Summary

shell-quote's parse() finalizes its token list with a reduce that uses
Array.prototype.concat as the accumulator. Each prev.concat(arg) copies the entire growing
array, so parse() runs in O(n²) in the number of tokens. An unauthenticated attacker who
can submit a string to any code path that calls parse() on it can block the single-threaded
Node.js event loop for tens of seconds with a small input — a denial of service. The trigger
needs no shell metacharacters (plain space-separated words suffice), so input filters that
only screen for ;, |, $, or backticks do not help.

Root cause

parse.js (lines 200–203), in parseInternal — this path runs on every parse() call:

}).reduce(function (prev, arg) { // finalize parsed arguments
    // TODO: replace this whole reduce with a concat
    return typeof arg === 'undefined' ? prev : prev.concat(arg);
}, []);

prev.concat(arg) allocates a new array and copies all of prev on every iteration, so
producing an N-token result costs 1 + 2 + … + N = O(N²) copies. A second acc.concat(s)
reduce in the module.exports wrapper (lines 211–224, reached only when env is a function)
has the same shape. The maintainer's own // TODO: replace this whole reduce with a concat
already flags the construct.

Proof of Concept

const { parse } = require('shell-quote');
const ms = fn => { const t = process.hrtime.bigint(); fn(); return Number(process.hrtime.bigint()-t)/1e6; };
for (const N of [16000, 32000, 64000, 128000]) {
  console.log(N, 'tokens ->', ms(() => parse('x '.repeat(N))).toFixed(0), 'ms');
}

Measured on shell-quote@1.8.4, Node v24:

Time grows ~×4 per 2× input → confirmed O(n²). A ~128 KB input blocks the event loop ~15 s;
~256 KB → ~57 s; a few hundred KB more → minutes.

poc.js

Impact

parse() is synchronous on the main thread; while it copies arrays quadratically the entire
event loop is blocked and the process serves no other requests. Any service that calls parse()
on attacker-influenced input (command parsers, chat-ops / bot command handlers, REPLs,
build-script / arg-string splitters) can be driven to a sustained DoS with a single small
request. No code execution and no data disclosure — availability only.

End-to-end confirmation: a minimal HTTP server that calls parse() on the request body, hit
with one POST of 'x '.repeat(32000) (~63 KB), froze for ~4.5 s. An out-of-process probe
client issuing harmless GET /ping requests (normally ~1 ms) observed 27 consecutive pings
stalled by up to 4374 ms during that single request — i.e. every concurrent client was denied
service for the whole parse. Scaling the body to a few hundred KB extends the outage to minutes.

This is the same class as several accepted 2026 advisories for quadratic-parser DoS on
untrusted input (e.g. markdown-it CVE-2026-48988, js-yaml CVE-2026-53550,
python-multipart CVE-2026-53539). It is distinct from the known shell-quote
command-injection issues (CVE-2021-42740, CVE-2016-10541, CVE-2026-9277), which are all in
quote(), not parse().

Suggested remediation

Replace the O(n²) concat-in-reduce with a linear flatten that pushes into the
accumulator instead of reallocating and copying it on every iteration. Apply the
same shape to the wrapper's acc.concat(s) reduce. A defensive input-length cap on
parse() is a cheap additional stop-gap.

Maintainer note (edit): the originally-suggested Array.prototype.flat() is
ES2019 / Node 11+, but shell-quote declares engines: node >= 0.4, so .flat()
would silently drop support for older runtimes. The fix instead flattens one-level
array tokens with forEach/push — and deliberately not push.apply(...), since
spreading a large array into function arguments can exceed the engine's argument
count limit. Output is byte-identical to the current code across strings, undefined
holes, one-level array tokens, and {op}/{comment}/{op:'glob'} objects, and
finalizing is now linear (1,024,000 tokens in ~150 ms vs ~57 s for 128,000 before).
Thanks for the clear report and PoC — the analysis and reproduction were spot on.

Disclosure

Found by source audit + wall-clock confirmation against 1.8.4 (and verified the same code is
present on main). Reported privately here; no public disclosure until a fix is available.