CVE-2026-2391 is a vulnerability in the 'qs' library, a popular Node.js module for parsing URL query strings. The issue stems from improper input validation (CWE-20) related to the 'arrayLimit' option when the 'comma' parsing feature is enabled. Normally, 'arrayLimit' restricts the maximum number of elements parsed into an array to prevent resource exhaustion. However, when 'comma: true' is set, the library parses comma-separated values (e.g., '?param=a,b,c') into arrays by splitting the string on commas before enforcing the 'arrayLimit'. The enforcement check occurs only after this splitting, but the code returns the split array immediately, bypassing the limit check entirely. This allows attackers to submit a single query parameter with an arbitrarily large number of commas, resulting in the creation of very large arrays in memory. The excessive memory allocation can exhaust server resources, causing denial-of-service conditions. This bypass is similar to a previously fixed bracket notation bypass (CVE-2025-15284). The vulnerability does not require authentication or user interaction and can be triggered remotely via HTTP requests. The default 'comma' option is false, so only applications explicitly enabling it are vulnerable. The vulnerability affects all versions of 'qs' prior to the fix and is rated medium severity with a CVSS 4.0 score of 6.3, reflecting network attack vector, low complexity, no privileges or user interaction required, and limited impact on availability. No known exploits are currently reported in the wild.

For European organizations, this vulnerability poses a risk primarily to web applications and APIs built on Node.js that use the 'qs' library with the 'comma' option enabled for parsing query parameters. Exploitation can lead to denial-of-service attacks by exhausting server memory, causing application crashes or severe performance degradation. This can disrupt business operations, degrade user experience, and potentially cause downtime. Industries with high web traffic or critical online services—such as finance, healthcare, e-commerce, and government portals—are particularly vulnerable. The impact is availability-focused, with no direct confidentiality or integrity compromise. However, denial-of-service incidents can indirectly affect data availability and service reliability. Since the attack vector is network-based and requires no authentication, attackers can exploit this vulnerability remotely and anonymously. European organizations relying on Node.js applications with unpatched 'qs' dependencies may face increased risk of service disruption, especially if they process complex query strings or enable the 'comma' parsing feature for convenience or legacy reasons.

1. Immediate upgrade: Update the 'qs' library to the latest patched version where this vulnerability is fixed. Monitor official repositories and advisories for the release addressing CVE-2026-2391. 2. Configuration audit: Review application code and configurations to identify usage of 'qs' with 'comma: true'. Disable the 'comma' option if not strictly necessary, as it is not enabled by default. 3. Input validation: Implement additional server-side validation to limit the length and complexity of query parameters before parsing. Reject or sanitize inputs with excessive commas or unusually large parameter sizes. 4. Rate limiting and WAF: Deploy rate limiting and Web Application Firewall (WAF) rules to detect and block suspicious requests containing abnormally large comma-separated values. 5. Monitoring and alerting: Set up monitoring for abnormal memory usage or application crashes that could indicate exploitation attempts. 6. Dependency management: Use tools like npm audit and software composition analysis (SCA) to detect vulnerable versions of 'qs' in the software supply chain. 7. Incident response: Prepare response plans for DoS incidents, including quick rollback or patch deployment capabilities. These steps go beyond generic advice by focusing on configuration review, input validation, and proactive detection tailored to this specific vulnerability.