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Reject __proto__ array hash keys and use plain assignment #328

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Reject __proto__ array hash keys and use plain assignment #328

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5446161
Use plain assignment for array hash keys except __proto__
keichi Oct 18, 2026
d13b033
Reject __proto__ keys in associative arrays instead of defining them
keichi Oct 18, 2026
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11 changes: 7 additions & 4 deletions lib/binary_parser.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1356,10 +1356,13 @@ export class Parser {
}

if (isHash) {
// defineProperty so that keys like "__proto__" become own properties
ctx.pushCode(
`Object.defineProperty(${lhs}, ${item}.${key}, { value: ${item}, enumerable: true, writable: true, configurable: true });`,
);
// Reject "__proto__" since assigning it would replace the prototype
const hashKey = ctx.generateTmpVariable();
ctx.pushCode(`var ${hashKey} = ${item}.${key};`);
ctx.pushCode(`if (${hashKey} === "__proto__") {`);
ctx.generateError(`"Invalid key: __proto__"`);
ctx.pushCode("}");
ctx.pushCode(`${lhs}[${hashKey}] = ${item};`);
} else {
ctx.pushCode(`${lhs}.push(${item});`);
}
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12 changes: 2 additions & 10 deletions test/composite_parser.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -299,7 +299,7 @@ function compositeParserTests(
},
});
});
it("should store __proto__ key as an own property of associative arrays", () => {
it("should reject __proto__ key of associative arrays", () => {
const parser = Parser.start().array("a", {
type: Parser.start()
.uint8("l")
Expand All @@ -313,16 +313,8 @@ function compositeParserTests(
...Array.from("__proto__", (c) => c.charCodeAt(0)),
1,
]);
const result = parser.parse(buffer);

expect(Object.keys(result.a)).toStrictEqual(["__proto__"]);
expect(Object.getPrototypeOf(result.a)).toBe(Object.prototype);
expect(result.a.evil).toBeUndefined();
expect(result.a["__proto__"]).toStrictEqual({
l: 9,
k: "__proto__",
evil: 1,
});
expect(() => parser.parse(buffer)).toThrow("Invalid key: __proto__");
});
it("should use formatter to transform parsed array", () => {
const parser = Parser.start().array("data", {
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