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fix(agent-script): prevent Object globals from mutating host prototypes #315

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fix(agent-script): prevent Object globals from mutating host prototypes #315

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21 changes: 21 additions & 0 deletions packages/agent-script/src/interp/globals/object-array.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,27 @@ describe("createObjectArrayGlobals", () => {
await expect(getClosure(getProperty(globals.Array, "from")).call(["10", globals.Number])).resolves.toEqual([1, 0]);
});


it("blocks assign and freeze on host prototype objects", async () => {
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@poe-code-agent poe-code-agent Bot May 15, 2026

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Please add coverage for the sandbox object-literal/null-prototype path here too. The current test covers Object.prototype, but not {} from the interpreter, which is the regression path for Object.assign({}, source) and Object.freeze({}).

const globals = createObjectArrayGlobals({
budget: new Budget()
});

expect(() =>
getClosure(getProperty(globals.Object, "assign")).call([
Object.prototype as unknown as SandboxObject,
{
polluted: true
}
])
).toThrowError("Object.assign(target, ...sources) requires an object or array target.");

expect(await getClosure(getProperty(globals.Object, "freeze")).call([Object.prototype as unknown as SandboxObject])).toBe(
Object.prototype
);
expect(Object.isFrozen(Object.prototype)).toBe(false);
});

it("treats sandbox coercion helpers as opaque Object sources", async () => {
const globals = createObjectArrayGlobals({
budget: new Budget()
Expand Down
27 changes: 19 additions & 8 deletions packages/agent-script/src/interp/globals/object-array.ts
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Expand Up @@ -37,13 +37,7 @@ export function createObjectArrayGlobals(options: { budget: Budget }): ObjectArr
name: "fromEntries"
}),
freeze: createSandboxClosure({
call: ([value]) => {
if (typeof value === "object" && value !== null) {
Object.freeze(value);
}

return value;
},
call: ([value]) => freezeSandboxValue(value),
name: "freeze"
}),
assign: createSandboxClosure({
Expand Down Expand Up @@ -80,6 +74,15 @@ export function createObjectArrayGlobals(options: { budget: Budget }): ObjectArr
};
}

function freezeSandboxValue(value: SandboxValue): SandboxValue {
if (!isAssignableSandboxTarget(value)) {
return value;
}

Object.freeze(value);
return value;
}

function assignSandboxValues(target: SandboxValue, sources: readonly SandboxValue[]): SandboxValue {
if (target === null || target === undefined) {
throw new TypeError("Object.assign(target, ...sources) requires a non-null target.");
Expand All @@ -105,7 +108,15 @@ function assignSandboxValues(target: SandboxValue, sources: readonly SandboxValu
function isAssignableSandboxTarget(
value: SandboxValue
): value is SandboxObject & Record<string, SandboxValue> | SandboxValue[] & Record<string, SandboxValue> {
return typeof value === "object" && value !== null && !isSandboxClosure(value) && !isSandboxPromise(value);
if (typeof value !== "object" || value === null || isSandboxClosure(value) || isSandboxPromise(value)) {
return false;
}

if (Array.isArray(value)) {
return true;
}

return Object.getPrototypeOf(value) === Object.prototype;
Comment on lines +115 to +119
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@poe-code-agent poe-code-agent Bot May 15, 2026

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Object literals are created with Object.create(null) in interpreter.ts, so this now rejects ordinary script code like Object.assign({}, { ok: true }) and Object.freeze({}). I verified run('return Object.assign({}, { ok: true }).ok') now throws this TypeError. Please allow sandbox null-prototype objects while still blocking the host prototypes.

Suggested change
if (Array.isArray(value)) {
return true;
}
return Object.getPrototypeOf(value) === Object.prototype;
if (Array.isArray(value)) {
return value !== Array.prototype;
}
const prototype = Object.getPrototypeOf(value);
return prototype === Object.prototype || (prototype === null && value !== Object.prototype);

}

async function arrayFromSandboxValues(args: readonly SandboxValue[], budget: Budget): Promise<SandboxValue> {
Expand Down
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