GHSA-mh99-v99m-4gvg

### Summary

`expand()` bounds the *number* of results it produces (the `max` option, `100_000` by default) but not their *length*. By chaining many brace groups, an attacker keeps the result count under `max` while making every result grow with the number of groups. Building `max` long results — plus the intermediate arrays combined at each brace group — exhausts memory and crashes the Node process with an **uncatchable** out-of-memory error. `try/catch` around `expand()` does not help: the fatal error terminates the process.

A ~7.5 KB input (`'{a,b}'.repeat(1500)`) is enough to crash a default Node process.

### Details

For `N` chained brace groups such as `'{a,b}'.repeat(N)`:

- the result count is `2^N`, immediately capped at `max` (`100_000`), so the `max` protection appears to hold, but - each result is `N` characters long, so the total output size is `max × N` characters, which grows without bound in `N`.

`expand_` combines each brace set with the fully-expanded tail:

```js const post = m.post.length ? expand_(m.post, max, false) : [''] ... for (let j = 0; j < N.length; j++) { for (let k = 0; k < post.length && expansions.length < max; k++) { const expansion = pre + N[j] + post[k] // grows one group longer per level ... expansions.push(expansion) } } ```

The loop guard `expansions.length < max` limits how many strings are built, but nothing limits how long they get. Each recursion level materializes another array of up to `max` strings, one character longer than the level below, and — because V8 represents `pre + N[j] + post[k]` as a cons-string (rope) that references `post[k]` — those intermediate strings stay reachable through the whole chain. Memory therefore scales with `max × N`.

Measured on `5.0.7` (`'{a,b}'.repeat(N)`, default `max`):

| groups (N) | input bytes | result count | peak RSS | |---|---|---|---| | 20 | 100 | 100,000 | ~80 MB | | 50 | 250 | 100,000 | ~214 MB | | 100 | 500 | 100,000 | ~409 MB | | 300 | 1,500 | 100,000 | ~1,148 MB | | 1500 | 7,500 | — | **OOM crash** |

### Proof of concept

```js const { expand } = require('brace-expansion')

// ~7.5 KB input — crashes the process with a fatal, uncatchable OOM: // FATAL ERROR: ... JavaScript heap out of memory try { expand('{a,b}'.repeat(1500)) } catch (e) { // never reached — the process is already dead } ```

### Impact

Any application that passes attacker-influenced strings to `brace-expansion.expand()` — directly, or transitively via `minimatch` / `glob` brace patterns — can be crashed by a small request. Because the failure is a fatal V8 out-of-memory error rather than a thrown exception, it cannot be caught and it takes down the whole worker/process, denying service.

### Remediation

Upgrade to a patched release. The fix bounds the total number of characters a single `expand()` call may accumulate (`EXPANSION_MAX_LENGTH`, default `4_000_000`, configurable via a new `maxLength` option), applied inside the output-building loops so intermediate arrays are bounded too. Once the limit is reached, output is truncated — consistent with how `max` already truncates — instead of growing without bound. The limit sits well above any realistic expansion (100,000 results hitting `max` measure ~1M characters), so legitimate input is unaffected.

After the fix, `'{a,b}'.repeat(1500)` returns a bounded, truncated result in ~0.7 s using ~340 MB and never crashes, including under a constrained 512 MB heap.

The fix bounds memory but the algorithm still rebuilds intermediate arrays at each level (roughly `O(N × maxLength)` work on this input class). A streaming rewrite that produces output in `O(total output size)` can be a non-urgent follow-up.

If immediate upgrade isn't possible, avoid passing untrusted input to `expand()` / glob brace patterns, or pass a small explicit `max` **and** `maxLength`.