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fix: prevent prototype hijack in theme safeMerge (CWE-1321) #5027

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fix: prevent prototype hijack in theme safeMerge (CWE-1321) #5027

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8 changes: 8 additions & 0 deletions src/theme/provider.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,11 @@ export const isPlatformColorSentinel = (v: unknown): boolean =>
typeof v === 'object' &&
('resource_paths' in v || 'semantic' in v || 'dynamic' in v);

// Keys that would otherwise let a crafted `overrides` object (e.g. parsed via
// `JSON.parse` from an external theme config) hijack the prototype of the
// merged output through bracket-notation assignment below.
const UNSAFE_KEYS = new Set(['__proto__', 'constructor', 'prototype']);

export const safeMerge = <T,>(base: T, overrides: unknown): T => {
if (
!base ||
Expand All @@ -45,6 +50,9 @@ export const safeMerge = <T,>(base: T, overrides: unknown): T => {
}
const out: Record<string, unknown> = { ...(base as Record<string, unknown>) };
for (const key of Object.keys(overrides as Record<string, unknown>)) {
if (UNSAFE_KEYS.has(key)) {
continue;
}
out[key] = safeMerge(
(base as Record<string, unknown>)[key],
(overrides as Record<string, unknown>)[key]
Expand Down