js-toml has silent type confusion via falsy-primitive duplicate-key bypass
Date: 2026-06
Affected: npm/js-toml
Attack vector: other
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-50029
CWE: CWE-697
CVSS: 5.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)

### 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();


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