENGTH ||
-    actionDescriptor[to] !== '"'
-  ) {
+  if (!mightBeServerReferenceId(actionId)) {
     return true
   }
 
@@ -1473,15 +1484,13 @@ function isInvalidActionIdFieldName(
   // The field name must always start with $ACTION_ID_ but since it is
   // the id is extracted from the key of the field we have already validated
   // this before entering this function
-  if (
-    actionIdFieldName.length !==
-    $ACTION_ID_.length + ACTION_ID_EXPECTED_LENGTH
-  ) {
+  const actionId = actionIdFieldName.slice($ACTION_ID_.length)
+  if (!mightBeServerReferenceId(actionId)) {
     // this field name has too few or too many characters
+    // or it is otherwise in the wrong format
     return true
   }
 
-  const actionId = actionIdFieldName.slice($ACTION_ID_.length)
   const entry = serverModuleMap[actionId]
 
   if (entry == null) {
diff --git a/packages/next/src/server/app-render/manifests-singleton.ts b/packages/next/src/server/app-render/manifests-singleton.ts
index 5df89c9881..e58e85aa70 100644
--- a/packages/next/src/server/app-render/manifests-singleton.ts
+++ b/packages/next/src/server/app-render/manifests-singleton.ts
@@ -5,6 +5,8 @@ import { InvariantError } from '../../shared/lib/invariant-error'
 import { normalizeAppPath } from '../../shared/lib/router/utils/app-paths'
 import { pathHasPrefix } from '../../shared/lib/router/utils/path-has-prefix'
 import { removePathPrefix } from '../../shared/lib/router/utils/remove-path-prefix'
+import { mightBeServerReferenceId } from '../../shared/lib/server-reference-info'
+import { wellKnownProperties } from '../../shared/lib/utils/reflect-utils'
 import { workAsyncStorage } from './work-async-storage.external'
 
 export interface ServerModuleMap {
@@ -16,6 +18,37 @@ export interface ServerModuleMap {
   }
 }
 
+export function getActionNotFoundError(actionId: string | null): Error {
+  return new Error(
+    `Failed to find Server Action${actionId ? ` "${actionId}"` : ''}. This request might be from an older or newer deployment.\nRead more: https://nextjs.org/docs/messages/failed-to-find-server-action`
+  )
+}
+
+export function getInvalidServerReferenceIdError(id: string): Error {
+  // `id` is arbitrary client-provided input. Unlike the not-found case, it has
+  // not passed the length gate and can reach this error via a malformed server
+  // reference in an action payload, so it may be of any length and contain
+  // control characters. `JSON.stringify` escapes newlines and quotes so it
+  // can't forge log lines, and truncating overly long ids prevents log
+  // flooding. Ids at or below the cap are logged in full so that we only add an
+  // ellipsis to ids that are meaningfully longer than the truncated length.
+  const encoded = JSON.stringify(
+    id.length > MAX_LOGGED_SERVER_REFERENCE_ID_LENGTH
+      ? id.slice(0, TRUNCATED_SERVER_REFERENCE_ID_LENGTH) + '…'
+      : id
+  )
+
+  return new Error(
+    `The Server Reference ID did not match the expected format. Received ${encoded}.\nRead more: https://nextjs.org/docs/messages/failed-to-find-server-action`
+  )
+}
+
+// Ids at or below the cap are logged in full. Longer ids are truncated to the
+// shorter length and marked with an ellipsis, so the cap leaves headroom over
+// the truncated length rather than ellipsizing ids that are barely too long.
+const MAX_LOGGED_SERVER_REFERENCE_ID_LENGTH = 100
+const TRUNCATED_SERVER_REFERENCE_ID_LENGTH = 90
+
 // This is a global singleton that is, among other things, also used to
 // encode/decode bound args of server function closures. This can't be using a
 // AsyncLocalStorage as it might happen at the module level.
@@ -179,48 +212,57 @@ function createProxiedClientReferenceManifest(
  * runtime, workers, etc. that React doesn't need to know.
  */
 function createServerModuleMap(): ServerModuleMap {
-  return new Proxy(
-    {},
-    {
-      get: (_, id: string) => {
-        const workers =
-          getServerActionsManifest()[
-            process.env.NEXT_RUNTIME === 'edge' ? 'edge' : 'node'
-          ]?.[id]?.workers
+  return new Proxy(Object.create(null) as ServerModuleMap, {
+    get: (target, id: string | symbol, receiver) => {
+      // React's debug serialization can probe the module map like a plain object.
+      // These probes are not server reference lookups.
+      if (typeof id !== 'string') {
+        return Reflect.get(target, id, receiver)
+      }
 
-        if (!workers) {
-          return undefined
-        }
+      if (wellKnownProperties.has(id)) {
+        return Reflect.get(target, id, receiver)
+      }
 
-        const workStore = workAsyncStorage.getStore()
+      if (!mightBeServerReferenceId(id)) {
+        throw getInvalidServerReferenceIdError(id)
+      }
 
-        let workerEntry:
-          | { moduleId: string | number; async: boolean }
-          | undefined
-
-        if (workStore) {
-          workerEntry = workers[normalizeWorkerPageName(workStore.page)]
-        } else {
-          // If there's no work store defined, we can assume that a server
-          // module map is needed during module evaluation, e.g. to create a
-          // server action using a higher-order function. Therefore it should be
-          // safe to return any entry from the manifest that matches the action
-          // ID. They all refer to the same module ID, which must also exist in
-          // the current page bundle. TODO: This is currently not guaranteed in
-          // Turbopack, and needs to be fixed.
-          workerEntry = Object.values(workers).at(0)
-        }
+      const workers =
+        getServerActionsManifest()[
+          process.env.NEXT_RUNTIME === 'edge' ? 'edge' : 'node'
+        ]?.[id]?.workers
 
-        if (!workerEntry) {
-          return undefined
-        }
+      if (!workers) {
+        throw getActionNotFoundError(id)
+      }
 
-        const { moduleId, async } = workerEntry
+      const workStore = workAsyncStorage.getStore()
+
+      let workerEntry: { moduleId: string | number; async: boolean } | undefined
+
+      if (workStore) {
+        workerEntry = workers[normalizeWorkerPageName(workStore.page)]
+      } else {
+        // If there's no work store defined, we can assume that a server
+        // module map is needed during module evaluation, e.g. to create a
+        // server action using a higher-order function. Therefore it should be
+        // safe to return any entry from the manifest that matches the action
+        // ID. They all refer to the same module ID, which must also exist in
+        // the current page bundle. TODO: This is currently not guaranteed in
+        // Turbopack, and needs to be fixed.
+        workerEntry = Object.values(workers).at(0)
+      }
 
-        return { id: moduleId, name: id, chunks: [], async }
-      },
-    }
-  )
+      if (!workerEntry) {
+        throw getActionNotFoundError(id)
+      }
+
+      const { moduleId, async } = workerEntry
+
+      return { id: moduleId, name: id, chunks: [], async }
+    },
+  })
 }
 
 /**
diff --git a/packages/next/src/shared/lib/server-reference-info.ts b/packages/next/src/shared/lib/server-reference-info.ts
index b8d27b08f7..6dd8687957 100644
--- a/packages/next/src/shared/lib/server-reference-info.ts
+++ b/packages/next/src/shared/lib/server-reference-info.ts
@@ -4,6 +4,12 @@ export interface ServerReferenceInfo {
   hasRestArgs: boolean
 }
 
+export const SERVER_REFERENCE_ID_LENGTH = 42
+
+export function mightBeServerReferenceId(id: string): boolean {
+  return id.length === SERVER_REFERENCE_ID_LENGTH
+}
+
 /**
  * Extracts info about the server reference for the given server reference ID by
  * parsing the first byte of the hex-encoded ID.