GHSA-mh99-v99m-4gvg

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 |...