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[bugfix] Fix locale('__proto__') corrupting the global locale - #6442
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mattjohnsonpint merged 2 commits intoSep 14, 2026
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The internal locale registry is a plain object, so loadLocale() checked whether a locale was already loaded with locales[name] !== undefined. For names like __proto__, constructor, or prototype, that lookup walks the prototype chain and returns an inherited value instead of a real locale entry, which then gets stored as the global locale and breaks every later formatting call until a valid locale is set again. Switch both lookups in loadLocale to hasOwnProp so an unrecognized name is always treated as not found, the same way it would be for any other unknown locale string.
mattjohnsonpint
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Thanks for the contribution and for identifying this bug. I added a small follow-up to guard the final locale registry lookup as well. Without that guard, browser builds could still resolve inherited properties such as I also expanded the tests to cover the standard |
mattjohnsonpint
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Sep 14, 2026
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Reported in #6440.
loadLocale()checks whether a name is already loaded withlocales[name] !== undefined, and sincelocalesis a plain object, that lookup falls through toObject.prototypefor names like__proto__,constructor, orprototype. The result then gets assigned as the global locale bygetSetGlobalLocale, and every later call toformat/calendar/fromNowthrows becauseObject.prototypeobviously isn't a realLocale.Repro against current develop:
The fix swaps both lookups in
loadLocaleto use the existinghasOwnProphelper (already used elsewhere in this file) instead of a plain property check, so an own-property-only lookup treats these names the same as any other locale that was never registered: not found, warn, keep the current locale.Added a regression test in
src/test/moment/locale.jscovering__proto__,constructor,prototype, andtoString. Ranpnpm validate(lint + full suite, 3930 tests / 164926 assertions) clean.