[system] Prevent prototype pollution in cssVarsParser#48822
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July 20, 2026 16:45
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assignNestedKeyswalked and wrote through key paths without guarding__proto__/constructor/prototype, so a theme containing such a nested key (e.g. one parsed from untrusted JSON) could write ontoObject.prototype. Added the same key blocklist already used byfastDeepAssign(#48580).Blast radius is very small in practice: the theme passed to
createTheme/extendTheme/cssVarsParseris developer-authored config and essentially never comes from user input — this is defensive hardening on the exportedunstable_cssVarsParsersurface.