smol-toml: Quadratic-time parse() from parseKey rescanning to end of document on each key line

Summary

parse() has a quadratic-time path in parseKey, reachable on default options with ordinary valid input. For every key line and table-header line, parseKey (dist/struct.js, lines 58 and 86) finds the dotted-key separator with ctx.s.indexOf('.', ctx.p), where ctx.s is the whole document. When a key has no . ahead of it, that search runs all the way to the end of the input, and the result is then clamped back to the line terminator endPtr - so everything scanned past the current line is wasted. parseKey runs once per line, so a document of N dot-free keys costs O(n^2).

The most ordinary TOML shape triggers it: a flat list of key = value lines, or a repeated [[a]] table. No dotted keys, no special options, valid input throughout.

Proof of concept

import { parse } from 'smol-toml'

let doc = ''
for (let i = 0; i < 256000; i++) doc += 'k' + i + ' = 1\n'

console.time('parse')
parse(doc) // ~2.8 MB of valid TOML, default options
console.timeEnd('parse')

Doubling the line count roughly quadruples the time:

Impact

Any service that runs parse() on attacker-supplied TOML can be stalled. The work is synchronous, so it blocks the whole event loop, and the quadratic is unbounded: a ~7 MB body pins a core for about a minute, larger bodies for several.

Patches

Version 1.9.0 uses a different implementation for parsing keys which is strictly linear.

Workarounds

Limit the maximum document size accepted when parsing arbitrary documents.