CVE-2025-48866 is a high-severity denial of service (DoS) vulnerability affecting versions of the open source web application firewall (WAF) engine ModSecurity prior to 2.9.10. ModSecurity is widely used as a cross-platform WAF for Apache, IIS, and Nginx web servers to protect web applications from various attacks. The vulnerability arises from the `sanitiseArg` (and its alias `sanitizeArg`) action within ModSecurity rules. This action is designed to sanitize input arguments, but due to improper handling, it can be exploited to add an excessive number of arguments in a loop. This excessive resource consumption leads to a denial of service condition by exhausting platform resources such as CPU and memory, causing the WAF process or the hosting web server to become unresponsive or crash. The vulnerability is similar to a previously disclosed issue (CVE-2025-47947) and is tracked under CWE-1050, which relates to excessive resource consumption within a loop. The CVSS 3.1 base score is 7.5, indicating a high severity, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no impact on confidentiality or integrity (C:N/I:N), but a high impact on availability (A:H). No known exploits are currently reported in the wild, and the issue is fixed in ModSecurity version 2.9.10. Until upgrading, the recommended workaround is to avoid using rules that contain the `sanitiseArg` or `sanitizeArg` actions to prevent triggering the vulnerability.

For European organizations deploying ModSecurity as part of their web application security stack, this vulnerability poses a significant risk of denial of service. An attacker can remotely trigger the excessive resource consumption without authentication or user interaction, potentially causing the WAF or the underlying web server to become unavailable. This can lead to service outages, impacting business continuity, customer access, and potentially causing financial and reputational damage. Since ModSecurity is often deployed in front of critical web applications, the disruption could affect e-commerce platforms, government portals, financial services, and other sectors reliant on continuous web availability. The lack of confidentiality or integrity impact means data breaches are unlikely from this vulnerability alone, but the availability impact can be severe. Additionally, denial of service conditions can be leveraged as part of multi-stage attacks or to distract security teams. The absence of known exploits in the wild suggests that proactive patching can effectively mitigate risk before widespread exploitation occurs.

European organizations should prioritize upgrading ModSecurity to version 2.9.10 or later, where this vulnerability is fixed. If immediate upgrading is not feasible, administrators should audit their ModSecurity rulesets to identify and disable any rules that use the `sanitiseArg` or `sanitizeArg` actions, as these are the vectors triggering the excessive resource consumption. Monitoring WAF logs for unusual spikes in argument processing or resource usage can help detect attempted exploitation. Implementing rate limiting and web server resource controls can also help mitigate the impact of potential DoS attempts. Organizations should ensure their incident response and business continuity plans account for potential WAF outages. Finally, maintaining an up-to-date inventory of ModSecurity deployments and versions across the infrastructure will facilitate timely patch management and vulnerability remediation.