Affected: npm: brace-expansion

### Summary brace-expansion's expand() exhibits exponential-time - O(2ⁿ) - behavior in the number of consecutive non-expanding {} groups. A short, all-ASCII input (~90 bytes/30 groups) blocks the calling thread for minutes; a slightly longer input hangs it effectively indefinitely. Because the dominant consumers run on Node's single-threaded event loop, one small input can fully stall a worker/process.

In `expand_`, `post` is computed unconditionally at the top of the function, before the early-return branches that don't use it: ```js const post = m.post.length ? expand_(m.post, max, false) : ['']; // always recurses ... if (!isSequence && !isOptions) { if (m.post.match(/,(?!,).*\}/)) { str = m.pre + '{' + m.body + escClose + m.post; return expand_(str, max, true); // restart — `post` discarded } return [str]; } ```

For input like a{},{},…, the first {} is non-expanding, so control reaches the {a},b} rewrite branch - but `expand_` has already recursed into post over the entire remaining tail, only to throw the result away. Each level therefore spawns two recursive expansions over essentially the same remaining work: `T(n) = 2·T(n−1) ⇒ O(2ⁿ)`.

The max option does not mitigate this: max only bounds the output-building loops; neither the post recursion nor the rewrite recursion consults it. Measured on 5.0.6:

| groups (n) | input bytes | time | |---|---|---| | 20 | 60 | 130 ms | | 24 | 72 | 1.9 s | | 26 | 78 | 7.8 s | | 30 (PoC) | 90 | ~2 min |

### Pro…