diff --git a/packages/core/src/i18n/locale_data_api.ts b/packages/core/src/i18n/locale_data_api.ts
index 21bd136501a6..910a8e169ae5 100644
--- a/packages/core/src/i18n/locale_data_api.ts
+++ b/packages/core/src/i18n/locale_data_api.ts
@@ -102,11 +102,20 @@ export function getLocalePluralCase(locale: string): (value: number) => number {
  */
 export function getLocaleData(normalizedLocale: string): any {
   if (!(normalizedLocale in LOCALE_DATA)) {
-    LOCALE_DATA[normalizedLocale] =
+    const globalLocaleData =
       global.ng &&
       global.ng.common &&
       global.ng.common.locales &&
       global.ng.common.locales[normalizedLocale];
+    // Only cache global locale data when an entry is actually found, to avoid
+    // caching missing lookups. In SSR this cache is process-wide across requests,
+    // so caching `undefined` would retain attacker-controlled locale identifiers
+    // indefinitely. It would also make the `in` check above short-circuit on
+    // subsequent lookups and skip the global fallback.
+    if (globalLocaleData !== undefined) {
+      LOCALE_DATA[normalizedLocale] = globalLocaleData;
+    }
+    return globalLocaleData;
   }
   return LOCALE_DATA[normalizedLocale];
 }
diff --git a/packages/core/test/i18n/locale_data_api_spec.ts b/packages/core/test/i18n/locale_data_api_spec.ts
index e6367f8fa0cd..81be3955007d 100644
--- a/packages/core/test/i18n/locale_data_api_spec.ts
+++ b/packages/core/test/i18n/locale_data_api_spec.ts
@@ -85,6 +85,16 @@ describe('locale data api', () => {
       expect(findLocaleData('de-CH')).toEqual(localeDeCH);
     });
 
+    it('should not cache missing global locale data lookups', () => {
+      const localeEnNZ: any[] = ['en-NZ'];
+
+      expect(findLocaleData('en-NZ')).toEqual(localeEn);
+
+      global.ng.common.locales['en-nz'] = localeEnNZ;
+
+      expect(findLocaleData('en-NZ')).toBe(localeEnNZ);
+    });
+
     it('should find the parent LOCALE_DATA if the exact locale is not available and the parent locale is on the global object', () => {
       expect(findLocaleData('de-BE')).toEqual(localeDe);
     });