Details
### 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
```js
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:
| lines | size | parse() |
|---|---|---|
| 32k | 0.3 MB | 0.3 s |
| 64k | 0.7 MB | 1.0 s |
| 128k | 1.4 MB | 3.5 s |
| 256k | 2.8 MB | 14 s |
### 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.
