GHSA-q2hr-2g5m-vwhr brace-expansion: Quadratic-time expansion of the `{a},b}` rewrite causes CPU denial of service

• npm brace-expansion: >= 4.0.0, < 5.0.12 → fixed in 5.0.12 • npm brace-expansion: >= 3.0.0, < 3.0.9 → fixed in 3.0.9 • npm brace-expansion: >= 2.0.0, < 2.1.7 → fixed in 2.1.7 • npm brace-expansion: < 1.1.21 → fixed in 1.1.21

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.