PT-2026-53025 · Npm · Js-Toml

Published

2026-06-26

·

Updated

2026-06-26

·

CVE-2026-50029

CVSS v3.1

5.3

Medium

VectorAV: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 (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] || [];

Fix

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Weakness Enumeration

CWE-697

Related Identifiers

CVE-2026-50029
GHSA-M34P-749J-X6M6

Affected Products

Js-Toml