brace-expansion: DoS via exponential-time expansion of consecutive non-expanding {} groups

### 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. Source Code Location: https://github.com/juliangruber/brace-expansion Affected Packages: - npm:brace-expansion, affected >= 3.0.0, < 5.0.7, patched in 5.0.7 - npm:brace-expansion, affected < 1.1.16, patched in 1.1.16 - npm:brace-expansion, affected >= 2.0.0, < 2.1.2, patched in 2.1.2 CWEs: - CWE-400: Uncontrolled Resource Consumption - CWE-407: Inefficient Algorithmic Complexity CVSS: - Primary: score 5.3, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L - CVSS_V4: score 7.7, CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:P/S:N/AU:Y/R:U/V:D/RE:M/U:Amber - CVSS_V3: score 5.3, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L References: - https://github.com/juliangruber/brace-expansion/security/advisories/GHSA-3jxr-9vmj-r5cp - https://nvd.nist.gov/vuln/detail/CVE-2026-13149 - https://github.com/juliangruber/brace-expansion/pull/122 - https://github.com/juliangruber/brace-expansion/pull/123 - https://github.com/juliangruber/brace-expansion/commit/835d6be91201122d9adffb0c0c8c094189ace265 - https://github.com/juliangruber/brace-expansion/commit/c7e33ec13ac1a684c116720843ce24e208611754 - https://github.com/juliangruber/brace-expansion/commit/d74e63030c012e3b7ae81657b8d665619cd51b95 - https://github.com/juliangruber/brace-expansion/releases/tag/v1.1.16 - https://github.com/juliangruber/brace-expansion/releases/tag/v2.1.2 - https://github.com/juliangruber/brace-expansion/releases/tag/v5.0.7 - https://www.npmjs.com/package/brace-expansion - https://github.com/advisories/GHSA-3jxr-9vmj-r5cp