toml-node: Quadratic-complexity DoS via long dotted keys or table paths in toml.parse()

Summary

toml.parse() is vulnerable to a quadratic-complexity (O(n²)) denial-of-service via a single long dotted key, table header, or inline-table key. A ~100KB crafted document causes a multi-second synchronous hang; a ~200KB document crashes the Node.js process outright with an uncatchable out-of-memory error. This is a separate, unpatched code path from the previously fixed stack-overflow DoS (GHSA-82x6-q7mm-w9cf) — it requires no array/inline-table nesting at all, and is confirmed unaffected by the maxDepth option that fixed that earlier issue.

Affected component / version

• Package: toml (npm), repository BinaryMuse/toml-node
• Functions: assign(), deepRef(), setNestedKey() in lib/compiler.js
• Confirmed present in the audited checkout at commit 1b2ac40 (2026-07-16), one commit past the v5.0.0 tag (a changelog-only change) — i.e. present in v5.0.0, the latest release. Not exhaustively verified against every prior release, but the affected functions are unrelated to v5.0.0's BigInt/Temporal changes (per CHANGELOG.md), so the defect likely predates that release as well.

Details

toml.parse(input, options) (index.js:5-9) forwards the caller-supplied string to parser.parse() and then compiler.compile() with no length or structural validation. Inside lib/compiler.js, three separate functions each walk a dotted key's segments in a for loop and, on every iteration, recompute the full path-prefix string from scratch:

• assign() (lines 69-107, the sink at line 81): var intermediatePath = makeFullPath(keys.slice(0, i + 1)); — reached for top-level dotted-key assignments like a.b.c = 1.
• deepRef() (lines 247-281, the sink at line 252): the identical pattern via pathKey(keys.slice(0, i + 1)) — reached from table headers [a.b.c] and array-of-tables [[a.b.c]] via setPath()/addTableArray().
• setNestedKey() (lines 184-202, the sink at line 187): the identical pattern — reached from inline-table dotted keys like t = { a.b.c = 1 }.

keys.slice(0, i + 1) allocates a new array of length i; pathKey()/makeFullPath() (lines 311-317) then does .map(quoteDottedString).join("."), also O(i). Since this runs on every one of the n segments of a dotted key, total work is O(1 + 2 + ... + n) = O(n²), not O(n). The string is computed unconditionally on the happy path (only ever used if an error is about to be thrown), so there is no early exit that makes this cheap in practice.

No limit exists anywhere on dotted-key segment count: dot_ended_table_key_part+ and inline_dot_key_part+ in src/toml.pegjs (lines 122, 311) are unbounded + repetitions, parsed iteratively rather than through the grammar's value rule — so the existing MAX_DEPTH/maxDepth nesting guard (src/toml.pegjs:141-151, enforced in the generated lib/parser.js:1022) does not apply to key length at all. We confirmed this directly: setting options.maxDepth: 1 (the strictest possible value) against a 16,000-segment key produced no measurable timing difference from the unrestricted case.

Proof of Concept

const toml = require('toml');

// n-segment dotted key: "k0.k1.k2. ... .k{n-1} = 1"
function segKey(n) {
  const parts = [];
  for (let i = 0; i < n; i++) parts.push('k' + i);
  return parts.join('.') + ' = 1\n';
}

// ~100.9 KB input -- takes ~6.6-6.8 seconds
toml.parse(segKey(16000));

// ~200 KB input -- crashes the Node process:
// "FATAL ERROR: Ineffective mark-compacts near heap limit - JavaScript heap out of memory"
toml.parse(segKey(32000));

Measured timings (Node v22, direct execution against this repo):

Doubling n from 8,000→16,000 roughly quadruples runtime (~4.2x), the signature of O(n²) rather than the expected O(n) (~2x). Independently reproduced with the identical scaling signature for the table-header variant ([k0. ... .k15999], exercising deepRef()) and the inline-table variant (t = { k0. ... .k15999 = 1 }, exercising setNestedKey()).

We also generalized this into a property-based fuzzing harness (fast-check, run under Node) that automatically tests the "doubling input size should not more than roughly double parse time" invariant across 12 structural dimensions of the grammar/compiler; it independently re-flags all three code paths above on every run, with no additional dimension confirmed vulnerable after follow-up investigation. The harness is available on request.

Impact

Node.js is single-threaded — a synchronous multi-second-to-crashing parse() call blocks the entire event loop of the process calling it, stalling every other request/operation that process is handling, not just the attacker's own. At the upper end this is not merely a hang: an uncaught, process-terminating out-of-memory crash from a ~200KB attacker-supplied string is a full denial of service requiring no authentication — just the ability to get one string parsed by an application using this library (config upload, API request body, imported manifest, or any of this library's intended use cases).

Suggested remediation

Build the path prefix as an incrementally-extended string as the loop walks each segment (append one already-quoted segment per iteration to a running string) instead of re-slicing an array and re-joining the entire prefix from scratch on every iteration. For example, in assign():

function assign(node) {
  var keys = node.key;
  var value = node.value;
  var off = node.offset;
  if (!Array.isArray(keys)) keys = [keys];
  var reduced = reduceValueNode(value);

  var target = context;
  var pathStr = pathKey(currentPath); // computed once, not per segment
  for (var i = 0; i < keys.length - 1; i++) {
    var k = keys[i];
    var quotedK = quoteDottedString(k);
    var intermediatePath = pathStr ? pathStr + "." + quotedK : quotedK; // O(new segment), not O(i)
    pathStr = intermediatePath;
    // ...same conditional logic as today, using intermediatePath
    target = target[k];
  }
  // ...
}

The identical treatment applies to deepRef() and setNestedKey(). As defense-in-depth, consider also capping total key-segment count or overall input size, the same way maxDepth already caps nesting.

Credit

Discovered and reported by SecDim Security Research secdim.com | @secdim | [email protected]