GHSA-m34p-749j-x6m6

### Summary

`js-toml`'s interpreter checks whether a key already exists in a parser-built container with `if (object[key])` instead of `if (key in object)`. When the prior value is a falsy primitive — `false`, `0`, `0n`, `0.0`, `-0`, or `""` — the duplicate-key branch is skipped and the value is silently overwritten by a later sub-table, dotted-key sub-table, or array-of-tables sharing the same name. Per the TOML 1.0.0 spec ("Defining a key multiple times is invalid"; "You cannot define any key or table more than once"), this should be a parse error.

The result is **structural type confusion of attacker-named keys** in the value returned by `load()`. A boolean-typed `false` (or numeric `0`) becomes a truthy object. Host applications that gate behavior on `if (config.flag)`, `if (!user.banned)`, `if (config.allowDelete)`, or `if (config.publicMode)` will silently take the truthy branch.

This is **distinct** from [GHSA-65fc-cr5f-v7r2](https://github.com/sunnyadn/js-toml/security/advisories/GHSA-65fc-cr5f-v7r2) (the 1.0.2 prototype-pollution fix). `Object.prototype` is **not** polluted. The `Object.create(null)` mitigation from 1.0.2 is intact; the bug here is in the duplicate-key state machine, not in container construction.

### Details

Two truthy checks are wrong:

`src/load/interpreter.ts:214` — `Interpreter.tryCreatingObject`

```js if (object[key]) { // falsy primitives slip through // duplicate-key logic } else { object[key] = createSafeObject(); // silently overwrites the prior falsy value ... } ```

`src/load/interpreter.ts:278` — `Interpreter.getOrCreateArray`

```js if (object[first] && !Array.isArray(object[first])) { // same flaw throw new DuplicateKeyError(); } object[first] = object[first] || []; // overwrites the prior falsy value ```

Both should use the `in` operator. Containers are created via `Object.create(null)`, so `in` is unambiguous (no inherited keys to worry about).

The bug is reachable through every parent-walking interpreter path:

- `assignValue` — dotted keys in `key = value` - `createTable` — `[stdTable]` headers - `getOrCreateArray` — `[[arrayOfTables]]` headers

### PoC

```toml isAdmin = false [isAdmin] forced = "yes" ```

```js import { load } from 'js-toml';

const config = load(` isAdmin = false [isAdmin] forced = "yes" `);

console.log(JSON.stringify(config)); // {"isAdmin":{"forced":"yes"}}

console.log(config.isAdmin ? 'BYPASS' : 'safe'); // BYPASS

if (config.isAdmin) { // attacker reaches admin-only code } ```

### Impact

Spec-violating input acceptance leading to structural type confusion. (CWE-697)

### Suggested fix

in `src/load/interpreter.ts`

```diff export class Interpreter extends BaseCstVisitor { ignoreImplicitDeclared, ignoreExplicitDeclared ) { - if (object[key]) { + if (key in object) { if ( !isPlainObject(object[key]) || (!ignoreExplicitDeclared && ``` ```diff export class Interpreter extends BaseCstVisitor { return this.getOrCreateArray(keys, object[first], idx + 1); }

- if (object[first] && !Array.isArray(object[first])) { + if (first in object && !Array.isArray(object[first])) { throw new DuplicateKeyError(); }

object[first] = object[first] || []; ```