Details
### Summary
Expanding `{a},b}`-shaped input takes time quadratic in the number of literal `}` characters, blocking the event loop.
Bash preserves a quirk where a brace group followed by a comma set still expands (`{a},b}`). The parser implements this by rewriting the string and restarting the scan. Each pass absorbs exactly one `}` and re-scans from the beginning, so `n` trailing braces cost `n` full passes.
### Reproduction
```js
const build = n => '{a}' + '}'.repeat(n) + ',z}'
for (const n of [8000, 16000, 32000, 64000, 128000]) {
const t = Date.now()
expand(build(n))
console.log(n, Date.now() - t + 'ms')
}
```
| n | input | time | results |
|---|---|---|---|
| 8,000 | 8 KB | 110 ms | 2 |
| 16,000 | 16 KB | 446 ms | 2 |
| 32,000 | 32 KB | 1.7 s | 2 |
| 64,000 | 64 KB | 6.9 s | 2 |
| 128,000 | 128 KB | **27.7 s** | 2 |
`ms/n^2` is flat at ~1.7 and each doubling of `n` costs exactly 4.0x - quadratic. 128 KB of input blocks the event loop for nearly half a minute to produce two results.
### Mechanism
Instrumenting the rewrite branch confirms it runs exactly `n + 1` times, once per literal `}`, each re-scanning the whole string.
There is a second multiplier. The rewrite replaces the group's closing `}` with the internal `escClose` sentinel, which is `'\0CLOSE' + Math.random() + '\0'` - about 25 characters. The working string therefore *grows* by ~25 characters on every pass:
| n | input length | final string length |
|---|---|---|
| 1,000 | 1,006 | 26,006 |
| 8,000 | 8,006 | 208,006 |
So the input is inflated roughly 26x, and that factor multiplies both the quadratic constant and peak memory. This makes it partly a memory-pressure issue as well as a CPU one.
### Why `max` and `maxLength` do not help
The cost is in parsing, before the result set exists. The payload yields 2 results regardless of size, so neither bound is ever reached.
### Impact
An application passing an untrusted pattern to `expand()`, directly or through `minimatch` / `glob`, can have its event loop blocked for tens of seconds by a payload well under minimatch's 65,536-character cap. For a single-threaded Node server that is a full stall, not just a slow request.
Degraded availability rather than a crash - the process recovers once the expansion completes.
### Affected versions
Verified affected on 1.1.18, 2.1.4, 3.0.6 and 5.0.9, all within a few percent of each other (~460-490 ms at n=16,000).
### Patch
The rewrite loop gets an iteration bound. Past the cap the remaining string is treated as non-expanding and returned literally, consistent with the existing `max` / `maxLength` caps, which truncate rather than throw.
Note this bounds the number of passes, not the cost of each: worst-case work remains proportional to `cap x input length`. The cap is set low enough that the residual is bounded in practice, and far above what any realistic `{a},b}` input needs.
### Severity note
Scored 5.3 Medium (`A:L`) for consistency with GHSA-3jxr-9vmj-r5cp, the other algorithmic-complexity advisory on this package (CWE-407), which uses the same vector. The stack-exhaustion advisories on this package score `A:H` because they crash the process outright; this one stalls it.
EPSS — exploit probability
Low0.30%
estimated chance of real-world exploitation in the next 30 days — higher than 20.5% of every CVE FIRST.org scores
Refreshed 9/30/2026 — via FIRST.org's EPSS model, not CVSS — this measures likelihood of exploitation, not how severe it would be.
