Summary
js-toml has silent type confusion via falsy-primitive duplicate-key bypass
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
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
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 inkey = valuecreateTable,[stdTable]headersgetOrCreateArray,[[arrayOfTables]]headers
PoC
isAdmin = false
[isAdmin]
forced = "yes"
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)
CVE-2026-50029 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.1.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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in src/load/interpreter.ts
export class Interpreter extends BaseCstVisitor {
ignoreImplicitDeclared,
ignoreExplicitDeclared
) {
- if (object[key]) {
+ if (key in object) {
if (
!isPlainObject(object[key]) ||
(!ignoreExplicitDeclared &&
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] || [];
Frequently Asked Questions
- What is CVE-2026-50029? CVE-2026-50029 is a medium-severity security vulnerability in js-toml (npm), affecting versions <= 1.1.1. It is fixed in 1.1.2.
- How severe is CVE-2026-50029? CVE-2026-50029 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of js-toml are affected by CVE-2026-50029? js-toml (npm) versions <= 1.1.1 is affected.
- Is there a fix for CVE-2026-50029? Yes. CVE-2026-50029 is fixed in 1.1.2. Upgrade to this version or later.
- Is CVE-2026-50029 exploitable, and should I be worried? Whether CVE-2026-50029 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-50029 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-50029? Upgrade
js-tomlto 1.1.2 or later.