brace-expansion: DoS via unbounded expansion length causing an out-of-memory process crash

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:

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

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.