CVE-2026-2391
Published: 12 February 2026
Summary
CVE-2026-2391 is a medium-severity Improper Input Validation (CWE-20) vulnerability in Qs Project Qs. Its CVSS base score is 6.3 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 16.0th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 SC-5 (Denial-of-service Protection) and SI-10 (Information Input Validation).
Deeper analysis
CVE-2026-2391 is a denial-of-service vulnerability in the qs JavaScript library, a popular query string parser used in Node.js applications. The issue arises because the arrayLimit option fails to enforce limits on comma-separated values when the comma option is explicitly set to true. In this configuration, qs parses strings like ?param=a,b,c into arrays but performs the arrayLimit check (default: 20) and optional throwOnLimitExceeded after the comma-splitting logic in lib/parse.js, allowing bypass. This permits creation of arbitrarily large arrays from a single parameter, leading to excessive memory allocation and exhaustion.
Remote, unauthenticated attackers can exploit this vulnerability by crafting a malicious query string with a parameter containing millions of commas, such as ?param=,,,,,,,,..., targeting endpoints that use qs.parse with comma: true enabled. Although comma: true is not the default, applications that configure it for parsing comma-separated arrays are susceptible. Successful exploitation results in denial of service through memory exhaustion, as demonstrated in the proof-of-concept where an arrayLimit of 5 is bypassed to create an array of 26 elements without throwing an error. The CVSS v3.1 base score is 3.7 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L), reflecting low impact but network accessibility with high attack complexity due to the required configuration.
The GitHub security advisory GHSA-w7fw-mjwx-w883 and the commit f6a7abff1f13d644db9b05fe4f2c98ada6bf8482 address this bypass by fixing the enforcement discrepancy in qs, similar to the prior bracket notation fix in v6.14.1 for GHSA-6rw7-vpxm-498p (CVE-2025-15284). Security practitioners should update to the patched version of qs and review applications for use of comma: true, considering disabling it or setting strict arrayLimit with throwOnLimitExceeded where feasible.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-6720
Vulnerability details
### Summary The `arrayLimit` option in qs does not enforce limits for comma-separated values when `comma: true` is enabled, allowing attackers to cause denial-of-service via memory exhaustion. This is a bypass of the array limit enforcement, similar to the bracket…
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notation bypass addressed in GHSA-6rw7-vpxm-498p (CVE-2025-15284). ### Details When the `comma` option is set to `true` (not the default, but configurable in applications), qs allows parsing comma-separated strings as arrays (e.g., `?param=a,b,c` becomes `['a', 'b', 'c']`). However, the limit check for `arrayLimit` (default: 20) and the optional throwOnLimitExceeded occur after the comma-handling logic in `parseArrayValue`, enabling a bypass. This permits creation of arbitrarily large arrays from a single parameter, leading to excessive memory allocation. **Vulnerable code** (lib/parse.js: lines ~40-50): ```js if (val && typeof val === 'string' && options.comma && val.indexOf(',') > -1) { return val.split(','); } if (options.throwOnLimitExceeded && currentArrayLength >= options.arrayLimit) { throw new RangeError('Array limit exceeded. Only ' + options.arrayLimit + ' element' + (options.arrayLimit === 1 ? '' : 's') + ' allowed in an array.'); } return val; ``` The `split(',')` returns the array immediately, skipping the subsequent limit check. Downstream merging via `utils.combine` does not prevent allocation, even if it marks overflows for sparse arrays.This discrepancy allows attackers to send a single parameter with millions of commas (e.g., `?param=,,,,,,,,...`), allocating massive arrays in memory without triggering limits. It bypasses the intent of `arrayLimit`, which is enforced correctly for indexed (`a[0]=`) and bracket (`a[]=`) notations (the latter fixed in v6.14.1 per GHSA-6rw7-vpxm-498p). ### PoC **Test 1 - Basic bypass:** ``` npm install qs ``` ```js const qs = require('qs'); const payload = 'a=' + ','.repeat(25); // 26 elements after split (bypasses arrayLimit: 5) const options = { comma: true, arrayLimit: 5, throwOnLimitExceeded: true }; try { const result = qs.parse(payload, options); console.log(result.a.length); // Outputs: 26 (bypass successful) } catch (e) { console.log('Limit enforced:', e.message); // Not thrown } ``` **Configuration:** - `comma: true` - `arrayLimit: 5` - `throwOnLimitExceeded: true` Expected: Throws "Array limit exceeded" error. Actual: Parses successfully, creating an array of length 26. ### Impact Denial of Service (DoS) via memory exhaustion.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Direct mapping to application exploitation causing endpoint DoS via memory exhaustion from crafted input.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly enforces validation of query-string input to reject or truncate excessively long comma-separated values before qs.parse allocates unbounded arrays.
Requires mechanisms that limit resource consumption caused by crafted query parameters containing millions of commas when comma:true is enabled.
Ensures memory and process resources remain available by constraining allocation that would otherwise result from the arrayLimit bypass in lib/parse.js.