NodeJS/brace-expansion/2.1.5

brace-expansion: Quadratic-time expansion of the `{a},b}` rewrite causes CPU denial of service

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

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.

brace-expansion: DoS via uncontrolled recursion on nested brace groups causing stack exhaustion

Summary

expand_() recurses once per level of brace nesting. Deeply nested input exhausts the native stack and crashes the process.

This is distinct from CVE-2026-14257 / GHSA-mh99-v99m-4gvg, which made the tail iterative (recursion on m.post, driven by how many groups are chained). Nesting depth drives a different recursion that the tail fix never touched, so the documented constant-stack-depth guarantee only ever covered chained input, not nested input.

It is also distinct from GHSA-6j4f-fj2g-mc7p, which fixed recursion in parseCommaParts(). Both payloads below still crash with that fix applied.

Two recursion sites

Comma members. Each alternative of a brace set is expanded by a recursive call, so nesting a set inside every alternative recurses once per level:

expand('{a,'.repeat(4000) + 'z' + '}'.repeat(4000))
// RangeError: Maximum call stack size exceeded

Crashes at depth 3,907 - about 15.6 KB of input.

Single set. A brace set whose body parses to a single part is expanded by a recursive call before being re-wrapped (x{{a,b}}y -> x{a}y x{b}y), which recurses once per nesting level:

expand('{'.repeat(3200) + 'a,b' + '}'.repeat(3200))
// RangeError: Maximum call stack size exceeded

Crashes at depth 3,125 - about 6.25 KB of input. This is the cheapest stack-exhaustion payload known against this package: roughly a quarter the input of GHSA-6j4f-fj2g-mc7p (29 KB), and about a tenth of minimatch's MAX_PATTERN_LENGTH (65,536).

Why max and maxLength do not help

Both crashes happen while recursing into sub-expansions, before the result set grows. The payloads produce almost no output - the single-set case yields 2 results - so neither bound is ever the limiter. expand(payload, { max: 1, maxLength: 1 }) still overflows.

Impact

Any application passing an untrusted string to expand(), directly or through minimatch / glob as a user-supplied glob pattern, can be crashed. In Node a RangeError the application does not catch terminates the process, so a server globbing user input is exposed to remote unauthenticated denial of service.

Availability only. No code execution, no data exposure.

Affected versions

Verified affected on 1.1.18, 2.1.4, 3.0.6 and 5.0.9, at near-identical depths on every line (single set: 3,125 on all four; comma members: 3,907-4,102). Not a regression from any recent fix - the gap predates them.

Patch

A maxDepth bound (default EXPANSION_MAX_DEPTH) is threaded through expand_(). Past the cap a group is treated as non-expanding and returned literally, which is how the parser already handles a group that cannot expand. This matches the existing max / maxLength caps, which truncate rather than throw, so expand() continues never to throw on any input.

The default sits far above any realistic nesting depth and well below the crash threshold.