CVE-2026-13149: High severity npm brace-expansion vulnerability

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 |

Proof of concept js const { expand } = require('brace-expansion'); // 30 non-expanding groups, ~90 bytes — blocks for minutes: expand('a{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{}');

Impact

Any application that passes attacker-influenced strings to brace-expansion.expand() - directly or transitively via minimatch/glob brace patterns - can be driven into a multi-minute-to-indefinite CPU hang by a tiny request, denying service on that thread/process.

Remediation

Upgrade to a patched release. The fix: 1. Defers computing post until after the early-return branches (and computes it locally in the $-suffix branch), so post is only expanded when a brace set actually expands and the value is used. This alone removes the exponential. 1. Converts the {a},b} rewrite from recursion to an in-function loop, so a long run of rewrites cannot grow the call stack.

Verified: the PoC drops from ~2 min to 0.55 ms, 5,000 groups complete in ~344 ms, and output is identical to 5.0.6 across a behavioral-equivalence suite (sequences, padding, $-prefix, a{},b}c, {},a}b, x{{a,b}}y, etc.). Post-fix complexity is ~O(n²) on this input class - acceptable for the security fix; a linear rewrite can be a non-urgent follow-up.

If immediate upgrade isn't possible, avoid passing untrusted input to expand() / glob brace patterns, or run such expansion under a timeout/worker.