[utils] Prevent prototype pollution in fastDeepAssign#48580
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JSON.parse creates a literal `__proto__` own property without triggering the prototype setter. `for (const key in source)` enumerates that key, and `target[key] = ...` then triggers the setter on Object.prototype, polluting the global prototype. This affects every consumer of `unstable_fastDeepAssign` (the merge used by `sx`, `compose`, and `breakpoints` in `@mui/system`), where the source object can be application-provided at runtime. Skip `__proto__`, `constructor`, and `prototype` in `mergeObject` and `cloneObject`. Adds regression tests.
Janpot
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romgrk
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May 27, 2026
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Summary
Closes the prototype-pollution code-scanning alert against
fastDeepAssign.JSON.parsecreates a literal__proto__own property without triggering the prototype setter;for (const key in source)then enumerates that key andtarget[key] = ...triggers the setter onObject.prototype, polluting the global prototype.unstable_fastDeepAssignis exported from@mui/utilsand used by@mui/systemforsx/compose/breakpoints, where the source object can be application-provided at runtime — so this is reachable from real consumer code, not just tests.The fix skips
__proto__,constructor, andprototypein bothmergeObjectandcloneObject. Same pattern used by reputable deepmerge implementations.